Bypassing the Transistor Paradigm

Current computer technology is based on solid state transistors, in which binary information is carried by switching current between "on" and "off" states. When these devices are shrunk to the molecular scale, they run up against fundamental physical limits, most importantly regarding energy dissipation; as a result, superdense transistor devices would melt as soon as they were switched on. In this Perspective,we discusses the advantages of an alternative route to molecular electronics, quantum-dot cellular automata , which store binary information in a charge configuration instead of a current switch.

How Electronic Components Work

Electronic gadgets have become an integral part of our lives. They have made our lives more comfortable and convenient. From aviation to medical and healthcare industries, electronic gadgets have a wide range of applications in the modern world. In fact, the electronics revolution and the computer revolution go hand in hand.

Most gadgets have tiny electronic circuits that can control machines and process information. Simply put, electronic circuits are the lifelines of various electrical appliances. This guide explains in detail about common electronic components used in electronic circuits and how they work.

In this article I will provide an overview on electronic circuits. Then I will provide more information on 7 different types of components. For each type I'll discuss the composition, how it works, and the function & significance of the component.

Electronic Circuit Overview

An electronic circuit is a structure that directs and controls electric current to perform various functions including signal amplification, computation, and data transfer. It comprises several different components such as resistors, transistors, capacitors, inductors, and diodes. Conductive wires or traces are used to connect the components to each other. However, a circuit is complete only if it starts and ends at the same point, forming a loop.

The Elements of an Electronic Circuit

The complexity and the number of components in an electronic circuit may change depending on its application. However, the simplest circuit consists of three elements, including a conducting path, a voltage source, and a load.

Element 1: Conducting Path

The electric current flows through the conducting path. Though copper wires are used in simple circuits, they are rapidly being replaced by conductive traces. Conductive traces are nothing but copper sheets laminated onto a non-conductive substrate. They are often used in small and complex circuits such as Printed Circuit Boards (PCB).

Element 2: Voltage Source

The primary function of a circuit is to allow electric current to pass through it safely. So, the first key element is the voltage source. It is a two-terminal device such as a battery, generators or power systems that provide a potential difference (voltage) between two points in the circuit so that current can flow through it.

Element 3: Load

A load is an element in the circuit that consumes power to perform a particular function. A light bulb is the simplest load. Complex circuits, however, have different loads such as resistors, capacitors, transistors, and transistors.

Electronic Circuit Facts

Fact 1: Open Circuit

As mentioned before, a circuit must always form a loop to allow the current to flow through it. However, when it comes to an open circuit, the current can’t flow as one or more components are disconnected either intentionally (by using a switch) or accidentally (broken parts). In other words, any circuit that does not form a loop is an open circuit.

Fact 2: Closed Circuit

A closed circuit is one that forms a loop without any interruptions. Thus, it is the exact opposite of an open circuit. However, a complete circuit that doesn’t perform any function is still a closed circuit. For example, a circuit connected to a dead battery may not perform any work, but it is still a closed circuit.

Fact 3: Short Circuit

In the case of short-circuit, a low-resistance connection forms between two points in an electric circuit. As a result, the current tends to flow through this newly formed connection rather than along the intended path. For example, if there is a direct connection between the battery’s negative and positive terminal, the current will flow through it rather than passing through the circuit.

However, short circuits usually lead to serious accidents as the current can flow at dangerously high levels. Hence, a short circuit can damage electronic equipment, cause batteries to explode, and even start a fire in commercial and residential buildings.

Fact 4: Printed Circuit Boards (PCBs)

Most electronic appliances require complex electronic circuits. That’s why designers have to arrange tiny electronic components on a circuit board. It comprises a plastic board with connecting copper tracks on one side and lots of holes to affix the components. When the layout of a circuit board is printed chemically onto a plastic board, it is called a printed circuit board or PCB.

**Figure 1:** Printed Circuit Board. [Image Source]

Fact 5: Integrated Circuits (ICs)

Though PCBs can offer a lot of advantages, most modern instruments such as computers and mobiles require complex circuits, having thousands and even millions of components. That’s where integrated circuits come in. They are the tiny electronic circuits that can fit inside a small silicon chip. Jack Kilby invented the first integrated circuit in 1958 at Texas Instruments. The sole purpose of ICs is to increase the efficiency of the electronic devices, while reducing their size and manufacturing cost. Over the years, integrated circuits have become increasingly sophisticated as technology continues to evolve. That’s why personal computers, laptops, mobiles phones, and other consumer electronics are getting cheaper and better by the day.

**Figure 2:** Integrated Circuits. [Image Source]

Electronic Components

Thanks to modern technology, electronic circuit building process has been completely automated, especially for building ICs and PCBs. The number and arrangement of components in a circuit may vary depending on its complexity. However, it is built using a small number of standard components.

The following components are used to construct electronic circuits.

Component 1: Capacitor

Capacitors are widely used to build different types of electronic circuits. A capacitor is a passive two-terminal electrical component that can store energy in an electric field electrostatically. In simple terms, it works as a small rechargeable battery that stores electricity. However, unlike a battery, it can charge and discharge in the split of a second.

A. Composition

Capacitors come in all shapes and sizes, but they usually have the same primary components. There are two electrical conductors or plates separated by a dielectric or insulator stacked between them. Plates are composed of conducting material such as thin films of metal or aluminum foil. A dielectric, on the other hand, is a non-conducting material such as glass, ceramic, plastic film, air, paper, or mica. You can insert the two electrical connections protruding from the plates to fix the capacitor in a circuit.

B. How Does It Work?

When you apply a voltage over the two plates or connect them to a source, an electric field develops across the insulator, causing one plate to accumulate positive charge while negative charge gets collected on the other. The capacitor continues to hold its charge even if you disconnect it from the source. The moment you connect it to a load, the stored energy will flow from the capacitor to the load.

Capacitance is the amount of energy stored in a capacitor. The higher the capacitance, the more energy it can store. You can increase the capacitance by moving the plates closer to each other or increasing their size. Alternatively, you can also enhance the insulation qualities to increase the capacitance.

C. Function and Significance

Though capacitors look like batteries, they can perform different types of functions in a circuit such as blocking direct current while allowing alternating current to pass or smooth the output from a power supply. They are also used in electric power transmission systems to stabilize voltage and power flow. One of the most significant functions of a capacitor in the AC systems is power factor correction, without which you can’t provide sufficient amount of starting torque to single phase motors.

Filters Capacitor Applications

If you are using a microcontroller in a circuit to run a specific program, you don’t want its voltage to drop as that will reset the controller. That’s why designers use a capacitor. It can supply the microcontroller with the necessary power for a split second to avoid a restart. In other words, it filters out the noise on the power line and stabilizes the power supply.

Hold-Up Capacitor Applications

Unlike a battery, a capacitor releases its charge rapidly. That’s why it is used to provide power to a circuit for a short while. Your camera batteries charge the capacitor attached to the flash gun. When you take a flash photograph, the capacitor releases its charge in a split second to generate a flash of light.

Timer Capacitor Applications

In a resonant or time-dependent circuit, capacitors are used along with a resistor or inductor as a timing element. The time required to charge and discharge a capacitor determines the operation of the circuit.

Component 2: Resistor

A resistor is a passive two-terminal electrical device that resists the flow of current. It is probably the simplest element in an electronic circuit. It is also one of the most common components as resistance is an inherent element of nearly all electronic circuits. They are usually color-coded.

A. Composition

A resistor is not a fancy device at all because resistance is a natural property possessed by almost all conductors. So, a capacitor consists of a copper wire wrapped around an insulating material such as a ceramic rod. The number of turns and the thinness of copper wire are directly proportional to the resistance. The higher the number of turns and thinner the wire, the higher the resistance.

You can also find resistors made of a spiral pattern of a carbon film. Hence, the name carbon film resistors. They are designed for lower-power circuits because carbon film resistors are not as precise as their wire-wound counterparts. However, they are cheaper than wired resistors. Wire terminals are attached to the both ends. As resistors are blind to the polarity in a circuit, the current can flow through in either direction. So, there is no need to worry about attaching them in a forward or a backward direction.

B. How Does It Work?

A resistor may not look like much. One may think it doesn’t do anything except consume power. However, it performs a vital function: controlling the voltage and the current in your circuit. In other words, resistors give you control over the design of your circuit.

When electric current starts flowing through a wire, all the electrons start moving in the same direction. It’s just like water flowing through a pipe. Less amount of water will flow through a thin pipe because there is less room for its movement.

Similarly, when the current passes through a thin wire in a resistor, it becomes progressively harder for the electrons to wiggle through it. In short, the number of electrons flowing through a resistor goes down as the length and thinness of the wire increases.

C. Function and Significance

Resistors have plenty of applications, but the three most common ones are managing current flow, dividing voltage, and resistor-capacitor networks.

Limiting the Flow of Current

If you don’t add resistors to a circuit, the current will flow at dangerously high levels. It can overheat other components and possibly damage them. For example, if you connect an LED directly to a battery, it would still work. However, after some time the LED will heat up like a fireball. It will eventually burn as LEDs are less tolerant to heat.

But, if you introduce a resistor in the circuit, it will reduce the flow of current to an optimal level. Thus, you can keep the LED on longer without overheating it.

Dividing Voltage

Resistors are also used to reduce the voltage to the desired level. Sometimes, a particular part of a circuit such as a microcontroller may need a lower voltage than the circuit itself. This is where a resistor comes in.

Let’s say your circuit runs off of a 12V battery. However, the microcontroller needs only a 6V supply. So, to divide the voltage in half, all you have to do is place two resistors of equal resistance value in series. The wire in between the two resistors will have halved the voltage of your circuit where the microcontroller can be attached. Using appropriate resistors, you can lower the voltage within the circuit to any level.

Resistor-Capacitor Networks

Resistors are also used in combination with capacitors to build ICs that contain resistor-capacitor arrays in a single chip. They are also known as RC filters or RC networks. They are often used to suppress electromagnetic Interference (EMI) or Radio Frequency Interference (RFI) in various instruments, including input/output ports of computers and laptops, Local Area Networks (LANs), and Wide Area Networks (WANs), among others. They are also used in machine tools, switchgears, motor controllers, automated equipment, industrial appliances, elevators, and escalators.

Component 3: Diode

A diode is a two-terminal device that allows electric current to flow in only one direction. Thus, it is the electronic equivalent of a check valve or a one-way street. It is commonly used to convert an Alternating Current (AC) into a Direct Current (DC). It is made either of a semiconductor material (semiconductor diode) or vacuum tube (vacuum tube diode). Today, however, most diodes are made from semiconductor material, particularly silicon.

A. Composition

As mentioned earlier, there are two types of diodes: vacuum diodes and semiconductor diodes. A vacuum diode consists of two electrodes (cathode and anode) placed inside a sealed vacuum glass tube. A semiconductor diode comprises p-type and n-type semiconductors. It is, therefore, known as a p-n junction diode. It is usually made of silicon, but you can also use germanium or selenium.

B. How Does It Work?

Vacuum Diode

When the cathode is heated by a filament, an invisible cloud of electrons, called space charge, forms in the vacuum. Though electrons are emitted from the cathode, the negative space charge repels them. As electrons can’t reach the anode, no current flows through the circuit. However, when the anode is made positive, the space charge vanishes. As a result, current starts flowing from the cathode to the anode. Thus, electric current within the diode flows only from the cathode to the anode and never from the anode to the cathode.

P-N Junction Diode

A p-n junction diode comprises p-type and n-type semiconductors of silicon. The p-type semiconductor is usually doped with boron, leaving holes (positive charge) in it. The n-type semiconductor, on the other hand, is doped with antimony, adding a few extra electrons (negative charge) in it. So, electric current can flow through both semiconductors.

When you put p-type and n-type blocks together, the extra electrons from the n-type combine with the holes in the p-type, creating a depletion zone without any free electrons or holes. In short, current can no longer pass through the diode.

When you connect the battery’s negative terminal to the n-type silicon and the positive terminal to p-type (forward-bias), current starts to flow as electrons and holes can now move across the junction. However, if you reverse the terminals (reverse-bias), no current flows through the diode because holes and electrons are pushed away from each other, widening the depletion zone. So, just like a vacuum diode, a junction diode can also allow current to pass in one direction only.

C. Function and Significance

Though diodes are one of the simplest components in an electronic circuit, they have unique applications across industries.

AC to DC Conversion

The most common and important application of a diode is the rectification of AC power to DC power. Usually, a half-wave (single diode) or a full-wave (four diodes) rectifier is used to convert AC power into DC power, particularly in household power supply. When you pass AC power supply through a diode, only half the AC waveform passes through it. As this voltage pulse is used to charge the capacitor, it produces steady and continuous DC currents without any ripples. Different combinations of diodes and capacitors are also used to build various types of voltage multipliers to multiply a small AC voltage into high DC outputs.

Bypass Diodes

Bypass diodes are often used to protect solar panels. When the current from the rest of the cells passes through a damaged or dusty solar cell, it causes overheating. As a result, the overall output power decreases, creating hot spots. The diodes are connected parallel to the solar cells to protect them against this overheating problem. This simple arrangement limits the voltage across the bad solar cell while allowing the current to pass through undamaged cells to the external circuit.

Voltage Spike Protection

When the power supply is suddenly interrupted, it produces a high voltage in most inductive loads. This unexpected voltage spike can damage the loads. However, you can protect expensive equipment by connecting a diode across the inductive loads. Depending on the type of security, these diodes are known by many names including snubber diode, flyback diode, suppression diode, and freewheeling diode, among others.

