THE CENTRAL PROCESSING UNIT ( C P U)Is the brain or the heart of a computer. Is also known as processor and consist of three units namely -
i) Control Unit ( C U)
ii) Arithmetic logic Unit ( A L U)
iii) Main Memory unit ( M M U)
The system unit is the core of a computer system. Usually it's a rectangular box placed on or underneath your desk. Inside this box are many electronic components that process data. The most important of these components is the central processing unit (CPU), or microprocessor, which acts as the "brain" of your computer. Another component is random access memory (RAM), which temporarily stores information that the CPU uses while the computer is on. The information stored in RAM is erased when the computer is turned off.
Almost every other part of your computer connects to the system unit using cables. The cables plug into specific ports (openings), typically on the back of the system unit. Hardware that is not part of the system unit is sometimes called a peripheral device. Peripheral devices can be external such as a mouse, keyboard, printer, monitor, external Zip drive or scanner or internal, such as a CD-ROM drive, CD-R drive or internal modem. Internal peripheral devices are often referred to as integrated peripherals. There are two types according to shape: tower and desktop. Tower System Unit Desktop System Unit
A motherboard (mainboard, system board, planar board or logic board) is the main printed circuit board found in computers and other expandable systems. It holds many of the crucial electronic components of the system, such as the central processing unit (CPU) and memory, and provides connectors for other peripherals.
Motherboard
TYPES OF PROCESSORS
I) Comples Instruction Set Computers (CISC)
ii) Reduced Instruction Set Computers (RISC)
FUNCTIONS OF CENTRAL PROCESSING UNIT
- Process data
- Control sequence of operaions within the computers
- It gives command to all parts of a computer
- It control the use of the main memory in storing of data and instructions
- it provides temporary storage (RAM) and permanent storage(ROM) of data
THE CONTROL UNIT
Is the center of operations for the computer system, it directs the activities of the computer system.
H. XO signal is active or passive
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Example 1: Your device outputs a 4…20 mA signal which is measured by a PR module.
Disconnect the two current loop wires from the PR module input terminal.
Using a voltmeter, measure for DC voltage present on the wires.
If no voltage is present, then your device is creating a passive current signal. This means a loop excitation source, (typically 12...24 VDC), must be used to power the current loop and measure the signal. Many PR modules have an input excitation source which powers a passive current loop, allowing our module to both measure and energize your device.
If you measure voltage on the wires, (typically 12...24 VDC), then the current signal created by your device is active. Do NOT use our loop excitation source when measuring an active current signal.
Example 2: A PR module outputs a 4…20 mA signal which is measured by your device.
Disconnect the two current loop wires from our output terminal.
Using a voltmeter, measure for DC voltage present on the wires.
If no voltage is present, then your device has a passive input. Therefore, the PR module must provide the excitation source needed to power and regulate current flow on the loop.
If you measure voltage on the wires, (typically 12...24 VDC), then your device has an active input. Therefore, the PR module should only regulate the level of current flowing through the loop, it should NOT power the loop.
List of PR devices by active/passive input and output measurement type
A PR module that measures:
an active input signal, simply measures the active input current. It does not excite the loop.
a passive input signal, excites the current loop, excites other loop powered devices wired in the loop, and measures the current.
A PR module the creates:
an active output signal, regulates the current level, excites the current loop, and excites other loop powered devices wired in the loop (e.g. a loop powered display). As an example, the output from the PR module can be connected directly to a passive input card on a PLC.
a passive output signal, only regulates the loop current level. The loop is powered by an external excitation source. As an example the output from the PR module can be connected directly to an active (powering) input card on a PLC.
Signal conditioning is a set of operations performed on a signal that makes it suitable for interfacing with other devices or systems. Signal conditioners are the actual devices that perform this operation. These devices have an input and an output. Normally the input is a sensor that measures a variable, not necessarily and electrical signal.
The signal conditioning process is also known as a transfer function because the final effect is to convert an input signal (or measurement) into a suitable output signal. For instance, when a temperature sensor measures the temperature of a system or environment, the output of the sensor (temperature) is not suitable to be an input signal to an electrical system. Therefore, the temperature measurement must be converted into an electrical signal.
The following diagram shows the evolution of a signal from the sensor through the central processing unit and to the output or load:
The Signal Chain. Image credit: Electronic Design
Signal Conditioner Functions
Signal conditioners provide filtering, amplification, converting, and/or other processes required to make sensor outputs suitable for reading by computer boards. They are used primarily for data acquisition, in which sensor signals must be normalized and filtered to levels suitable for analog-to-digital conversion. The digital signal is then available to be analyzed or interpreted by a computerized device.
Filtering
Filters can be constructed from either active or passive components. A passive filter uses only resistors, capacitors, and inductors with a maximum gain of one. An active filter uses passive components and active components like operational amplifiers and transistors. They have a higher gain with sharper frequency response curves.
The filter architecture can be analog or digital.
Analog (RC) - Analog filters are designed with resistors and capacitors. They are used for analog signals only, and are often used in low-noise requirement applications.
Digital (FIR, IIR) - Digital filters are designed with solid-state components and used for digital signals and quantized signals from a sample-and-hold amplifier. This category includes finite impulse response (FIR) and infinite impulse response (IIR) filters. Digital filtering can approach ideal bandpass characteristics.
Filter Type
The function of the filter is to separate the signal's frequency spectrum into valid data while blocking noise. The standard types of filter responses are low-pass, high-pass, band-pass, and band-reject (or notch filter). Filters are selected based on the frequency of the signal to be analyzed.
Low-pass filters block high frequency components; or allow the passage of low frequency signals. A simple passive low-pass filter can be constructed with only a resistor and a capacitor.
Band-pass filters allow the passage of signals within a range of frequencies and blocks signals with frequencies below the smallest frequency in the range and above the highest frequency in the range. If the range (band) of frequencies is between f1 and f2 then the filter allows the passage of signals with frequencies between f1 and f2 only.
Band-pass Filter Response
Image Credit: Electronic Engineering Dictionary
Band-notch filters, also known as a band-reject filters, allow the passage of all frequencies with the exception of signals within a range of frequencies.
Band notch filter. Image Credit: CircuitsToday.com
Amplification
Amplification is a process which increases (amplifies) the signal for possessing or digitization. Signal amplifiers often include electronic components that amplify signals without producing significant amounts of thermal noise. In some applications a signal must be amplified or attenuated in order to drive a circuit or a system. There are many types of amplifiers used in signal conditioning including the following:
Voltage followers have a unity gain, so the output signal is a reproduction of the input signal. This type of amplifier is mainly used as an impedance matching device.
Isolation amplifiers are designed specifically to isolate high DC levels from the data acquisition device while passing the relatively small AC or differential signal. The inputs and outputs are electrically isolated.
Instrumentation amplifiers are differential amplifiers that have been optimized for use with DC signals. They are characterized by high gain, high common mode rejection ratio (CMRR), and high input impedance.
Sample-and-hold amplifiers freeze analog voltage instantly. During this process the HOLD command is issued and analog voltage is available for an extended period.
Signal Converting
In many instances it is required to convert a signal from one type to another, in order to accommodate the driving input of circuits. Some important signal converters are:
Current loop converters convert an analog or digital signal to a current loop output such as 4-20 mA or 0-20 mA.
Charge convertersconvert the charge output from a piezoelectric, capacitive or other charge-producing sensor to a signal such as analog voltage or current.
Digital Conversion
Data acquisition is the digitizing and processing of multiple sensor or signal inputs for the purpose of monitoring, analyzing and/or controlling systems and processes. Analog sensors and signals are first normalized by the use of filters, amplifiers and signal converters. The next wave of the signal chain is the exchange of this signal to a digitized format. The two most important conversion functions in this phase of the process are analog-to-digital conversion and digital-to-analog conversion.