Signal Demodulation

They are also used in the process of signal modulation because diodes can remove the negative element of an AC signal efficiently. The diode rectifies the carrier wave, turning it into DC. The audio signal is retrieved from the carrier wave, a process called audio-frequency modulation. You can hear the audio after some filtering and amplification. Hence, diodes are commonly found in radios to extract the signal from the carrier wave.

Reverse Current Protection

Reversing polarities of a DC supply or incorrectly connecting the battery can cause a substantial current to flow through a circuit. Such a reverse connection can damage the connected load. That’s why a protective diode is connected in series with the positive side of the battery terminal. The diode becomes forward-biased in the case of correct polarity and the current flows through the circuit. However, in the event of a wrong connection, it becomes reverse-biased, blocking the current. Thus, it can protect your equipment from potential damage.

Component 4: Transistor

One of the most crucial components of an electronic circuit, transistors have revolutionized the field of electronics. These tiny semiconductor devices with three terminals have been around for more than five decades now. They are often used as amplifiers and switching devices. You can think of them as relays without any moving parts because they can turn something ‘on’ or ‘off’ without any movement.

**Figure 6:** Transistors [Image Source]

A. Composition

In the beginning, Germanium was used to build transistors which were extremely temperature-sensitive. Today, however, they are made from Silicon, a semiconductor material found in the sand because Silicon transistors are much more temperature-tolerant and cheaper to manufacture. There are two different types of Bipolar Junction Transistors (BJT), NPN and PNP. Each transistor has three pins called Base (b), collector (c), and emitter (e). NPN and PNP refer to the layers of semiconductor material used to make the transistor.

B. How Does It Work?

When you sandwich a p-type silicon slab between two n-type bars, you get an NPN transistor. The emitter is attached to one n-type, while the collector is attached to the other. The base is attached to the p-type. The surplus holes in the p-type silicon act as barriers, blocking the flow of the current. However, if you apply a positive voltage to the base and the collector and negatively charge the emitter, electrons start flowing from the emitter to the collector.

The arrangement and number of p-type and n-type blocks remain inverted in a PNP transistor. In this type of transistor, one n-type is sandwiched between two p-type blocks. As voltage allocation is different, a PNP transistor works differently. An NPN transistor requires a positive voltage to the base, while a PNP requires a negative voltage. In short, the current must flow away from the base to turn a PNP transistor on.

C. Function and Significance

Transistors function as both, switches and amplifiers in most electronic circuits. Designers often use a transistor as a switch because unlike a simple switch, it can turn a small current into a much larger one. Though you can use a simple switch in an ordinary circuit, an advanced circuit may need varying amounts of currents at different stages.

Transistors in Hearing Aids

One of the most well-known applications of transistors is the hearing aid. Usually, a small microphone in the hearing aid picks up the sound waves, converting them into fluctuating electrical pulses or currents. When these currents pass through a transistor, they are amplified. The amplified pulses then pass through a speaker, converting them into sound waves once again. Thus, you can hear a substantially louder version of the surrounding noise.

Transistors in Computers and Calculators

We all know that computers store and process information using the binary language of “zero” and “one.” However, most people don’t know that transistors play a critical role in making something called logic gates, which are the backbones of computer programs. Transistors are often hooked up with logic gates to build a unique piece of an arrangement called a flip-flop. In this system, the transistor remains ‘on’ even if you remove the base current. It now flips on or off whenever new current passes through it. Thus, a transistor can store a zero when it’s off or a one when it’s on, which is the working principle of computers.

Darlington Transistors

A Darlington transistor is made of two PNP or NPN polar junction transistors placed together. It is named after its inventor Sidney Darlington. The sole purpose of a Darlington transistor is to deliver a high current gain from a low base current. You can find these transistors in instruments that require a high current gain at a low frequency such as power regulators, display drivers, motor controllers, light and touch sensors, alarm systems, and audio amplifiers.

IGBT and MOSFET Transistors

The Insulated-Gate Bipolar Transistor (IGBT) transistors are often used as amplifiers and switches in various instruments including electric cars, trains, refrigerators, air-conditioners, and even stereo systems. On the other hand, Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFET) are commonly used in integrated circuits to control a device’s power levels or for storing data.

Component 5: Inductor

An inductor, also known as a reactor, is a passive component of a circuit having two terminals. This device stores energy in its magnetic field, returning it to the circuit whenever required. It was discovered that when two inductors are placed side by side without touching, the magnetic field created by the first inductor affects the second inductor. It was a crucial breakthrough that led to the invention of the first transformers.

A. Composition

It is probably the simplest component, comprising just a coil of copper wire. The inductance is directly proportional to the number of turns in the coil. Sometimes, however, the coil is wound around a ferromagnetic material such as iron, laminated iron, and powdered iron to increase the inductance. The shape of this core can also increase the inductance. Toroidal (donut-shaped) cores provide better inductance compared to solenoidal (rod-shaped) cores for the same number of turns. Unfortunately, it is difficult to join inductors in an integrated circuit, so they are usually replaced by resistors.

B. How Does It Work?

Whenever the current passes through a wire, it creates a magnetic field. However, the unique shape of the inductor leads to the creation of a much stronger magnetic field. This powerful magnetic field, in turn, resists alternating current, but it lets direct current flow through it. This magnetic field also stores energy.

Take a simple circuit comprising a battery, a switch, and a bulb. The bulb will glow brightly the moment you turn the switch on. Add an inductor to this circuit. As soon you turn the switch on, the bulb changes from bright to dim. On the other hand, when the switch is turned off, it becomes very bright, just for a fraction of a second before turning off completely.

As you turn the switch on, the inductor starts using the electricity to create a magnetic field, temporarily blocking the current flow. But, only DC current passes through the inductor as soon as the magnetic field is complete. That’s why the bulb changes from bright to dim. All this time, the inductor stores some electrical energy in the form of magnetic field. So, when you turn the switch off, the magnetic field keeps the current in the coil steady. Thus, the bulb burns brightly for a while before turning off.

C. Function and Significance

Though inductors are useful, it is difficult to incorporate them into electronic circuits due to their size. As they are bulkier compared to other components, they add a lot of weight and occupy plenty of space. Hence they are usually replaced by resistors in integrated circuits (ICs). Still, inductors have a wide range of industrial applications.

Filters in Tuned Circuits

One of the most common applications of inductors is to select the desired frequency in tuned circuits. They are used extensively with capacitors and resistors, either in parallel or series, to create filters. The impedance of an inductor increases as the frequency of signal increases. Thus, a stand-alone inductor can only act as a low-pass filter. However, when you combine it with a capacitor, you can create a notched filter because the impedance of a capacitor decreases as the frequency of signal increase. So, you can use different combinations of capacitors, inductors, and resistors to create various types of filters. They are found in most electronics including televisions, desktop computers, and radios.

Inductors as Chokes

If an alternate current flows through an inductor, it creates an opposite current flow. Thus, it can convert an AC supply into a DC. In other words, it chokes the AC supply but allows the DC to pass through it, hence the name ‘choke.’ Usually, they are found in power supply circuits that need to convert AC supply to DC supply.

Ferrite Beads

A ferrite bead or ferrite choke is used to suppress high-frequency noise in electronic circuits. Some of the common uses of ferrite beads include computer cables, television cables, and mobile charge cables. These cables can, sometimes, act as antennas, interloping with audio and video output of your television and computer. So, inductors are used in ferrite beads to reduce such radio frequency interference.

Inductors in Proximity Sensors

Most proximity sensors work on the principle of inductance. An inductive proximity sensor comprises four parts including an inductor or coil, an oscillator, a detection circuit and an output circuit. The oscillator generates a fluctuating magnetic field. Whenever an object comes into the proximity of this magnetic field, eddy currents start to build up, reducing the sensor’s magnetic field.

The detection circuit determines the strength of the sensor, while output circuit triggers the appropriate response. Inductive proximity sensors, also called contactless sensors, are cherished for their reliability. They are used at traffic lights to detect the traffic density and also as parking sensors in cars and trucks.

Induction Motors

An induction motor is probably the most common example of the application of inductors. Usually, in an induction motor, inductors are placed in a fixed position. In other words, they are not allowed to align with the nearby magnetic field. An AC power supply is used to create a rotating magnetic field which then rotates the shaft. The power input controls the speed of rotation. Hence, inductions motors are often used in fixed speed applications. The induction motors are very reliable and robust because there is no direct contact between the motor and the rotor.

Transformers

As mentioned earlier, the discovery of inductors led to the invention of transformers, one of the fundamental components of power transmission systems. You can create a transformer by combining the inductors of a shared magnetic field. They are usually used to increase or decrease voltages of the power lines to the desired level.

Energy Storage

Just like a capacitor, an inductor can also store energy. However, unlike a capacitor, it can store energy for a limited time. As the energy is stored in a magnetic field, it collapses as soon as the power supply is removed. Still, inductors function as reliable energy storage device in switch mode power supply such as desktop computers.

Component 6: Relay

A relay is an electromagnetic switch that can open and close circuits electromechanically or electronically. You need a relatively small current to operate a relay. Usually, they are used to regulate low currents in a control circuit. However, you can also use relays to control high electric currents. A relay is the electrical equivalent of a lever. You can switch it on with a small current to turn on (or leverage) another circuit using large current. Relays are either electromechanical relays or solid-state relays.

A. Composition

An Electromechanical Relay (EMR) comprises a frame, coil, armature, spring, and contacts. The frame supports various parts of the relay. The armature is the moving part of a relay switch. A coil (mostly copper wire), wound around a metal rod generates a magnetic field that moves the armature. Contacts are the conducting parts that open and close the circuit.

A Solid-State Relay (SSR) consists of an input circuit, a control circuit, and an output circuit. The input circuit is the equivalent of a coil in an electromechanical relay. The control circuit acts as a coupling device between input and output circuits, while the output circuit performs the same function as the contacts in an EMR. Solid-state relays are becoming increasingly popular as they are cheaper, faster, and reliable compared to electromechanical relays.

B. How Does It Work?

Whether you are using an electromechanical relay or a solid-state relay, it is either a Normally Closed (NC) or a Normally Opened (NO) relay. In case of an NC relay, the contacts remain closed when there is no power supply. However, in a NO relay, the contacts remain open when there is no power supply. In short, whenever current flows through a relay, the contacts will either open or close shut.

In an EMR, power supply energizes the relay coil, creating a magnetic field. The magnetic coil attracts a ferrous plate mounted on the armature. When the current stops, the armature is released into its resting position by spring action. An EMR can also have single or multiple contacts within a single package. If a circuit uses only one contact, it is called a Single Break (SB) circuit. A Double Break Circuit (DB), on the other hand, comes with tow contacts. Usually, single break relays are used to control low power devices such as indicator lamps, while double break contacts are used to regulate high-power devices such as solenoids.

When it comes to operating an SSR, you need to apply a voltage higher than the specified pickup voltage of the relay to activate the input circuit. You have to apply a voltage less than the stipulated minimum dropout voltage of the relay to deactivate the input circuit. Control circuit transfers the signal from the input circuit to the output circuit. The output circuit switches on the load or performs the desired action.

C. Function and Significance

As they can control a high current circuit by a low current signal, most control processes use relays as the primary protection and switching devices. They can also detect fault and irregularities occurring in the power distribution systems. Typical applications include telecommunication, automobiles, traffic control systems, home appliances, and computers among others.

Protective Relays

Protective relays are used to trip or isolate a circuit if any irregularities are detected. Sometimes, they can also set off alarms when a fault is detected. Types of protection relays depend on their function. For example, an overcurrent relay is designed to identify the current exceeding a predetermined value. When such current is detected, the relay operates tripping a circuit breaker to protect the equipment from potential damage.

A distance relay or impedance relay, on the other hand, can detect abnormalities in the ratio of current and voltage rather than monitoring their magnitude independently. It swarms into action when the V/I ratio falls below a predetermined value. Usually, protective relays are used to protect equipment such as motors, generators, and transformers, and so on.

Automatic Reclosing Relay

An automatic reclosing relay is designed to cause multiple reclosures of a circuit breaker that is already tripped by a protective relaying. For example, when there is a sudden voltage drop, the electrical circuit in your home may experience several brief power outages. These outages occur because a reclosing relay is trying to switch on the protective relay automatically. If it succeeds the power supply will be restored. If not, there will be a complete blackout.

Thermal Relays

The thermal effect of electrical energy is the working principle of a thermal relay. In short, it can detect the rise the ambient temperature and switch on or off a circuit accordingly. It consists of a bimetallic strip which heats up if an overcurrent passes through it. The heated strip bends and closes the No contact, tripping the circuit breaker. The most common application of thermal relay is overload protection of electric motor.

Component 7. Quartz Crystal

Quartz crystals have several applications in the electronics industry. However, they are mostly used as resonators in electronic circuits. Quartz is a naturally occurring form of silicon. However, it is now produced synthetically to meet the growing demand. It exhibits the piezoelectric effect. If you apply physical pressure on one side, the resulting vibrations generate an AC voltage across the crystal. Quartz crystal resonators are available in many sizes according to the required applications.

A. Composition

As mentioned earlier, quartz crystals are either synthetically manufactured or occur naturally. They are often used to make crystal oscillators to create an electrical signal with a precise frequency. Usually, the shape of quartz crystals is hexagonal with pyramids at ends. However, for practical purposes, they are cut into rectangular slabs. The most common types of cutting formats include X cut, Y cut, and AT cut. This slab is placed between two metal plates called holding plates. The outer shape of a quartz crystal or crystal oscillator can be cylindrical, rectangular or square.

B. How Does It Work?

If you apply an alternating voltage to a crystal, it causes mechanical vibrations. The cut and the size of the quartz crystal determine the resonant frequency of these vibrations or oscillations. Thus, it generates a constant signal. Quartz oscillators are cheap and easy to manufacture synthetically. They are available in the range from a few KHz to a few MHz. As they have a higher quality factor or Q factor, crystal oscillators are remarkably stable with respect to time and temperature.

C. Function and Significance

The exceptionally high Q factor enables you to use quartz crystals and the resonant element in oscillators as well as filters in electronic circuits. You can find this highly reliable component in radio frequency applications, as oscillator clock circuits in microprocessor boards, and as a timing element in digital watches as well.

Quartz Watches

The problem with traditional coil spring watches is that you have to keep winding the coil periodically. Pendulum watches, on the other hand, depend on the force of gravity. Thus, they tell time differently at different sea levels and altitudes due to changes in the gravitational force. The performance of quartz watches, however, is not affected by any of these factors. Quartz watches are battery-powered. Usually, a tiny crystal of quartz regulates the gears that control the second, the minute, and the hour hands. As quartz watches use very little energy, the battery can often last longer.

Filters

You can also use quartz crystals in an electronic circuit as filters. They are often used to filter out unwanted signals in radios and microcontrollers. Most basic filters consist of a single quartz crystal. However, advanced filters may comprise more than one crystal to match the performance requirements. These quartz crystal filters are far superior to the ones manufactured using LC components.

Conclusion

From communicating with your loved ones living across continents to making a hot cup of coffee, electronic gadgets touch almost every aspect of our lives. However, what makes these electronic gadgets finish seemingly time-consuming tasks in just a few minutes? Tiny electronic circuits are the foundation of all electronic equipment.

Bypass Caps: Decouple Your Way to Cleaner Power

In modern electronics it is rare to encounter electronics with few active components. Increasing IC counts in circuits introduces interesting design challenges. In this post we will discuss bypass capacitors and their role in preventing noise and voltage glitches on power supply rails.

All ICs need power which for most applications is supplied from a single voltage regulator. Powering multiple chips from a single regulator is not as simple as dropping your chips on a PCB and connecting them to the power supply. ICs do not present a constant load on the power supply, instead they have varying loads depending on their function. Since the voltage regulator’s job is to keep its output voltage constant, variations in the load will represent themselves as changing current.

Lets take a look at some hypothetical (and highly exaggerated) examples of different chips and the current waveforms they demand from the power supply.

Figure 1: Hypothetical current waveforms for various integrated circuits

LED Driver: Differing current levels when IC Turns on LEDs

Microcontroller: Current spikes corresponding to the clock frequency

Opamp: Current is proportional to the load it drives

It is not uncommon for a circuit to have a single voltage regulator supplying the example chips above and more. As the complexity of a circuit increases, so does the complexity of the load presented on the power supply.

THE PROBLEM:

So what happens when we connect all of these chips to the same power supply? Lets take a look at a theoretical PCB to find out.

Figure 2: Example PCB

This example PCB shows a voltage regulator which supplies power to the the three chips mentioned above. Power traces are shown in green stemming from the voltage regulator. (All other traces are not shown for clarity).

In any circuit there will inevitably be some distance between the voltage regulator and the chips that it provides power to. In an ideal world, the copper traces that span the distance between the regulator and ICs would have no impedance and act as a short circuit. In reality, all traces will exhibit some non-zero impedance and will have an impact on the trace voltage when current flows through them.

Considering the current waveforms presented above, as they pass through the impedance of the traces they induce voltage spikes or noise (remember ohm’s law?). This noise is not due to the voltage regulator or the traces picking up stray electromagnetic waves in the environment, but simply due to the chips pulling the current they need to operate – something that they are supposed to do!

To make matters worse, the way traces are routed on the PCB can compound the issue. On our example pcb, the current that supplies all three ICs is forced to pass through a single trace before splitting into individual traces to the chips. Because the current for all three chips pass through this one segment, the induced noise on this trace is the sum of the induced voltages. The voltage noise that is induced on the trace is then at risk of being coupled into our ICs and potentially anything further down the line.

Figure 3: Example PCB – All IC current flowing through one trace

In this example with only three chips and it is easy to see how this can become an issue. Imagine what would happen if we had more chips pulling more complex current waveforms? It is certainly possible for the induced noise on a power supply line to become large enough to compromise the performance of some if not all of the ICs connected to the supply or the components that are connected themselves to the ICs.

THE SOLUTION:

Luckily, the solution to this problem is both cheap and simple. By placing a low impedance path connected to the traces that feed power to the ICs we can create a filter that “bypasses” or “decouples” noise on the supply line. The easiest way to accomplish this is to add a capacitor across the power supply + and – lines. These capacitors are typically called bypass capacitors for reasons that will become clear soon.

Below is an image of a schematic showing a bypass capacitor connected across the power supply lines near an IC.

Figure 4: Schematic representation of bypass cap

Let’s take a look at the effects of a bypass capacitor on the transient current flowing into this IC.

Figure 5: Current loop with and without bypass cap

The image on the left shows the flow of current in a circuit that has no bypass cap on the IC. Any transient spikes in current will be subjected to any impedance on the traces and can cause problematic voltage droops or noise.

The image on the right shows the same circuit with a bypass capacitor installed across the power supply lines near the IC. Since the capacitor has the ability to source large currents quickly, the transients can be supplied directly from the capacitor instead of having to be sourced from the power supply. This can be seen with the current loop on the right in red. The addition of this capacitor helps “bypass” the transient current spikes from the voltage regulator and helps keep the voltage on the traces steady by sourcing the quick current transients.

By acting as a local transient current source, the capacitor prevents noise and voltage glitches from being coupled into other ICs that are connected on the power supply traces. Because of this, the capacitors are often called “decoupling capacitors” in addition to the name “bypass capacitor” since they decouple the current demands of one component in the circuit from the rest of the circuit.

It is important to note that the names “decoupling capacitor” and “bypass capacitor” were given to describe the function of the capacitors in the circuit and not a specific part. There are no capacitors that are specifically called decoupling caps or bypass caps as any cap can perform the function although some caps are more suited to the job than others as we will explore later.

LAYOUT CONSIDERATIONS:

Bypass caps should always be placed as close to the power pins as possible. Since the intent of the cap is to provide as low of an impedance path as possible and to decouple the current path for transients, keeping the traces from the cap to the IC short is very important. Every power pin on an IC should be bypassed with its own unique cap.

Figure 6: Example bypass cap layout

BYPASS CAP VALUE AND SIZE:

Considering the impedance of the traces on the PCB, the input impedance of the IC, and the operating frequency of the IC, an optimal value of bypass capacitor does exist but going through the rigors of calculating the value is rarely useful or necessary. In many cases the datasheet for an IC will give a recommended value for the capacitor. If no value is suggested, 0.1uF (100nF) is widely accepted as a standard go-to value. For most designs, a standard ceramic 0402, 0603, or 0805 package size will typically work well as a bypass cap.

RECAP

Bypass capacitors are an absolute must to avoid issues with noise on power supply traces and cross talk between devices on a PCB. Every IC in your designs should have bypass caps placed close to their power pins to provide low impedance paths for reducing the impact of current transients.

XO ___XO Control systems

Introduction

An instrumented control system is an electrical, electronic, or programmable electronic system (E/E/PES) which may perform some or all of the following functions:

Monitoring, recording and logging of plant status and process parameters;
Provision of operator information regarding the plant status and process parameters;
Provision of operator controls to affect changes to the plant status;
Automatic process control and batch/sequence control during start-up, normal operation, shutdown, and disturbance. i.e. control within normal operating limits;
Detection of onset of hazard and automatic hazard termination (i.e. control within safe operating limits), or mitigation;
Prevention of automatic or manual control actions which might initiate a hazard.

These functions are normally provided by, alarm, protection (trip, interlocks and emergency shutdown), and process control systems.

These engineered systems are individually and collectively described as control systems, and may be independent, or share elements such as the human interface, plant interface, logic, utilities, environment and management systems.

The human interface may comprise a number of input and output components, such as controls, keyboard, mouse, indicators, annunciators, graphic terminals, mimics, audible alarms, and charts.

The plant interface comprises inputs (sensors), outputs (actuators), and communications (wiring, fibre optic, analogue/digital signals, pneumatics, fieldbus, signal conditioning, barriers, and trip amplifiers).

The logic elements may be distributed, and linked by communications, or marshalled together and may be in the form of relays, discrete controllers or logic (electronic, programmable or pneumatic), distributed control systems (DCS), supervisory control and data acquisition (SCADA), computers (including PCs), or programmable logic controllers (PLC). The logic elements may perform continuous control functions, or batch or change of state (e.g. start-up/shut-down) sequences. It should also be noted that logic functions may be distributed to be undertaken within smart sensors or actuators.

Utilities are the power supplies and physical elements required for the systems, such as electricity and instrument air.

Environment is the physical accommodation and surroundings in which the control systems (including the operator) are required to work, including physical accommodation or routings, environmental conditions (humidity, temperature, flammable atmospheres), and external influences such as electromagnetic radiation and hazards which might affect the operation of the control system during normal or abnormal conditions such as fire, explosion, chemical attack etc.

Modern instrumented control systems are generally electrical, electronic or programmable electronic systems (E/E/PES), but some purely pneumatic systems may still be in operation.

Safety related systems

A control system or device is deemed to be safety related if it provides functions which significantly reduce the risk of a hazard, and in combination with other risk reduction measures, reduces the overall risk to a tolerable level, or if it is required to function to maintain or achieve a safe state for the equipment under control (EUC).

These functions are known as the safety functions of the system or device and are the ability to prevent initiation of a hazard or detect the onset of a hazard, and to take the necessary actions to terminate the hazardous event, achieve a safe state, or mitigate the consequences of a hazard.

All elements of the system which are required to perform the safety function, including utilities, are safety related, and should be considered part of the safety related system.

Safety related control systems may operate in low demand mode, where they are required to carry out their safety function occasionally (not more than once/year) or in high demand (more than once/year) or continuous mode where failure to perform the required safety function will result in an unsafe state or place a demand on another protective system. The likelihood of failure of a low demand system is expressed as probability of failure on demand, and as failure rate per hour for high/continuous demand systems.

Safety related control systems operating in continuous or high demand mode where the E/E/PES is the primary risk reduction measure have been known as HIPS (high integrity protective systems). However, use of such systems does not circumvent the need for a hierarchical approach to risk reduction measures such as inherent safety, and careful consideration of prevention of common mode failures by use of diverse technology and functionality (such as relief valves), independent utilities and maintenance and test procedures, physical separation, and external risk reduction (such as bunds). Measures should favour simple technological solutions rather than complex ones. The lowest failure rate which can be claimed for high integrity systems operating in continuous or high demand mode is 10^-9 dangerous failures per hour.

It should be noted that control systems for equipment under control which are not safety related as defined above may also contribute to safety and should be properly designed, operated and maintained. Where their failure can raise the demand rate on the safety related system, and hence increase the overall probability of failure of the safety related system to perform its safety function, then the failure rates and failure modes of the non-safety systems should have been considered in the design, and they should be independent and separate from the safety related system.

A control system operating in continuous or high demand mode, for which a failure rate of less than 10^-5/hr is claimed in order to demonstrate a tolerable risk, provides safety functions, and is safety related.

In some circumstances, the safety function may require the operator to take action, in which case, he/she is part of the safety related system and will contribute significantly to the probability of failure on demand (PFD). Typically, in a well designed system, a figure of 10^-1 is assumed for the probability of an operator failing to take correct action on demand. Where exceptional care has been taken in design of human factors such as alarm management, instructions and training, and where such arrangements are monitored and reviewed, then a probability of failure on demand of not better than 10^-2 may be achievable. Any supporting hardware or software, such as alarm systems, would also need the requisite integrity level).

System	Claimed failure rate or probability of failure on demand
Non-safety related system	Not better than 10^-5/hr
Operator action	10^-1/demand (typical) 10^-2/demand (best)
High integrity protective system	Not better than 10^-9/hr

System integrity

The integrity required of a safety related system depends upon the level of risk reduction claimed for the safety function to be performed.

Safety integrity is the probability that safety related system will satisfactorily perform the required safety function under all stated conditions within a stated period of time when required to do so.

Safety integrity is therefore a function of performance and availability.

Performance is the ability of the system or device to perform the required safety function in a timely manner under all relevant conditions so as to achieve the required state.

Availability is the measure of readiness of the system to perform the required safety function on demand, and is usually expressed in terms of probability of failure on demand.

Performance and availability depend on:

Proper design or selection, installation and maintenance and testing of the plant interfaces, including sensors actuators and logic, for the required duty and full range of process and environmental conditions under which they will be required to operate, including, where necessary, any excursions beyond the safe operating limits of the plant;
Accuracy and repeatability of the instrumentation;
Speed of response of the system;
Adequate margins between normal and safe operating limits and the system settings;
Reliability;
Survivability from the effects of the hazardous event or other external influences such as power system failure or characteristics, lightning, electromagnetic radiation (EMR), flammable, corrosive or humid atmospheres, temperature, rodent attack, vibration physical impact, and other plant hazards;
Independence (the ability of the system to act alone, without dependence on other protective measures, control systems or common utilities or to be influenced by them.