Analog-to-Digital ConverterAn analog-to-digital (ADC) converter is a device that accepts, as input, an analog signal and at the output, produces an equivalent digital signal. Most of our sensors and transducers produce analog signals that have to be converted to digital signal in order to be processed by computers or other digital device. There are several types of analog-to-digital (ADC) converters including: direct conversion, successive-approximation, integrating and sigma-delta ADCs.
Direct-conversion ADCs. Image Credit: HITEQUEST
Digital-to-Analog ConverterA digital-to-analog (DAC) converter produces the reverse operation of an ADC. These devices accept digital signals and convert them to analog signals (normally voltages).
Selection Criteria
After defining the function of the signal conditioner, the form factor, device specifications, signal inputs, sensor inputs, excitation, outputs and user interface are important parameters to consider when searching for signal conditioners.
Form Factor
Common form factors for signal conditioners include circuit board, panel or chassis mount, modular bay or slot system, rack mount, DIN rail, and stand-alone.
Printed circuit boards(PCBs) attach to enclosures or plug into computer backplanes.
Panel or chassis mounts are used to install the device in cabinets, enclosures, or panels with bolts.
Modular style units include stackable units that dock in bays, slots, or boxes.
Rack units that fit inside a standard 19” telecommunications rack.
Devices can be designed for mount on a Standard Deutsches Institute for Normung (DIN) rail, which is a German standard.
Benchtop or freestanding devices often feature full casings or cabinets and integral interfaces.
Device Specifications
Device specifications that are important to consider when searching for signal conditioning products include analog input channels, digital I/O channels, and accuracy.
Analog signals are a wave signal which means that the value changes steadily over time and can have any value in a range. Signal converters with analog inputs can have multiple channels. Channels are either single-end or differential.
Single-end inputs have only one low wire shared by all inputs. For example a board could have 2 single-end inputs; there will be two input lines and one ground line. Single-end inputs are less expensive and allow for twice the number of inputs in the same size wiring connector since they only require one analog input and one ground input which is shared by all the inputs. They save space and are easier to install. When single-ended outputs are available, suppliers often specify the maximum number of analog channel outputs as twice the number of differential outputs.
Differential channels have two inputs. The voltage is the signal processed between the two inputs. The board will have one signal and one ground pin for each input to allow for measurement voltage difference between two signals tied to the same ground. Differential channels provide excellent common-mode noise rejection. This type of input should be used when EMI, RIF or noise is present.
Digital signals do not have 'in between' values. They are an on or off signal producing a square wave. Digital signals break down the information into binary code, which is a series of 1sand 0s. The data receiver reassembles the code into useful information. Digital signal allows users to send more information in a smaller space.
Analog and Digital Signals Compared
Image credit: Being Ministry
Accuracy is defined as the difference (error) between the true value and the indication expressed as percent of the span. Accuracy, which is represented as a percentage of a full-scale measurement range, depends on signal conditioning linearity, hysteresis, and temperature considerations. It includes the combined effects of method, observer, apparatus and environment.
Static accuracy is the combined effects of Linearity, Hysteresis, and Repeatability. It is expressed as +/- percentage of full scale output. The static error band is a good measure of the accuracy that can be expected at constant temperature.
Linearity is the deviation of a calibration curve from a specified straight line. One way to measure linearity is to use the least squares method, which gives a best fit straight line. The best straight line (BSL) is a line between two parallel lines that enclose all output vs. pressure values on the calibration curve.
Repeatability is the ability of a transducer to reproduce output readings when the same pressure is applied to the transducer repeatedly, under the same conditions and in the same direction.
Hysteresis is the maximum difference in output at any pressure within the specified range, when the value is first approached with increasing and then with decreasing pressure. Temperatures hysteresis is the sensor's ability to give the same output at a given temperature before and after a temperature cycle.
Hysteresis
Image credit: Showa Measuring Instruments Co. LTD
Signal Inputs
The input signal can have a variety of specifications as it enters the converter. The type of converter selected depends largely on the type of input signal from the system and the desired output signal. The input signal could have properties such as:
DC type voltage and/or current
AC type voltage and/or current
Frequency waveforms for varying frequency, pulse or specialized waveforms.
Chargewhich comes from a piezoelectric device and usually requires conditioning.
Sensor Inputs
Sensor inputs can be accelerometer, thermocouple, thermistor, RTD, strain gauge or bridge, and LVDT or RVDT. Specialized inputs include encoder, counter or tachometer, timer or clock, relay or switch, and other specialized inputs.
Excitation
Sensors can be classified as either active or passive devices. Passive devices like thermocouples can generate a signal without a power supply. Active sensors need a power supply in order to control the flow of electrons and make a measurement. In some cases active sensors are powered by the signal conditioner. The output from the signal conditioner that powers the device is referred to as an excitation source. Signal excitation can either be a voltage or current output. The following schematic showcases how an excitation voltage powers a Wheatstone bridge:
Wheatstone Bridge. Image credit: Maxim
Outputs
Outputs for signal conditioning products can be voltage, current, frequency, timer or counter, relay, resistance or potentiometer, and other specialized outputs.
User Interface
Signal converters have several user interfaces available that allow the user to make adjustments to the system.
A front panel is a local interface with integral controls, a keypad, and/or display on the panel of the unit
Computer programmable converters are interfaced with a separate supervisory or host computer.
Touch screens have a visual display which interacts with the user through touch. The user can directly put in information through the contact-sensitive screen.
Remote and handheld devices can be mobile while the user enters program parameters.
I. XO Capacitor Uses & Applications ______________________________________________
Capacitors are used in virtually every area of electronics, and they perform a variety of different tasks. Although capacitors operate in the same way whatever the capacitor application or use, there are several different uses for capacitors in circuits.
In order to select the right kind of capacitor it is necessary to have an understanding of the particular capacitor application so that its properties can be matched to the given use to which it is to be put.
Each form of capacitor has its own attributes and this means that it will perform well in a particulate capacitor use or application.
Choosing the right capacitor for a given application is all part of the design process for a circuit. Using the wrong capacitor can easily mean that a circuit will not work.
Capacitors can be used in a variety of different ways in electronics circuits. Although their mode of operation remains exactly the same, the different forms of capacitor can be used to provide a variety of different circuit functions.
Coupling capacitor use
In this capacitor application, the component is allow AC signals to pass from one section of a circuit to another while blocking any DC static voltage. This form of capacitor application is often required when connecting two stages of an amplifier together.
It is possible that a static voltage will be present, say on the output of one stage, and only the alternating signal, audio frequency, radio frequency or whatever is required. If the DC components of the signal at the output of the first stage were present at the input of the second, then the bias and other operating conditions of the second stage would be altered.
Transistor circuit with input and output coupling capacitors
Even when using operational amplifiers where the circuit has been designed to provide small offset voltages, it is often wise to use coupling capacitors because of the high levels of DC gain present. Without a coupling capacitor, the high levels of DC gain could mean that the operational amplifier would run into saturation.
For capacitor applications of this nature it is necessary to ensure that the impedance of the capacitor is sufficiently low. Typically the value of the capacitor is chosen to be the same as the impedance of the circuit, normally the input impedance of the second circuit. This gives a drop in response of 3dB at this frequency.
IMPORTANT PARAMETERS FOR COUPLING CAPACITOR USES
PARAMETER
NOTES ON CAPACITOR USE
Capacitor rated voltage
Must be greater than the peak voltage across the capacitor. Normally the capacitor will be able to withstand the supply rail voltage with margin in hand to ensure reliability.
Capacitance value
High enough to pass lowest frequencies with little or no attenuation.
Tolerance
Wide tolerance capacitors can often be used because the exact value is not important.
Dielectric
Some capacitors, for example electrolytic capacitors have a limited frequency response. This should be taken into account. Also for high impedance applications, electrolytic capacitors should not be used as they have a relatively high level of leakage which may offset the operation of the second stage.
Decoupling capacitor use
In this application, the capacitor is used to remove any AC signals that may be on a DC bias point, power rail, or other node that needs to be free of a particular varying signal.