The following measures are required to ensure adequate performance and availability of the safety related system:

Protection against random failures by hardware reliability, fault tolerance (e.g. by redundancy) and fault detection (diagnostic coverage, and proof testing);
Protection against systematic and common mode failures by a properly managed safety lifecycle, independence from common utilities, common management systems and other protective systems, and by diversity. The lifecycle includes hazard and risk evaluation, specification, design, validation, installation, commissioning, operation, maintenance, and modification and are detailed in BS IEC 61508.

Integrity levels

History

Historically, little industry guidance has been available for qualifying or quantifying safety integrity levels to achieve to achieve a requisite risk reduction.

Guidance related to determination of suitable integrity of programmable electronic systems in terms of configuration, reliability (quantitative and qualitative), and quality has been available in the HSE document Programmable Electronic Systems in safety related applications (PES 1 and 2) since 1987.

Additional guidance has also been available in EEMUA 160 Safety related instrument systems for the process industries.

However, most major companies will have developed internal standards which relate safety related system integrity to required risk reduction. These standards are likely to address the design process, system configuration, and demonstration that the required risk reduction has been achieved by qualitative or quantitative analysis of the failure rate of the design. They will also have procedures to ensure that the integrity is maintained during commissioning, operation, maintenance, and modification.

The latest applicable standard is BS IEC 61508 ‘Functional safety of electrical/electronic/programmable electronic safety-related systems’ which is in 7 parts. Parts 1, 3, 4, are published as British Standards, Part 5 is issued as an international IEC standard, and Parts 2, 6 and 7 remain in draft form.

Underlying philosophy

Integrity levels for safety related systems may be determined from the hazard and risk analysis of the equipment under control. A number of different methodologies are available, but the process includes identification of hazards and the mechanisms which can initiate them, risk estimation (likelihood of occurrence), and risk evaluation (overall risk based on likelihood and consequences). The risk estimation provides a measure of the risk reduction required to reduce the risk to a tolerable level.

Hazard identification results in the identification of safety functions which are required to control the risk.

The safety functions may then be allocated to a number of different systems including E/E/PES, other technology and external measures.

For each system providing a safety function, a failure rate measure can be assigned which in turn determines the integrity required of the system. alternatively, a qualitative approach (based on the likelihood and consequence of the hazard, and the frequency and level of exposure and avoidability) may be used to define the required integrity.

Safety Integrity Levels

IEC 61508 assigns four software and hardware safety integrity levels (SILs) to required measures of risk reduction. Guidance is then provided on the system configuration, level of subsystem fault tolerance and diagnostic coverage, and safety life-cycle measures required to achieve the designated hardware SIL, and the software methods and life-cycle measures required to achieve the designated software SIL. It also provides guidance on qualitative methods for establishing the SIL level required. Part 2 of the standard places architectural constraints on the hardware configuration by setting minimum fault tolerance and diagnostic coverage requirements for each element or subsystem. It should be noted that IEC 61508 limits the risk reductions which can be claimed for a safety related E/E/PES which operated in low demand mode or continuous mode to no better than 10^-6 and 10^-9 respectively for SIL4.

The requirement is more demanding for subsystems which do not have well defined behaviour modes or behaviour (e.g. programmable systems). The standard requires that a reliability model of the system architecture be created and the reliability predicted and compared with the target safety integrity level to confirm that the required risk reduction has been achieved.

It is necessary to demonstrate that the required level of integrity has been achieved in the design, installation, operation and maintenance of the system.

It should be noted that the integrity of a safety related system is critically dependant upon the detection and correction of dangerous failures. Where there is a low level of diagnostic coverage, as is usually the case with lower integrity systems, then the integrity is critically dependent upon the proof test interval. Where there is a high level of diagnostic coverage to automatically reveal failures on-line, for example for high demand high integrity systems, then the integrity is also heavily dependant upon the frequency of diagnostic checks, and the mean time to repair the equipment, which includes the diagnostic test interval.

SIL levels are now being quoted for proprietary subsystems (and certified by test bodies). Quoted SILs should be associated with proof test intervals, diagnostic coverage and fault tolerance criteria. They are useful for evaluation of architectural constraints, but do not eliminate the requirement to confirm that the requires safety integrity level for the safety function provided by the system has been achieved. Software includes high level user application programmes and parameter settings.

Alarm systems

Alarm systems alert operators to plant conditions, such as deviation from normal operating limits and to abnormal events, which require timely action or assessment.

Alarm systems are not normally safety related, but do have a role in enabling operators to reduce the demand on the safety related systems, thus improving overall plant safety.

However, where a risk reduction of better than 10^-1 failures on demand is claimed then the alarm system, including the operator, is a safety related system which requires a suitable safety integrity level (SIL 1 or SIL 2 as defined by BS IEC61508).

EEMUA 191 ‘Alarm systems - a guide to design, management and procurement’ considers alarm settings, the human interface (alarm presentation), alarm processing and system management controls for both safety related and other alarm systems. It provides the following guidance in regard to safety related alarm systems:

The alarm system should be designed in accordance with IEC 61508 to SIL 1 or 2, with the designated reliability;
The alarm system should be independent from the process control system and other alarms unless it has also been designated safety related;
The operator should have a clear written alarm response procedure for each alarm which his simple, obvious and invariant, and in which he is trained;
The alarms should be presented in an obvious manner, distinguishable from other alarms, have the highest priority, and remain on view at all times when it is active;
The claimed operator workload and performance should be stated and verified.

Alarms which are not designated as safety should be carefully designed to ensure that they fulfil their role in reducing demands on safety related systems.

For all alarms, regardless of their safety designation, attention is required to ensure that under abnormal condition such as severe disturbance, onset of hazard, or emergency situations, the alarm system is remains effective given the limitations of human response. The extent to which the alarm system survives common cause failures, such as a power loss, should also be adequately defined.

Further guidance is available in EEMUA 191 ‘Alarm systems - a guide to design, management and procurement’, and CHID circular CC/Tech/Safety/9.

Alarm settings

The type of alarm and its setting should be established so as to enable the operator to make the necessary assessment and take the required timely action. Settings should be documented and controlled in accordance with the alarm system management controls.

Human interface (alarm presentation)

The human interface should be suitable. Alarms may be presented either on annunciator panel, individual indicators, VDU screen, or programmable display device.

Alarms lists should be carefully designed to ensure that high priority alarms are readily identified, that low priority alarms are not overlooked, and that the list remains readable even during times of high alarm activity or with repeat alarms.

Alarms should be prioritised in terms of which alarms require the most urgent operator attention.

Alarms should be presented within the operators field of view, and use consistent presentation style (colour, flash rate, naming convention).

Each alarm should provide sufficient operator information for the alarm condition, plant affected, action required, alarm priority, time of alarm and alarm status to be readily identified.

The visual display device may be augmented by audible warnings which should at a level considerably higher than the ambient noise at the signal frequency. Where there are multiple audible warnings, they should be designed so that they are readily distinguished from each other and from emergency alarm systems. They should be designed to avoid distraction of the operator in high operator workload situations. Where both constant frequency and variable frequency (including pulsed or intermittent) signals are used, then the later should denote a higher level of danger or a more urgent need for intervention.

Alarm processing

The alarms should be processed in such a manner as to avoid operator overload at all times (alarm floods). The alarm processing should ensure that fleeting or repeating alarms do not result in operator overload even under the most severe conditions. A number of alarm processing techniques include filtering, deadband, debounce timers, and shelving, are described in EEMUA 191 ‘Alarm systems - a guide to design, management and procurement’.

The presentation of alarms should not exceed that which the operator is capable of acting upon, or alternatively the alarms should be prioritised and presented in such a way that the operator may deal with the most important alarms without distraction of the others. Applicable alarm processing techniques include grouping and first-up alarms, eclipsing of lower grade alarms (e.g. suppression high alarm when the high-high activates) suppression of out of service plant alarms, suppression of selected alarms during certain operating modes, automatic alarm load shedding and shelving.

Care should be taken in the use of shelving or suppression to ensure that controls exist to ensure that alarms are returned to an active state when they are relevant to plant operation.

Alarm system management procedures

Management systems should be in place to ensure that the alarm system is operated, maintained and modified in a controlled manner. Alarm response procedures should be available, and alarm parameters should be documented.

The performance of the alarms system should be assessed and monitored to ensure that it is effective during normal and abnormal plant conditions. The monitoring should include evaluation of the alarm presentation rate, operator acceptance and response times, operator workload, standing alarm count and duration, repeat or nuisance alarms, and operator views of operability of the system. Monitoring may be achieved by regular and systematic auditing.

Matters which are not worthy of operator attention should not be alarmed.

Logging may be a suitable alternative for engineering or discrepancy events to prevent unnecessary standing alarms. A system for assessing the significance of such logged events to ensure timely intervention by maintenance personnel may be required.

Protection systems (Trips and Interlocks)

Protective tripping systems provide a defence against excursions beyond the safe operating limits by detecting a excursions beyond set points related to the safe operating limits (i.e. the onset of a hazard) and taking timely action to maintain or restore the equipment under control to a safe state. Trips should not be self resetting unless adequate justification has been made. Protective interlocks prevent those control actions which might initiate a hazard from being undertaken by an operator or process control system, and are by nature self-resetting.

Protection systems should indicate that a demand to perform a safety function has been made and that the necessary actions have been performed.

Independence

Protective systems should be sufficiently independent of the control system or other protective systems (electrical/electronic or programmable). Where there is an interface between systems (e.g. for indication, monitoring or shared components) or shared utilities (e.g. power), environment (e.g. accommodation, wiring routes) or management systems (maintenance procedures, personnel), then the method of achieving independence should be defined, and common cause failures adequately considered.

Measures to defend against common mode failures due to environmental interactions may include physical separation or segregation of system elements (sensors, wiring, logic, actuators or utilities) of different protective systems.

Independence will also be required for protection against systematic and common mode faults. Measures may include use of diverse technology for different protective systems. Where more than one E/E/PES protective system is used to provide the required risk reduction for a safety function, then adequate independence should be achieved by diverse technology, construction, manufacturer or software as necessary to achieve the requires safety integrity level.

Dependence on utilities

The action required from the protective system depend upon the nature of the process. The actions may be passive in nature, such as simple isolation of plant or removal of power, or they may be active in that continued or positive action is required to maintain or restore a safe state, for example by injection of inhibitor into the process, or provision of emergency cooling.

Active protective measures have a high dependence upon utilities, and may be particularly vulnerable to common mode failures. The scope of the protective system therefore includes all utilities upon which it depends, and they should have an integrity consistent and contributory to that of the remainder of the system.

Measures taken to defend against common mode failure of utilities will be commensurate with the level of safety integrity required, but may include standby or uninterruptable/reservoir supplies for electricity, air, cooling water, or other utilities essential for performance of the safety function. Such measures should themselves be of sufficient integrity.

Survivability and external influences

The protective system should be adequately protected against environmental influences, the effects of the hazard against which it is protecting, and other hazards which may be present. Environmental influences include power system failure or characteristics, lightning (BS 6651), electromagnetic radiation (EMR) (BS 6667, IEC 61000), flammable atmospheres (BS 5345, BS EN 60079, BS 6467, BS 7535, BS EN 50281), corrosive or humid atmospheres, ingress of water or dust (BS EN 60529), temperature, rodent attack, chemical attack, vibration physical impact, and other plant hazards.

Degradation of protection against environmental influences during maintenance and testing should have been considered and appropriate measures taken. e.g. Use of radios by maintenance personnel may be prohibited during testing of a protective system with the cabinet door open where the cabinet provides protection against EMR.

Protection against random hardware faults

The architecture of the protective system should be designed to protect against random hardware failure. It should be demonstrated that the required reliability has been achieved commensurate with the require integrity level. Defensive measures may include high reliability elements, automatic diagnostic features to reveal faults, and redundancy of elements (e.g. 2 out of 3 voting for sensors) to provide fault tolerance.

Protection common mode failures

Diversity of elements is not effective for protection against random hardware faults, but is useful in defence against common mode failures within a protective system.

Protection systematic failures

Protection against systematic hardware and software failures may be achieved by appropriate safety lifecycles (see IEC 61508, Out of Control).

Sensing

Sensors include their connection to the process, both of which should be adequately reliable. A measure of their reliability is used in confirming the integrity level of the protective system. This measure should take into account the proportion of failures of the sensor and its process connection which are failures to danger.

Dangerous failures can be minimised by a number of measures such as:

Use of measurement which is as direct as possible, (e.g. pneumercators provide an inferred level measurement but actually measure back pressure against a head and are sensitive to changes in density due to temperature variations within the process, and to balance gas flow, upon which they are dependant);
Control of isolation or bleed valves to prevent uncoupling from the process between proof tests or monitoring such that their operation causes a trip;
Use of good engineering practice and well proven techniques for process connections and sample lines to prevent blockage, hydraulic locking, sensing delays etc.;
Use of analogue devices (transmitters) rather than digital (switches);
Use of positively actuated switches operating in a positive mode together with idle current (de-energise to trip);
Appropriate measures to protect against the effects of the process on the process connection or sensor, such as vibration, corrosion, and erosion;
Monitoring of protective system process variable measurement (PV) and comparison against the equivalent control system PV either by the operator or the control system.