As the name of this capacitor use indicates, it used to decouple the node from the varying signal on it.
Transistor circuit with line and collector decoupling capacitors
In this circuit there are two ways in which the capacitor is used for decoupling. C3 is used to decouple any signal that may be on the voltage rail. Also the combination C4, R5 is used to ensure that the collector signal does not leak through on the signal rail. The time constant of C4 and R5 is generally the dominating factor and the time constant should be chosen to be longer than the lowest frequency anticipated.
IMPORTANT PARAMETERS FOR DECOUPLING CAPACITOR USES
PARAMETER
NOTES ON CAPACITOR USE
Capacitor rated voltage
Must be greater than the peak voltage across the capacitor. Normally the capacitor will be able to withstand the voltage of the node with some margin in hand to ensure reliability.
Capacitance value
High enough to pass lowest frequencies with little or no attenuation.
Tolerance
Wide tolerance capacitors can often be used because the exact value is not important.
Dielectric
Some capacitors like electrolytic capacitors have a relatively low upper frequency limit. Often to overcome this, a capacitor such as a ceramic capacitor with a smaller value may be used to provide the high frequency response, while a larger value electrolytic capacitor is used to pass the lower frequency components. The lower value ceramic or other capacitor still presents a low impedance at the higher frequencies because the reactive impedance is inversely proportional to the frequency.
RF coupling and decoupling applications
When using capacitors for RF applications, it is necessary to consider their RF performance. This can be different to the performance at lower frequencies. Performance issues like self-resonance, low tangent and the like become particularly important. At microwave frequencies the issue can be of great importance.
Many surface mount ceramic capacitors offer very good performance levels and these are often used.
Smoothing capacitor applications
This is effectively the same as a decoupling capacitor, but the term is normally used in conjunction with a power supply.
When an incoming line signal is taken through a transformer and a rectifier, the resulting waveform is not smooth. It varies between zero and the peak voltage. If applied to a circuit, this is most unlikely to operate as a DC voltage is normally needed. To overcome this, a capacitor is used to decouple or smooth the output voltage.
Rectifier circuit with smoothing capacitor
In this use, the capacitor charges up when the peak voltage exceeds that of the output voltage, and supplies charge when the rectifier voltage falls below the capacitor voltage.
In this capacitor use, the component decouples the rail and supplies charge where needed.
IMPORTANT PARAMETERS FOR SMOOTHING CAPACITOR USES
PARAMETER
NOTES ON CAPACITOR USE
Capacitor rated voltage
Must be greater than the peak voltage across the capacitor. The capacitor must be able to withstand the maximum peak rail voltage with some margin in hand to ensure reliability.
Capacitance value
Dependent upon the current required, but typically can be several thousand microfarads.
Tolerance
Wide tolerance capacitors can often be used because the exact value is not important.
Dielectric
Electrolytic capacitors are typically used because of the high values available.
Ripple current
In addition to the capacitor having sufficient capacitance to hold the required amount of charge, t must also be constructed in a way to be able to supply the current required. If the capacitor becomes too hot when delivering the current it may be damaged and fail. Ripple current ratings are particularly important on capacitors used for smoothing applications.
Capacitor use as a timing element
In this application a capacitor can be used with a resistor or inductor in a resonant or time dependent circuit. In this function the capacitor may appear in a filter, oscillator tuned circuit, or in a timing element for a circuit such as an a-stable, the time it takes to charge and discharge determining the operation of the circuit
LC or RC oscillators and filters are widely used in a host of circuits, and obviously one of the major elements is the capacitor.
In this particular capacitor use, one of the main requirements is for accuracy, and therefore the initial tolerance is important to ensure that the circuit operates on the required frequency. Temperature stability is also important to ensure that the performance of the circuit remains the same over the required temperature range.
IMPORTANT PARAMETERS FOR TIMING CAPACITOR USES
PARAMETER
NOTES ON CAPACITOR USE
Capacitor rated voltage
The actual peak voltage across the capacitor will vary according to the particular circuit and the rail voltage. It is necessary to assess each case on its own merits, noting that in some cases it may be higher than expected. In most cases it is unlikely to exceed the rail voltage.
Capacitance value
Dependent upon the frequencies used and the inductor or resistor needed to obtain the required operating frequency..
Tolerance
Close tolerance normally needed to ensure that the required operating frequency is obtained. In this application, capacitors with a good selection of values within each decade may be an advantage.
Dielectric
In many timing applications, the capacitor loss is important. High loss equates to low Q, and Q values should normally be as high as possible.
Temperature stability
The temperature stability of the capacitor should be high for these capacitor applications because the circuit will need to retain its frequency over the operating temperature range. If the value changes with temperature, even by a small amount, this can have a marked effect on the operation of the circuit.
Hold-up capacitor applications
In this particular capacitor application, the charge held by the capacitor is used to provide power for a circuit for a short while.
IMPORTANT PARAMETERS FOR HOLD-UP CAPACITOR USES
PARAMETER
NOTES ON CAPACITOR USE
Capacitor rated voltage
Must be able to withstand the maximum operating voltage with a good margin for reliability.
Capacitance value
Can be up to several Farads.
Tolerance
Super capacitors widely used for this capacitor application have a wide tolerance. Fortunately this is not an issue as it primarily affects the time the hold-up can be maintained.
Super capacitors are often used for battery hold up applications
Capacitor application choices
In addition to the function within a circuit, there is also the frequency of operation that is of importance. Some capacitors operate better at low frequencies, whereas others are better at high or radio frequencies.
Capacitors are used in virtually every electronics circuit that is built today. Capacitors are manufactured in their millions each day, but there are several different capacitor types that are available.
Each type of capacitor has its own advantages and disadvantages can be used in different applications.
Accordingly it is necessary to know a little about each capacitor type so that the correct one can be chosen for any given use or application.
Capacitor types
There are very many different capacitor types that can be bought and used in electronics circuits.
While the list below gives some of the major capacitor types, not all can be listed and described and there are some less well used or less common types that can be seen. However it does include most of the major capacitor types.
Ceramic capacitor: As the name indicates, this type of capacitor gains its name from the fact that it uses a ceramic dielectric. This gives the many properties including a low loss factor, and a reasonable level of stability, but this depends upon the exact type of ceramic used. Ceramic dielectrics do not give as high a level of capacitance per unit volume as some types of capacitor and as a result ceramic capacitors typically range in value from a few picofarads up to values around 0.1 µF.
For leaded components, disc ceramic capacitors are widely used. This type of ceramic capacitor is extensively for applications like decoupling and coupling applications. More highly specified capacitors, especially used in surface mount types of capacitor often have specific types of ceramic dielectric specified. The more commonly seen types include:
COG: Normally used for low values of capacitance. It has a low dielectric constant, but gives a high level of stability.
X7R: Used for higher capacitance levels as it has a much higher dielectric constant than COG, but a lower stability.
Z5U: Used for even higher values of capacitance, but has a lower stability than either COG or X7R.
Electrolytic capacitor: This type of capacitor is the most popular leaded type for values greater than about 1 microfarad, having the one of the highest levels of capacitance for a given volume. This type of capacitor is constructed using two thin films of aluminium foil, one layer being covered with an oxide layer as an insulator. An electrolyte-soaked paper sheet is placed between them and then the two plates are wound around on one another and then placed into a can.
Electrolytic capacitors are polarised, i.e. they can only be placed one way round in the circuit. If they are connected incorrectly they can be damaged, and in some extreme instances they can explode. Care should also be taken not to exceed the rated working voltage. Normally they should be operated well below this value.
This capacitor type has a wide tolerance. Typically the value of the component may be stated with a tolerance of -50% +100%. Despite this they are widely used in audio applications as coupling capacitors, and in smoothing applications for power supplies. They do not operate well at high frequencies and are typically not used for frequencies above 50 - 100 kHz. . . . . Read more about electrolytic capacitors.