Guidance on process connection is provided in BS 6739 British Standard Code of practice for instrumentation in process control systems: Installation design and practice.

Proof testing procedures should clearly set out how sensors are reinstated and how such reinstatement is verified after proof testing.

Maintenance procedures should define how sensors/transmitters are calibrated with traceability back to national reference standards by use of calibrated test equipment.

Other matters which will need to have been considered are:

Cross sensitivities of analysers to other fluids which might be present in the process;
Reliability of sampling systems;
Protection against systematic failures on programmable sensors/analysers. The measures taken will depend on the level of variability and track record of the software. ‘Smart’ transmitters with limited variability software which are extensively proven in use may require no additional measures other than those related to control of operation, maintenance, and modification, whereas bespoke software for an on-line analyser may require a defence in depth against systematic failures (BS IEC 61508 Part 3);
Signal conditioning (e.g. filtering) and which may affect the sensor response times;
Degradation of measurement signals (distance between sensor and transmitter may be important);
Accuracy, repeatability, hysteresis and common mode effects (e.g. effects of gauge pressure or temperature on differential pressure measurement);
Integrity of process connections and sensors for containment (sample or impulse lines, instrument pockets are often a weak link in process containment measures).

Use of ‘SMART’ instruments requires adequate diagnostic coverage and fault tolerance (see architectural constraints in IEC 61508 Part 2), and measures to protect against systematic failures (software design/integration, inadvertent re-ranging during maintenance). Measures may include use of equipment in non-smart mode (analogue signal output, no remote setting) and equipment of stable design for which there is an extensive record of reliability under similar circumstances.

Actuators and signal conversion

Actuators are the final control elements or systems and include contactors and the electrical apparatus under control, valves (control and isolation), including pilots valves, valve actuators and positioners, power supplies and utilities which are required for the actuator to perform its safety function, all of which should be adequately reliable. A measure of their reliability is used in confirming the integrity level of the protective system. This measure should take into account the proportion of failures of the actuator under the relevant process conditions which are failures to danger.

Actuators are frequently the most unreliable part of the tripping process.

Dangerous failures can be minimised by a number of measures such as:

Use of ‘fail-safe’ principles so that the actuator takes up the tripped state on loss of signal or power (electricity, air etc.). e.g. held open, spring return actuator;
Provision of uninterruptable or reservoir supplies of sufficient capacity for essential power;
Failure detection and performance monitoring (end of travel switches, time to operate, brake performance, shaft speed, torque etc.) during operation;
Actuator exercising or partial stroke shutoff simulation during normal operation to reveal failures or degradation in performance. Note this is not proof testing but may reduce probability of failure by improved diagnostic coverage (IEC 61508);
Overrating of equipment.

Other matters which should have been considered are:

Valves should be properly selected for their duty, and it should not be assumed that a control valve can satisfactorily perform isolation functions;
Actuators may also include programmable control elements (e.g. SMART instruments) particularly within positioners and variable speed drives and motor control centres. Modern motor control centres may use programmable digital addressing. This introduces a significant risk of introduction of systematic failure and failure modes which cannot be readily predicted. Such an arrangement should be treated with caution. It is normally reasonably practicable for trip signal to act directly upon the final contactor;
Potential for failure due to hydraulic locking between valves (e.g. trace heated lines between redundant shutoff valves).

Logic systems

Commonly, the logic systems for protective systems are electronic, but programmable and other technology systems (magnetic or fluidic/pneumatic) have been used.

The architecture of the logic system will be determined by the hardware fault tolerance requirements, for example dual redundant channels. Where a high level of integrity for the system is required (SIL3 or SIL4) then diverse hardware between channels may be employed. This should not be confused with diversity of independent protective systems.

Logic systems are likely to incorporate provisions for fault alarms and overrides, for which there should be suitable management control arrangements. They may also provide monitoring of input and output signal lines for detection of wiring (open circuit, short circuit) and sensors/actuators (stuck-at, out of range). Such monitoring may initiate an alarm, a trip action or, in a voting arrangement, disable the faulty element.

Software based systems should be adequately protected against systematic failures, for example by an appropriate hardware and software safety lifecycles, and suitable techniques and quality systems. Guidance is available in BS IEC 61508 Part 3, PES Parts 1 & 2, EEMUA 160, Out of Control, and IGasE SR15 - Programmable equipment in safety related applications.

Wiring and communications (signal transmission)

Transmitters, communications devices and wiring systems should be arranged to meet the requirements for survivability, protection against external influences and independence.

Independent systems or redundant channels should not share multicore cables with each other or power circuits, and may require diverse routes depending upon the safety integrity level to be achieved.

Measures to protect against failures include:

Use of fail-safe principles such as DC model (e.g. 4-20 ma loop) for analogue signal transmission diagnosis and alarm of out of range, abnormal, or fault states (such as stuck-at) with defined control system responses for both the sensor and transmitter;
Cable selection (screening etc.);
Protection of cables against fire, chemical attack, physical damage etc.;
Physical separation or segregation of cables and cable routes;
Routing in benign environments;
Use of optical fibres to protect against electrical interference;
Careful attention to lightning protection (BS 6651) of data links between buildings.

Use of fieldbus or other digital communication protocols in protective systems should be considered a novel approach requiring a thorough evaluation and demonstration of the safety integrity. EEMUA 189 'A guide to fieldbus applications in the process industry' provides limited guidance.

Utilities

Utilities which are required for the protective system to perform its safety function may include power supplies such as electricity, air, inhibitor materials and their propellants, inert gas such as nitrogen, cooling water, steam, pilot flames and their gases all of which should be adequately reliable. Measures such as redundancy, and uninterruptable/reservoir supplies, and availability monitoring (e.g. loss of air alarm) may be required. Confirmation that the designed capacity of reserves is adequate should be demonstrated by test.

Utilities may also introduce external influences into the protective systems (e.g. from electrical supplies) .

Measures to protect against external influences may include:

Under/Over voltage protection;
Overcurrent and short circuit protection;
Use of an uninterruptable power supply or voltage conditioning or filtering;
Careful attention to lightning protection (BS 6651) and equipotential bonding (BS 7671).

Proof testing

The probability of failure on demand, or the failure rate of a protective system is critically dependent upon the frequency of proof testing and its ability to detect previously unrevealed failures of the system. The proof test interval should therefore be established accordingly, and as a rule of thumb for low demand systems, should be an order of magnitude less than the mean time between failure of the system and the demand rate.

Proof test procedures should be available which specify the success/failure criteria and detail how the test will be performed safely, including any management arrangements, operating restrictions and competence of personnel.

The tests should be arranged to reveal all dangerous failures which have been unrevealed in normal operation including the following measures:

Tests performed at the conditions which would be expected at trip. (Where test under trip conditions cannot be performed, for example for safety reasons, then measures to ensure that potential failures at trip conditions will be revealed should be clarified);
End to end tests at appropriate intervals, including proving sample/impulse lines. (Different elements of the protective system may require proof testing at different intervals).

Operation

Procedures should be available which detail the operation of the protective system including:

Override management (authorisation, security, recording, monitoring and review of overrides, reset requirements);
Operating instruction for trips;
Instructions for response to equipment faults including fault alarms. (There should be procedural arrangements in place to ensure timely repair so that mean time to repair criteria can be met).

Maintenance

Procedures should be available for maintenance activities including:

Maintenance instructions;
Control of spares (segregation of faulty or non-conforming parts, identification to prevent interchange of similar parts etc.);
Competence of maintenance personnel;
Operating restriction during maintenance;
Control of software back-ups and memory media (E/EPROMS, floppy disks, files on hard disks on portable PCs etc.);
Post maintenance reinstatement and proof testing.

For systems where a high diagnostic coverage is claimed, for example high integrity high systems, the probability of failure (expressed as failure rate) is critically dependant upon the mean time to repair the faults revealed. For such systems, the repair performance should monitored and reviewed against the design criteria.

Modification

A management system for control of modifications should be available to ensure that:

Unauthorised modifications are prevented;
Authorised modifications are not ill conceived;
Safety verification to confirm that the required safety function and integrity have been maintained;
Designed and implementation is carried out by competent persons.

Remote diagnostic systems

Remote diagnostic systems have the potential to cause danger by initiating unexpected operations or by affecting safety functions by software/parameter modification or by diverting the control system processor from time critical functions.

The need for remote diagnosis should be justified, a risk assessment completed, and measures taken to ensure that safety is not affected by normal operation or malfunction of the diagnostic system, including the remote diagnostic terminal and software, communication link, and the control system diagnostic interface and software.

Consideration should be given to:

Security and control of access;
Communication between diagnostician and plant personnel;
Restricted mode of operation; passive (monitoring only), active (control/operator functions), interactive (software change possible);
Potential for operation outside restricted mode under fault conditions;
Protection of safety functions from unauthorised modification;
Change control;
Competence of personnel.

Whilst beyond the scope of HS(G)87 'Safety in the remote diagnosis of manufacturing plant and equipment', the publication provides a useful background to the subject.

Process control systems

Process control systems are primarily implemented for economic reasons. However, those which are not considered safety related should still be designed, installed, operated and maintained so that their failure does not place a rate demand in the protective system which was not anticipated in its design. Part 1 of BS IEC 61508 provides guidance. The dangerous failure modes of the control system should be determined and taken into account in overall safety system specification. The control system should also be sufficiently independent of the safety systems.

The control system may provide steady state or change of state (start-up, shutdown, batch) control functions. The latter may be implemented by automatic sequences or procedurally under manual control. Control systems should be implemented to provide stable control of the process under all expected normal and upset circumstances, including start-up and shutdown.

The system should be designed to prevent or verify operator commands which might place a demand upon the protective system.

The dangerous failure rate of the control system should be supported by operational experience of the system in a similar application, reliability analysis or reliability data from industry databases. The failure rate that may be claimed may not be less than 10^-5 dangerous failures/hour.

Consideration should be given to failure behaviour so as to minimise the demands placed on the protective systems such as under the following circumstances:

I/O power failure;
Main power failure;
I/O faults (open/short circuit, out of range);
Module/processor failure (I/O, controller, cell, supervisory);
Communications failure (at all levels of the architecture).

Consideration should also be given to change control and software back-up systems. As the control system provides control, monitoring and logging functions which significantly aid the operator, consideration should be given to survival of the control system during hazardous events and emergency response.

It should be noted that redundant (non-diverse), cross monitored control processors are extremely vulnerable to common mode failure.

It should be demonstrated that the process control system does not exercise safety functions during sequences and changes of state under its control. For example, where the control system batch sequence controls the mixing of quantities of materials or reagents which, if incorrect quantities are admitted, may result in an unintended reaction, then measures of sufficient safety integrity, other than the control system, should be taken to ensure that the residual risk is as low as reasonable practicable.

For the purposes of risk evaluation, failure of the control system (at not less than 10^-5 failures/hour or 10^-1 failures on demand) should be considered as part of the hazard initiation sequence rather than a risk reduction measure.

Exothermic reactions

Exothermic reactions are particularly demanding in terms of control and protection as they tend to be unstable with aggressive reaction kinetics, and may require risk reduction measures which are required continuously throughout the reaction stage and which rely on utilities such as cooling systems, agitation, inhibitor injection etc.

Thus, loss of any single utility may be a dangerous failure, and initiate a hazard (e.g. loss of agitator blades, and hence reduced cooling because of poorer heat transfer, giving rise to a runaway reaction).

The components of the utilities should be considered safety related and provide adequate protection against failure including common mode failures (e.g. loss of electricity) and systematic failures (e.g. failure to fill inhibitor stock vessel). Sufficient diagnostics should be provided to reveal such failures so that timely automatic or manual response can be initiated.

Diagnostics should be designed to reveal the failure as directly as possible, for example:

Agitator torque rather than shaft speed (which will not reveal blade loss);
Cooling water flow rather than pump stopped.

Their capacity and capability to deal with the most extreme reaction kinetics (e.g. worst case mixtures) and limiting conditions (e.g. maximum temperature/pressure achievable under worst case) should also be demonstrated.

Expert systems

Expert systems are normally employed as operator support tools. Use of an expert or other deductive or learning system for direct process control should be considered novel and adequate assessment of the risks provided .

XO___XO B?W Communication Systems/Telephone System

From the western perspective, the telephone is ubiquitous. However, the reality is that 80% of the world’s population have limited access to one.

To the average person, the telephone system is simply a ‘black box’. Relatively few people need to know how the PSTN works. The main consideration is that it works, and is relatively inexpensive. The PSTN has a designed reliability of 99.999% (called 5 nines reliability).

A Telephone Call

To make a telephone call, one simply picks up the handset, enters a number, and waits for the system to perform its magic:

Lifting the handset from its cradle releases a hook switch and causes a dc current to flow (20 - 120 ma). The central office monitors this loop current and interprets it as a request for service.

The office acknowledges the request for service by sending dial tone. This normally occurs in less time than it takes to pickup the handset and place it to the ear. Once dial tone has been received, the subscriber starts to dial.

In the past, when dialing, a rotary dial switch opened and closed the loop in a predetermined manner. If one was very coordinated, it was possible to perform the same task by flashing the hook switch. To assure the customer that the system is responding, dial tone is removed once dialing starts. In most systems today DTMF (Dual Tone Multi Frequency) signaling is used How Telephones Work

Depending upon the office type and digits received, a number of things might happen. In most cases, end-users are attached to what is called a class 5 or end-office. These are the most common types of telephone exchanges. Each class 5 office has one or more, three digit exchange numbers. These are the first three digits in an ordinary 7-digit telephone number.