Plastic film capacitors: There are two main formats for the construction of plastic film capacitors:
Metallised film: In this type of film capacitor the plastic film has a very thin layer of metallisation deposited into the film. This metallisation is connected to the relevant connection on one side of the capacitor or the other.
Film foil: This form of film capacitor has two metal foil electrodes that are separated by the plastic film. The terminals are connected to the end-faces of the electrodes by means of welding or soldering.
Plastic film capacitors can use a variety of dielectrics. Polycarbonate, polyester and polystyrene are some of the most common. Each has its own properties, allowing them to be used in specific applications. Their values may range anywhere from several picofarads to a few microfarads dependent upon the actual type. Polyester film capacitorNormally they are non-polar. In general they are good general-purpose capacitors that may be used for a variety of purposes, although their high frequency performance is not usually as good as that of the ceramic types. Some of the more common types include:
Mylar - Can introduce noise when used in applications where there is vibration.
Polycarbonate - Moderate level of loss which can increase with frequency. Very high insulation resistance.
Polyester - Moderate level of loss which can increase with frequency. Very high insulation resistance.
Polystyrene - tend to be very low loss but bulky. Have a temperature coefficient of around -150 ppm / C
Tantalum: Ordinary aluminium electrolytic capacitors are rather large for many uses. In applications where size is of importance tantalum capacitors may be used. These are much smaller than the aluminium electrolytics and instead of using a film of oxide on aluminium they us a film of oxide on tantalum. They do not normally have high working voltages, 35V is normally the maximum, and some even have values of only a volt or so.
Leaded tantalum capacitorLike electrolytic capacitors, tantalums are also polarised and they are very intolerant of being reverse biased, often exploding when placed under stress. However their small size makes them very attractive for many applications. . . . . Read more about tantalum capacitors.
Silver Mica: Silver mica capacitors are manufactured by plating silver electrodes directly on to the mica film dielectric. To achieve the required capacitance, several layers are used. Wires for the connections are added and then the whole assembly is encapsulated. The values of silver mica capacitors range in value from a few picofarads up to two or three thousand picofarads. Silver mica capacitorThis type of capacitor is not as widely used these days. However they can still be obtained and are used where stability of value is of the utmost importance and where low loss is required. In view of this one of their major uses is within the tuned elements of circuits like oscillators, or within filters. . . . . Read more about silver mica capacitors.
Supercap Super capacitors with capacitance levels of a Farad or more are now becoming more commonplace. These super capacitors are generally used for applications like memory hold up and the like. Super capacitor or supercapThey are too large for use in most circuits and their frequency response is limited, but they make ideal hold up capacitors, being able to provide residual current and voltage to retain memory for periods when power may be removed. . . . . Read more about super capacitors.
Capacitor types overview
It can be seen from even the selection of the most commonly used types of capacitor, that many forms are available. Each has its own advantages and disadvantages, and if the right one is chosen for each job, then it can perform very well in a circuit. It is for this reason when building circuits that it is important to use the right type of capacitor. If the wrong sort is used, then its performance many not be to the standard needed for the circuit.
The electrolytic capacitor is one of the mainstays of the capacitor industry.
The electrolytic capacitor is the most popular leaded type for values greater than about 1 µF, having the one of the highest levels of capacitance for a given volume.
Aluminium electrolytic capacitors have been in use for many years - in this way they have become a regular component in many designs.
Electrolytic capacitors are widely used as leaded components, often being found in applications from power supplies to audio where leaded devices can be used. Initially aluminium electrolytic capacitors were not popular in surface mount technology format because of the levels of heat experienced during soldering could damage them. Now with more development, surface mount electrolytic capacitors are widely used and provide good levels of reliability.
Electrolytic capacitor early development
The electrolytic capacitor has been in use for many years. Its early development and history can be traced back to the very early days or radio around the time when the first broadcasts of entertainment were being made. At the time, valve wireless sets were very expensive, and they had to run from batteries. However with the development of the indirectly heated valve or vacuum tube it became possible to use AC mains power.