If the central office includes the customer dialed exchange number, it will know that the call is local and the other party is connected to the same office. The office will therefor control the entire call setup and takedown.

If the first three digits do not correspond to an exchange handled by the end-office, it will have to find a trunk line to an office that can handle the call. This means that each office must know the exchange numbers of all the offices within its calling area, and how to get to them. The call setup and takedown will therefor be shared between the two exchanges. They must monitor the call in progress and inform each other of any change in call status.

If the first digit dialed is a one, the office will recognize this as a long distance call, and will start looking for a spare toll trunk. A toll office has a greater knowledge base as to where distant exchanges are located and how to get to them.

The telephone system must be intelligent enough to recognize that in a local call, only seven digits are usually required. Some very small exchanges however, allow local calls by omitting the exchange number and using only the last 4 digits or extension number. In large urban areas, it may be necessary to prefix local calls with a 3-digit area code. An international call may require up to 16 digits.

Once the entire number has been received, the office at each end of the connection must alert both parties as to what is happening. At the originating end, a ringing tone is sent to the speaker in the handset. At the terminating end, the office is generating a much larger ringing voltage to activate a bell.

The far-end-office monitors the line to determine if someone answers the ringing phone. This is done by examining the DC current drawn when the far-end customer lifts the handset, inducing loop current through the hook switch. The far-end-office must then disconnect the ringing before the handset reaches the ear, and signals back to the originating office that someone has picked up the phone. The origination office must then disconnect the ring back tone and complete the voice connection.

Both end-offices monitor their respective loop currents during the entire call to determine if one party hangs up. Once this happens, one end-office signals the other, and dial tone is placed on the loop. This alerts the remaining party that the connection has been terminated.

If the line is in use, the central office will not set up the connection and return a busy tone to the originator. By doing this switching, call processing, and transmission resources are not being tied up unnecessarily. However, there are a number of options such as call forwarding and call waiting which modify this process.

With call forwarding, a call to a busy number is routed to an answering service. With call waiting, the calling party hears a ringing tone, and the called party hears a beep, which they can either ignore or signal back to the office that the new call should be given priority over the existing call. If however, the call cannot be completed because the system itself is too busy, it returns a fast busy tone to the originator.

In a touch-tone environment, the same procedure is followed, except that tones are used to convey numbers to the local office instead of interrupting loop current. Some calling features, generally known as CLASS (Custom Local Area Signaling Services) , are available in areas with touch-tone service.

But where do the telephone wires go?

The telephone line goes to a terminal block in a service area interface. These are often located on a pole or small enclosure on the street. The service area interface bundles the subscriber drop cables into a single larger cable. These are in turn gathered together to form larger feeder cables. The entire wiring system somewhat resembles a huge tree.

Cables coming out of a central office may have hundreds or even thousands of pairs bundled together however by the time the cable gets to the end user, it is generally down to about 50 pairs. An individual subscriber consists of many cable sections spliced together. Bellcore claims that the average U.S. subscriber line has twenty-two splices.

Feeder cables enter the central office in a large underground room called a cable vault. Each feeder may contain hundreds of pairs of wires, and be pressurized in order to prevent moisture or ground water from entering and affecting the transmission characteristics of the wire. A typical vault may contain tens of thousands of wire pairs.

The cables pass through the vault and are terminated on the vertical side of the MDF (main distribution frame) . To protect the central office equipment from high voltage transients, such as lightning strikes, which may travel down the wire, the lines are surge protected by carbon blocks or gas tubes.

The horizontal side of the MDF, connects the incoming telephone lines to the peripheral equipment. All that is required to connect a line appearance to a specific interface is to place a jumper between the vertical and horizontal sides of the MDF.

Signals coming from an end-user are generally analog in nature. Consequently, the peripheral equipment converts the signals to digital form before passing them on to the rest of the network.

Incoming trunks from other central offices are comprised of specialized carrier systems. They may be either analog or digital, but all new systems are strictly digital.

Most end-user voice & data interfaces are multiplexed on to high-speed paths, which pass through the internal switching, network before being routed to outgoing lines or trunks. Incoming digital carrier systems may be accepted directly into the switching network through a cross-connect or may be demultiplexed prior to switching.

PSTN Hierarchy

Historically the telephone network was composed of a hierarchical structure consisting of 5 different office types. The most common of these is the class 5 end-office. An end-office connects directly to subscriber telephone sets and performs switching functions over a relatively small area. Telephone exchanges connect to subscribers by means of local loops or lines, generally one per customer. Telephone offices connect to each other by means of trunks.

A class 5 or end-office interconnects telephones throughout a small service area. Each end-office may contain several three-digit exchange numbers and is aware of other local exchange numbers held by other offices.

Calls between offices are routed over interoffice or tandem trunks. Long distance calls are routed to toll offices via toll trunks. The average class 5 office serves approximately 41,000 subscribers, and covers 30 square km in an urban environment.

Some nodes may have no customers at all, and may be connected only to other nodes. These inter-node or trunk connections are usually made by FDM or TDM transmission links.

Exchange Area Network

An exchange network consists of local and tandem exchanges connected by trunks. A tandem office interconnects class 5 offices by means of twisted pair, coax, microwave, or fiber optic carriers. Alternate routing paths between local exchanges are provided if the direct trunks are occupied.

An exchange area includes all of the offices, which are aware of each other, but do not involve long distance charges. In very large urban areas, there is an overlap between exchange areas, which may also cross over area code boundaries.

Long Haul Network

A long haul network consists of exchanges interconnected by toll offices. Toll offices keep track of long distance charges and are typically confined to national boundaries. These trunks consist of high capacity coax, microwave, or glass fiber.

Messages used to control the call setup and takedown can be sent by two basic methods. Traditionally, inter-office messages are sent over the same channel that will carry the voice path, but in newer systems, common channel signaling is being employed. In this method, the offices have dedicated facilities, which are used to send inter-office messages. There are some advantages to this, perhaps the notable being the added degree of difficulty encountered if one wants to defraud the system. When in-band signaling was used, it was possible for people to dial long distance calls without being charged, if they created the tones used to disable the toll circuit.

Interoffice Signaling

Trunks are used to interconnect the various levels of telephone exchanges. It is necessary for these links to exchange on a wide range of information including:

• Call related signaling messages

• Billing information

• Routing and flow control signals

• Maintenance test signals

There are two ways for telephone offices to communicate with each other and pass on routing information. Information can be conveyed in the same channel that will be used to convey the voice signal, or it may be completely disassociated with it.

CAS

The CAS (Channel Associated Signaling) approach uses the voice channel to send information through a trunk. For example, a 2600 Hz tone is used in interoffice trunks to signal on-hook. A major disadvantage of this system is that subscribers can bypass toll centers by injecting the appropriate tones. One way to avoid this problem is by using out-of- band signals on toll trunks. Since the customer’s signal must pass through an audio anti-aliasing filter, it is not possible to inject the out-of-band signaling tone.

A principle advantage of in-channel signaling is that the integrity of the voice path is checked each time a connection is established. Out-of-band signaling allows for continuous supervision of the connection throughout the call.

CCIS

The CCIS (Common Channel Interoffice Signaling) approach has the signaling information conveyed on a facility completely separated from the customer’s voice path. This allows for a faster, more efficient control, however the reliability of the CCS network must be considerably greater than that of the individual voice paths. The signaling channel may follow the same route as the final connection path, or it may be completely disassociated with it. STPs (Switch Transfer Point) are need in the network if the signaling path is disassociated, thus effectively creating two networks: a speech network and a signaling network overlay.

SS7

Virtually all calls requiring tandem or toll office routing are established and controlled by the SS7 signaling network.

The SS7 signaling network is a packet switching facility comprised primarily of STPs (Signaling Transfer Point) and SCPs (Service Control Point) connected to the PSTN SSP (Signal Switching Point). STPs are deployed in pairs and are the brains of the system. They determine which trunks and offices should be used in establishing inter-office connections.

The SCP is a database that keeps track of such things as: credit card authorization, virtual network subscriber listings, 800 number conversion tables, billing, and other special services.

Class 5 Office

A Generic Communication SwitchA telephone central office is often referred to as a switch because it switches or routes calls. Regardless of who makes them, all class 5 offices have the same objectives, and therefor have similar structures. The three major components found in any modern switching systems are; the central control, network, and peripherals.

The internal architecture of a telecommunications switch is somewhat like the organization of the entire system. The internal structure is often illustrated by the traditional pyramid or hierarchical arrangement. The control or brains of the operation are shown at the top, and the peripheral units that connect to the outside world are placed at the bottom.

Physically, the switch is simply a series of boxes, full of electronics:

The MTBF (mean time between failure) for any PSTN switch must be very long, since business would soon grind to a halt if telephone traffic was interrupted for a prolonged period, but more importantly, emergency services would be severely curtailed. For these reasons, large public switches have a great deal of redundancy built in. Redundancy is provided in two basic ways; hot standby and load sharing.

In the hot standby arrangement, two or more processors are fed with the identical information and are making decisions, however, only one of these processors is in charge and is executing decisions. In the event of a failure, the healthy unit assumes the full load. There is no degradation in performance, and no calls in progress should be lost.

In a load-sharing configuration, all processors are actively working but not to their full capacity. In the event of a failure, the defective processor is isolated from the system, and the others pick up the slack. There may be degradation in performance, and calls in progress on the defective processor may be lost.

Peripheral Layer

The periphery is the outermost layer in any system. It contains the interface to all outgoing lines and trunks.

Network Layer

The network switch does the actual routing of signals from one customer or port to another. Switching can be done in either the space or time domain. Initially, all switching was done by mechanical contacts in the space domain. Today most switches are digital in nature and operate mainly in the time domain. However, some degree of space switching is always required since signals must ultimately be routed from one line to another.

Networks always provide some form of concentration. That is to say, not all customers can be handled simultaneously. Statistical analysis shows that in the majority of cases, the switch needs only to handle about 20% of all the subscribers at one time. This is similar to highway systems where the roads are designed to handle not all vehicles simultaneously, but only a certain peak load.

File:Basic Telephone Call Procedures

Central Control

This contains the system intelligence and customer database. It knows who the customers are, what they want, and how to provide the service they require.

In a step x step [step by step] switch, the intelligence is fully distributed and there is no central control, whereas in a crossbar facility, all of the intelligence is resident in a central controller or computer. In all modern systems the intelligence is somewhat distributed, with various functional blocks contributing to the decision making process.

At onetime there was a sharp distinction between computers and telecom switches, but today this division is less clear, and central controllers may be regarded as a specialized computer.

BASIC TELEPHONE CALL PROCEDURES: The steps in completing a local telephone call are as follows:

Initially calling party goes in off hook condition.
Once the DC current flows into the loop,the switching machine sends a dial tone to the calling party in order to make sure that it access to the switching machine.
Now the caller starts dialing the destination telephone number either by rotary dialer or touch tone keypad.
The switching machine will locate the corresponding local loop for the destination number.
After locating,the switching machine will check for on hook or off hook condition of the destination subscriber soas to make dc current to flow.Simultaneously the switching machine provide a signal path through it for the two local loops.
The switching machine sends a busy dial tone to the called subscriber when the destination party is off hook.
If the destination part is off hook when the destination party answers to the call and completes the loop.
When the destination party answers the called party,DC current flows through the loop.The flow of DC current through the loop indicates the switching machine that destination party answered the call.As a result,the switching machine remove the ringing and ring back signals.
If both stations are on hook,switching machine recognizes on open circuit on that loop and drops the connection through the switching machine.

Specialized Components

There are some components associated with telephone systems that deserve special consideration.

Line Cards

Line cards are the single most common component in a telephone office. It is a very complex device that contains a wide range of technologies.

The (telephone) line interface is often referred to as a BORSCHT circuit.

This acronym describes the functional requirements of a standard telephone line interface. The tip and ring leads of the telephone set are wired through some protection devices to the line interface located in the peripheral module. This interface must perform the following functions:

B Battery feed

O Over voltage protection

R Ringing

S Supervision & Signaling

C Coding

H Hybrid

T Test

Many of these functions can be integrated into a single IC, often called a SLIC chip (subscriber line interface chip). SLICs have been available for the PBX market for over a decade. Recently however, they have also become available for the central office environment as well.

B - Battery Feed

Most domestic appliances are powered from an electric utility grid. The notable exception to this is the telephone. This is because the telephone should still operate in the event of a power failure. Indeed, the telephone is vital in case of disaster or emergency.

The telephone office provides a nominal -48 volt dc feed to power the phone. This magnitude is considered the maximum safe dc operating potential. It would not be in the telephone company’s best interest to provide a dc voltage, which could electrocute its customers, or it’s own employees. A negative potential was chosen to reduce corrosive action on buried cables.

Multi-function telephones cannot always be powered from the telephone exchange and often require an alternate power source. For this reason, sophisticated line interfaces such as ISDN SAA interfaces have a ‘fail to POTS’ mode. If the electric power fails, the complex phone cannot function to full capacity. The telephone exchange can sense the local power outage through the telephone loop and switches to POTS only service.

The POTS loop requires a nominal -48 v at 20 – 100 ma dc to maintain a voice and signaling path. The earpiece in the handset does not require biasing, but the carbon microphone does. Subscriber signaling is performed by temporarily placing a short circuit on the loop thus changing the loop current, which is then sensed at the central office.

There are several ways to provide loop current, the simplest being a resistor in series with a battery.

Another way to provide loop current is by an electronic current source.

Although this method is quite complex, it has become quite popular with the advent of high voltage bipolar technology. One of the more difficult requirements to meet is the 60-dB longitudinal line balance requirement. To achieve this, the impedance to ground on each side of the loop, must match within 0.1%. This is easy to do with laser trimmed thick film resistors, but a bit tricky with current sources.

A standard telephone requires a minimum of about 20 ma. This means that the maximum possible loop resistance is about 2000

\Omega

. In actual practice, the loop is generally limited to 1250 W. The maximum loop length is determined by the wire gauge.

O - Over-voltage Protection

The two major types of over-voltage that can occur are lightning strikes and power line contact. In both cases, the circuit must either recover or fail-safe. Under no circumstances can a surge be allowed to propagate further into the system, or create a fire.

Initial surge protection is provided at the MDF by gas tubes and/or carbon blocks, which arc if the applied voltage exceeds a few hundred volts. Since these devices take a finite time to respond, high-speed diodes are also used at the line circuit inputs.

R - Ringing

Ringing is often provided by means of a dedicated ringing generator that is connected onto the loop by means of a relay. It is possible to generate ringing voltages at the line interface if the current generators have a high enough voltage source available to them. Or alternately, a switching converter with step up capability can be place on the interface.

In Canada, the ringing voltage is a nominal 86 Vrms at 20 Hz, with a 2 second on and 4 second off cycle. On rural party lines, ringing codes of long and short rings are sometimes used.

In the U.S. there are a number of fully selective and semi-selective ringing methods used on party lines. One employs different frequencies ranging from about 16 – 66 Hz. In such cases, each telephone ringer is tuned to its own frequency. Other methods use positive and negative battery voltages or apply ringing on either the tip or ring side of the line with respect to ground.

S - Supervision & Signaling

The central office must supervise the loop in order to identify customer requests for service. A request for service is initiated by going off-hook. This simply draws loop current from the CO.

Loop current at the far-end is monitored during ringing to enable the CO to disconnect the ringing generator when the phone is answered. The office continues to monitor the loop current at both ends of the connection throughout the call, to determine when the call is terminated by hanging up.

Signaling is a way to inform the CO what the customer wants. The two basic signaling methods used in customer loops are dial pulse and touch-tone. It is interesting to note that preferred customer loop signaling method in analog exchanges is digital, while the preferred method in digital exchanges is analog!

MF Signaling Tones

Two tones are used to perform the signaling function to eliminate the possibility that speech be interpreted as a signal. At one time DTMF decoders were costly and bulky devices located in a common equipment bay, but today with the advent of LSI technology, this function can be performed on a chip. An example is the Mitel MT8865 DTMF filter, and MT8860 DTMF decoder.

Positions 11 to 14 are not presently being used.

C - Coding

Telecommunications signals are seldom linearly encoded, but rather are companded (a combination of compression & expansion). This allows for a more uniform S/N ratio over the entire range of signal sizes. Without companding, a 12 bit linear encoding scheme would be needed to obtain the same S/N ratio at low volume levels. It also reduces the noise and crosstalk levels at the receiver.

There are two principal international standards used to implement coding algorithms today: A-law, which is used in Europe, and µ-law, which is used in North America. These both involve 8 kHz sampling, and 8 bit A/D & D/A conversion, thus resulting in 64 kbps digital bit streams.

Although the actual compression algorithms are continuous functions, the codec approximates them by linear segments. A-law has 13 linear segments, and µ-law has 15 linear segments or cords.

Another important difference between the European and North American codecs, can be seen by the position of the decision threshold and its digital value.

When a telephone call is placed between Europe and North America, it is essential that all of these differences be accounted for. It is possible to regenerate the analog voice by passing it through the same type of codec that originally processed it, and then re-code with the other. An alternate approach is to use lookup tables that translate the binary values of one system to the other.

Since the highest frequency passed is about 3.4 kHz, a great deal of ingenuity is required to pass data at 4.8, 9.6 kbps or even higher. Note that these are well above the Nyquist rate but considerably below the Shannon-Hartley limit.

All modern telephone systems today employ codecs in the BORSCHT interface to digitize the incoming analog signals. It is ironic that although the telephone system has been updated to digital technology, the telephone set and loop has remained analog.

By international agreement, all voice codecs use an 8 kHz sampling rate. Since each transmitted sample is 8 bits long, the analog voice signal is encoded into a 64 kbps binary steam. This rate determines the basic channel data rate of most other digital communications systems.

By bypassing the codec, it is possible to send 64 kbps customer data through the telephone system. However, because of old style signaling schemes still in use, digital data rates are often limited to 56 kbps.

H - Hybrid

A diplexer performs a bi-directional 2-wire to 4-wire conversion. It allows two unidirectional electrical paths to be combined into a single bi-directional one, and vice versa. It is advantageous to separate transmit and receive portions of the signal since it is easier to make unidirectional amplifiers, filters, and logic devices.

One of the simplest ways to create an audio band hybrid is to use a transformer hybrid.

Single Core Transformer Hybrid

There are several ways to split transmit and receive paths, the simplest method uses a single core hybrid transformer.

The basic defining transformer equations are:

{\frac {V_{1}}{n_{1}}}={\frac {V_{2}}{n_{2}}}={\frac {V_{3}}{n_{3}}}=\cdots \quad \quad \quad and\quad \quad \quad I_{1}n_{1}+I_{2}n_{2}+I_{3}n_{3}+\cdots =0

For a single core hybrid with a center-tapped secondary, the impedance relationships for proper operation (conjugate matching) are:

for\quad \quad \quad n_{2}=n_{3}\quad \quad \quad Z_{2}=Z_{3}=2Z_{4}\quad \quad \quad and\quad \quad \quad Z_{1}=\left({\frac {n_{2}}{n_{1}}}\right)^{2}Z_{4}

Note what happens if the transformer is driven from one of the secondary windings:

let\quad \quad \quad n_{1}=n_{2}=n_{3}\quad \quad \quad \therefore I_{3}=-I_{1}-I_{2}

But I₁ and I₂ flow in the opposite directions, therefor:

I_{3}=-\left({-I}\right)_{1}-I_{2}\quad \quad \quad and\;if\quad \quad \quad \left|{I_{1}}\right|=\left|{I_{2}}\right|\quad \quad \quad then\quad \quad \quad I_{3}=0

This last requirement can be satisfied by adjusting the impedances Z₁ - Z₄ to make the currents equal. From this we observe that signals injected into any port emerge only at adjacent ports but not at the opposite one.

In a properly balanced single core hybrid the typical throughput or insertion loss is about 3.5 dB and the THL (trans hybrid loss) is about 25 dB.

Double Core Hybrid

When properly balanced, a 2-core network can achieve a THL of 50 dB while the insertion loss remains at about 3.5 dB. It has better performance than the single core device, but is bulkier and more expensive.

Balancing Networks

All telecom equipment is tested and characterized against standard impedance terminations. These impedances are based on line surveys and are approximate equivalent circuit representations of the outside cabling plant. For this reason, these networks vary from country to country. In North America, IRL (input return loss) is measured against:

T - Testing

In order to maintain a high degree of service (99.999%), the equipment must be capable of detecting and repairing faults before the customer is even aware that there may be a problem. As a result, a separate test buss and access relay is provided on a line interface. Tests may be performed in a bridged mode or with the loop and line card disconnected from each other.

Testing can be done in three basic directions:

• From the line interface looking out towards the subscriber loop

• From the loop connection looking into the line card

• From the central office side of the line card

These tests are generally automated and are conducted late at night when there is little chance that the customer will request service, thus interrupting the test. Some of the scheduled tests may include:

• Transmit and receive levels

• Transmit and receive frequency response

• Insertion loss

• Trans-hybrid loss

• Quantization distortion

• Aliasing distortion

Some other tests that may be performed when commissioning a line or when a complaint is lodged, include:

• Impulse noise test

• C-message noise

• Longitudinal balance

Repeaters

By placing two hybrids back to back, it is possible to create a bidirectional amplifier or repeater. The total gain in the 4-wire path within the repeater must not exceed the combined transhybrid loss of the transformers. If this happens, the circuit will oscillate or sing.

The total gain in the 4-wire path within the repeater must not exceed the combined transhybrid loss of the transformers. If this happens, the circuit will oscillate or sing.

Space Division Switching

The physical path between any two customers on a space switch is not shared with anyone else. Crosspoints made from electromechanical relays have been used to perform the interconnection, but newer systems use semiconductors.

In a crossbar matrix, the number of inputs and outputs do not have to be equal thus facilitating either concentration or expansion. In any case, a total of NxM crosspoints are required. Although only one contact is shown, many systems require two contacts if they keep the integrity of the tip and ring leads throughout the system.

The folded matrix treats inputs and outputs identically and requires only N(N-1)/2 crosspoints. The major disadvantage of this type of switch is the rapid increase of crosspoints as the system size increases. The number of crosspoints increases as N² and yet only a maximum of N crosspoints can be active at any given time. Failure of a single crosspoint prevents communication between the two devices sharing that crosspoint.

Multiple Stage Space Switch

One way to avoid the cost penalties associated with full matrices, is to organize the contacts into smaller groups. This impacts the call processing since the switch controller must manage several contacts per connection. Furthermore, the connection between any two subscribers may take any one of a number of paths, thus further complicating the decision making process.

The following sketch shows a simple three-stage space switch. It should be remembered that since a full duplex connection is required, a second switch supplying the return path must be provided.

The total number of crosspoints in the above illustration is:

PLACE EQUATIONS HERE

The number of crosspoints required is dramatically reduced in this multiple stage switch in comparison to a square matrix. The central stage allows several ways to make a connection between two subscribers therefor, single crosspoint failures can be bypassed. This results in a more flexible and reliable system, but demands a more complex control structure.

Blocking

A non-blocking network is capable of finding a path between any idle input to any idle output. This does not mean that the system be able to handle simultaneously all customer requests for service. In such a case, the system may overload but the customer is not able to distinguish the difference between blocking and overload.

The following illustration shows how blocking can occur. The solid lines represent connections in service.

PLACE ILLUSTRATION HERE

The connections in service are not necessarily the optimum routing and may have been forced by the prior connections. Note that in this case, it is not possible for the last customer on switch 1:1 to contact the last customer on switch 1:3 because there is not a free center switching stage common to both. To overcome this, an additional center stage can be added. To prevent blocking, 2n-1 center arrays are necessary.

The total number of crosspoints (NoC) in a non-blocking network is therefore:

PLACE EQUATIONS HERE

As the total number of lines in increase, the total number of crosspoints can be approximated by:

A non-blocking network of this type is known as a Clos switch, after its inventor. Its basic characteristics include:

• Expansion in the first switching stage

• An odd number of center stages

• Concentration in the last stage

Time Division Switching

Time domain switching is simply an application of time domain multiplexing and may be performed on analog or digital signals. Any number of inputs may be sequentially routed to a single output.

This technique increases the transmission link utilization and can be modified to support circuit switching. If a multiplexer is placed at the input, a demultiplexer is placed at the output. This system can be used to multiplex either analog or digital signals.

Each customer is assigned a unique switch, but all customers share the same internal signal path. For N customers there are 2N switches. TDM bus switching occurs when the input sequence is not the same as the output sequence.

Each customer is given access to the common structure for a brief moment. If higher data rates are needed, multiple inputs can be assigned, thus giving the customer more time to transmit a signal.

Time Slot Interchange [TSI]

If information can be arranged into a sequence, it can also be rearranged, much like shuffling a deck of cards. This is the task of the time slot interchange unit. Full duplex operation is achieved by interchanging time slots in both directions.

The incoming TDM channels are mapped sequentially into RAM while the outgoing channels are read out non-sequentially. This output address generator is simply a memory-mapped pointer governed by a central controller.

The required memory access time can be approximated as the inverse of the channel rate. The RAM width is determined by the number of bits in a channel, and the length by the number of channels in a frame.

Time Multiplexed Switching

Combining TDM and TSI allows a channel from one digital bit stream to be switched to any channel on another digital bit stream. A multistage time switch can consist of cascaded switching modules. To prevent blocking, 3 or more stages are required.

Although it appears that only time domain switching is used in this example, it is also known as a Time-Space-Time switch or simply TST. This is because the center stage is actually switching different input lines in space to a common output line. Since there is an ambiguity in the terminology, some manufacturers of telecommunications equipment may refer to this as a TTT switch if all of the signals are digital.

For small-scale switches, space switching is most efficient. However, as switch size increases, time domain switching tends to gain the advantage.

It is sometimes difficult to make direct comparisons between various telecommunications switches because the internal architectures may be quite different. However, it is possible to compare BHCA (busy hour call attempts) capacity or performance such as traffic intensity, under a specified set of circumstances.

Traffic Intensity is the product of average holding time and the calling rate and is expressed in CCS (hundred call seconds) or Erlangs.

CCS is defined as: (number of calls per hour)x(call holding time in seconds)/100seconds Therefore, 36 CCS = 1 Erlang

An erlang is a measure of the traffic intensity and is equal to the average number of simultaneous calls at any given moment. It represents the total circuit usage during any time interval, divided by that interval. It also corresponds to the minimum number of channels necessary to carry the traffic, if it could be scheduled.

Example

Imagine for a moment that there are 8 customers in a small telephone system making random calls:

These calls could conceivably be arranged as:

Three, 1 hour channels could carry this traffic and still have 25 minutes left over to spare. The traffic intensity is therefor less than 3 Erlangs:

Questions for Research

1. What is the purpose of dial tone?

2. What are the advantages of CAS and CCIS interoffice signaling?

3. What is the difference between a line and a trunk?

4. Why does the standard telephone exchange need to provide BORSCHT?

5. List the differences between North American and European codecs.

6. Under what circumstances can poor THL be tolerated?

7. What is the difference between an echo suppresser and an echo canceler?

8. Where do the telephony terms tip and ring originate? Tip and Ring come from switchboard days. The plug on operators cords had a center 'tip' for one conductor, and the outer 'ring' for the other conductor. 9. Define blocking.

10. Why are multiple stage space (or time) switches more practical than a single stage switch?

Active Bypass Diodes Improve Solar Panel Efficiency and Performance

The Schottky bypass diodes used in most cell-based solar panels serve as a protection mechanism that allows the panel to continue producing power when one of its cell strings is shaded or damaged. However, the characteristics of traditional diodes create energy losses that reduce the overall efficiency of a solar power system and, in some situations, may actually cause costly damage. To solve this problem, several manufacturers have introduced a new class of “active diodes” that use transistors to produce diode-like behavior, while allowing the solar panels they protect to operate with higher efficiency and better reliability. This article will explore the technology that underlies active diodes, look at the products currently on the market, and look at how they are changing the way solar panels are being designed and manufactured.

Figure 1: Solar arrays rely on bypass diodes to protect them from damage and minimize output power losses when a section of an individual panel or a larger part of the array experiences shading due to obstructions, clouds, snow, or other phenomena. (Courtesy of STMicroelectronics.)

Bypass diodes provide protection

To understand why bypass diodes are needed, let’s look at a typical solar power system. Each string in the system is typically comprised of 10 to 20 series-connected solar modules, each with 60 to 100+ (typically 72) cells that are, similarly, all connected in series. The result is a 1,000-odd cell string, with each cell attempting to produce current in direct proportion to the amount of sunlight it sees. If any of these cells become shaded, soiled or damaged, the entire string current is limited to that which the weakest link can support. Besides being susceptible to temporary performance losses when even a single cell is shaded, an array constructed using long-series strings can introduce other, more subtle problems into a solar-electric system.

A typical silicon cell has a forward voltage of 0.5 volts when optimally loaded, while thin-film processes such as CdTe or CIGS vary from 0.3 V to over 1 V. If, for some reason (such as shading), a cell cannot produce as much current as other nearby cells, then this same cell will now be forced into a reverse mode of operation where it is subjected to a negative voltage, which, depending on its position in the string, can be from 5 to 30 V. Although the cells are somewhat forgiving, if enough mismatch is present, the under-performing cell will be driven into the region of reverse breakdown. With 10 to 20 solar modules connected in series, the DC output of a modern solar system can easily be 400 V. Under these conditions, the shaded cell with 30 V applied across it may begin operating in reverse breakdown, with the functional cells in the rest of the string accounting for the remaining 370 V. In a typical system where string currents approach 10 A, a shaded cell is quickly transformed into a 300 W heating element, creating a hot spot that can easily damage the panel.

To prevent panel damage (and the occasional rooftop fire), solar manufacturers have traditionally arranged their panels into substrings of 12 to 24 cells, each with its own bypass path (Figure 2). The decision to use 12- to 24-cell groups is based on a comparison of the summation of the forward voltages versus the expected breakdown voltage of the weakest cell in the string. For example, in a group of 24 cells, each with a forward voltage of 0.5 V, an overall voltage of 12 V will be produced. To prevent a breakdown-induced “meltdown”, the bypass diode must be inserted at points that prevent the total number of diode drops that comprise the local loop’s voltage from exceeding an individual solar cell’s reverse breakdown threshold (usually 25 to 30 V).

Figure 2: When one or more of a solar panel’s substrings of PV cells experiences shading, its bypass diodes protect prevent damage from over-voltage conditions and electrically-induced overheating while allowing the remainder of the string to operate normally. (Courtesy of Microsemi Corp.)

Bypass diodes also provide some protection against DC arcing, which, unlike the arcs produced by the AC voltages found in most wiring, will not self-extinguish. Arcing in AC systems often self-clears at the “zero-crossing” of the 50/60-Hz waveform, while a DC-generated arc must be interrupted by a protection device or physically moving the discharge points further apart. Bypass diodes can provide limited protection against “series” arcs within the module itself, because they limit the local arcing voltage to 10 to 20 V – a very important secondary feature.

The bypass devices on early solar panels were silicon P/N diodes. They had forward junction voltages of 0.7 to 1.0 V and breakdown voltages of 500 to 600 V. The heat produced by the P/N diode’s junction losses was acceptable at low amperages but, as cell efficiencies improved and larger wafers raised typical string currents, the industry was forced to adopt Schottky diodes. Their lower forward junction voltages (0.4 to 0.5 V) cut power dissipation in half, which helped solve the heating problems but introduced several other difficulties. For example, Schottky diodes have much higher leakage currents than an equivalent P/N device, a problem that gets worse as the device’s temperature increases.

Schottky diodes also have much lower reverse breakdown voltages, typically 40 to 60 V less than the 600 V ratings of comparable P/N devices. The diodes’ junctions are also more susceptible to permanent damage by relatively low levels of transient energy. A diode that fails “open” can leave the substring it protects vulnerable to a destructive “hot spot” event during the next occurrence of shading or soiling. If the diode fails in a “shorted” mode, it will (at a minimum) dramatically reduce the panel’s output power.

Active solutions

Several companies have recently introduced an alternative to Schottky diodes in the form of a new category of so-called “lossless”, or “active” diodes. In truth, they are actually two-terminal FET-based switching circuits, designed as pin-compatible replacements for conventional diodes. Several manufacturers offer active bypass diodes for solar applications including Microsemi, STMicroelectronics, and Texas Instruments. Regardless of who makes it, the main components of an active diode (Figure 3) include:

Charge pump
Control logic and FET driver
MOSFET
Capacitor

Figure 3: Anatomy of an active solar bypass diode. (Courtesy of STMicroelectronics.)

When the solar array is shaded, the active bypass device operates as follows:

The shaded cells create an increase in local voltage, activating the bypass diode.
Initially, the FET body diode conducts, enabling the charge pump to start.
Once the capacitor is fully charged, the charge pump stops and the FET is activated.
The cycle repeats (Figure 4) when the energy in the capacitor is depleted, wherein the FET turns off and the charge pump is re-activated.

Figure 4: Active bypass diode operation. (Courtesy of Texas Instruments.)

Each manufacturer offers some combination of active diode elements in a multi-chip package, with the remainder as discrete external components. Microsemi's LX2400 IDEALSolar Bypass devices come in an LGA package that houses all the components required for an active bypass diode, as well as a lightning survivability circuit that provides a bi-directional, low-impedance path that lowers its power dissipation during a lightning strike.

The present generation of STMicroelectronics’ Cool Bypass Diodes (the SPV1001 family) is available in TO220, D2PAK, and PQFN packages, which integrate all the necessary elements except for the switching capacitor and power FET. The company’s second-generation products (available Q1 2013) will eliminate the need for an external capacitor and will be available in several higher and lower voltage ranges. Texas Instruments’ SM74611 Smart Bypass Diode (according to TI this part will be available and on the Digi-Key site December 2012) will be available in multi-chip module with an industry-standard D2Pak form factor that includes the capacitor and power FET.

Active dividends

As with nearly any new technology, active bypass diodes have a price disadvantage against the mature technology they are displacing. The first generation of active diodes cost roughly 2X to 3X what manufacturers pay for high-quality Schottky diodes. However, the prices of some active devices has declined by 10 to 20 percent over the past year, and smart designers are beginning to understand that the devices can offer significant dividends in terms of overall solution cost and added capabilities that repay their higher cost several times over.

The most obvious advantage active bypass diodes offer is dramatically lower losses in both their bypass and “off” modes. A typical device has a 40 to 50 mV forward voltage versus the Schottky’s 0.4 V, which translates into roughly a 10X reduction in power dissipation when running in bypass mode. This improves an array’s ability to operate efficiently when one or more panels in its string are subject to shading conditions (caused by neighboring buildings, trees, chimneys, etc.).

When the active bypass diodes of the strings in the affected modules operate in forward (bypass) mode, they produce roughly 80 percent less than an equivalent Schottky diode (Figure 5). When a PV array using active bypass devices experiences 10 percent shading (not uncommon in residential/commercial systems), it will produce up to 0.5 percent more energy than a system using conventional devices. Their low-leakage characteristics also provide a small, but measurable improvement in energy production during normal operations. Their reverse leakage current is roughly 0.3 µA (versus up to 100 µA for Schottky diodes), which adds roughly 0.01 percent to a typical system’s overall energy gain.

Figure 5: Power dissipation of a Schottky diode and an active diode in a junction box at 85°C ambient (per diode). (Courtesy of Texas Instruments.)

Active diodes’ reduced heat dissipation pays much bigger dividends by allowing the use of simpler, smaller heatsinks which can be housed in simpler, more compact junction boxes that cost less to manufacture. Their compact form factor and reduced cooling demands may even allow designers to mount the diodes directly on the solar panel’s bus bar, eliminating the need for a junction box and its associated wiring. This configuration could cut as much as $10 to $15 from a panel’s total system cost, savings that would repay the active diodes’ added price several times over. Similar savings can be realized by integrating the cool-running devices directly into the per-panel micro-inverters and optimizer boxes that are being offered by some manufacturers.

These devices may also be essential for new bypass architectures that support the emerging requirements for incorporating monitoring and safety shutdown capabilities into the solar panel itself. The genesis of these requirements is in part a report from Underwriters Laboratories Inc., titled “Firefighter Safety and Photovoltaic Installations Research Project,“2 which, among other issues, documented how water on a damaged solar array creates a potential danger zone with a radius of up to 20 ft. (and up to 1,000 ft. if the water contains salt). This research also concluded that turning off an array is not as simple as opening a disconnect switch. As long as the array is illuminated, parts of the array will always be energized, therefore generating lethal DC voltages. This and other studies are driving a new safety standard that will require solar panels used in some applications, such as utility-scale solar farms, to provide an electronic interlock that can be used to “kill” each panel’s output remotely. Since the bypass diode is already located within the solar string, the addition of a control input that could use the device’s power FET to disable the panel would provide the required safety mechanism at a minimal increase in cost.

Coming: Integrated diodes

The same low power and compact form factors have also aroused manufacturers’ interest in integrating them into the panel itself. At present, no commercial products are available that laminate the diodes onto the same backing that holds their PV cells. However, STMicro has experimented with on-panel diodes for some time, and has developed a PQFN package option for some members of the SPV100x family which boast a 0.75 mm thickness with the goal of making the lamination process easier and smoother (Figure 6). Integrated diodes also eliminate the added cost and failure points created by external wiring, making it increasingly feasible to use more diodes per panel, allowing the remaining substrings to continue producing power in partial shading conditions.

Figure 6: Active diodes’ low power dissipation and compact form factor makes it possible to mount them directly onto the solar panel substrate, eliminating the need for a junction box. (Courtesy of STMicroelectronics.)

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e- Bypass System working on communication

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Rabu, 05 September 2018

How Electronic Components Work

Electronic Circuit Overview

The Elements of an Electronic Circuit

Element 1: Conducting Path

Element 2: Voltage Source

Element 3: Load

Electronic Circuit Facts

Fact 1: Open Circuit

Fact 2: Closed Circuit

Fact 3: Short Circuit

Fact 4: Printed Circuit Boards (PCBs)

Fact 5: Integrated Circuits (ICs)

Electronic Components

Component 1: Capacitor

A. Composition

B. How Does It Work?

C. Function and Significance

Filters Capacitor Applications

Hold-Up Capacitor Applications

Timer Capacitor Applications

Component 2: Resistor

A. Composition

B. How Does It Work?

C. Function and Significance

Limiting the Flow of Current

Dividing Voltage

Resistor-Capacitor Networks

Component 3: Diode

A. Composition

B. How Does It Work?

Vacuum Diode

P-N Junction Diode

C. Function and Significance

AC to DC Conversion

Bypass Diodes

Voltage Spike Protection

Signal Demodulation

Reverse Current Protection

Component 4: Transistor

A. Composition

B. How Does It Work?

C. Function and Significance

Transistors in Hearing Aids

Transistors in Computers and Calculators

Darlington Transistors

IGBT and MOSFET Transistors

Component 5: Inductor

A. Composition

B. How Does It Work?

C. Function and Significance

Filters in Tuned Circuits

Inductors as Chokes

Ferrite Beads

Inductors in Proximity Sensors

Induction Motors

Transformers

Energy Storage

Component 6: Relay

A. Composition

B. How Does It Work?

C. Function and Significance

Protective Relays

Automatic Reclosing Relay

Thermal Relays

Component 7. Quartz Crystal

A. Composition

B. How Does It Work?

C. Function and Significance

Quartz Watches

Filters

Conclusion

Bypass Caps: Decouple Your Way to Cleaner Power

Introduction

Safety related systems

System integrity

Integrity levels

History

Underlying philosophy

Safety Integrity Levels