While it was fine for the heaters to run from an AC supply, the anode supply needed to be rectified and smoothed to prevent mains hum appearing on the audio. In order to be able to use a capacitor that was not too large Julius Lilienfield who was heavily involved in developing wireless sets for domestic use was able to develop the electrolytic capacitor, allowing a component with sufficiently high capacitance but reasonable size to be used in the wireless sets of the day.
Electrolytic capacitor symbols
The electrolytic capacitor is a form of polarised capacitor. The electrolytic circuit symbol indicates the polarity as it is essential to ensure that the capacitor is fitted into the circuit correctly and is not reverse biased.
Circuit symbol variants used for electrolytic capacitors
There is a variety of schematic symbols used for electrolytic capacitors. The first one '1' is the version that tends to be used in European circuit diagrams, while '2' is used in many US schematics, and '3' may be seen on some older schematics. Some schematic diagrams do not print the "+" adjacent to the symbol where it is already obvious which plate is which.
Electrolytic capacitor construction
This type of capacitor is constructed using two thin films of aluminium foil, one layer being covered with an oxide layer as an insulator. The use of the aluminium foil give rise to the fact that the capacitor is often referred to as the aluminium electrolytic capacitor.
An electrolyte-soaked paper sheet is placed between them and then the two plates are wound around on one another and then placed into a can.
Selection of leaded aluminium electrolytic capacitors
In the manufacture of the aluminium electrolytic capacitor, one of the first stages is to etch the foils to make them rougher to increase the surface area and hence the capacitance level that can be gained in a given area.
The next process is to form the anode. This entails chemically growing a thin layer of aluminium oxide, Al2O3onto the anode foil making it different from the cathode.
The capacitor element itself is wound on a winding machine. The four separate layers: the formed anode foil; paper separator, cathode foil; and paper separator are all brought in and wound together. The paper separators prevent the two electrodes from touching and shorting.
Electrolytic capacitor construction
When the assembly has been wound, it is taped to prevent unwinding.
Once the capacitor is wound, it is impregnated with the electrolyte. This may be done by immersion and under pressure.
The electrolyte used in aluminium electrolytic capacitors is a formulation developed to provide the required properties for the capacitor - voltage rating, operating temperature range and the like. It primarily consists of solvent and a salt (required to provide the electrical conduction). Common solvents include ethylene glycol, and common salt include ammonium borate and other ammonium salts.
Once this is complete the capacitor is placed into a can which is sealed to prevent evaporation of the electrolyte.
Electrolytic capacitor properties
Aluminium electrolytic capacitors provide a much higher level of capacitance for a given volume than most ceramic capacitors. This means that high value electrolytic capacitors can be relatively small. This is a significant advantage in many instances.
Markings on aluminium electrolytic capacitor>
Electrolytic capacitors are polarised, i.e. they can only be placed one way round in the circuit. If they are connected incorrectly they can be damaged, and in some extreme instances they can explode. Care should also be taken not to exceed the rated working voltage. Normally they should be operated well below this value.
The electrolytic capacitor has a wide tolerance. Typically the value of the component may be stated with a tolerance of -50% +100%. Despite this they are widely used in audio applications as coupling capacitors, and in smoothing applications for power supplies. They do not operate well at high frequencies and are typically not used for frequencies above 50 - 100 kHz.
Leaded electrolytic capacitor showing markings
Electrolytic capacitor electrical parameters
There are a number of parameters of importance beyond the basic capacitance and capacitive reactance when using electrolytic capacitors. When designing circuits using electrolytic capacitors it is necessary to take these additional parameters into consideration for some designs, and to be aware of them when using electrolytic capacitors.
Tolerance: Electrolytic capacitors have a very wide tolerance. Often capacitors may be quoted as -20% and +80%. This is not normally a problem in applications such as decoupling or power supply smoothing, etc. However they should not be used in circuits where the exact value is of importance.
ESR Equivalent series resistance: Electrolytic capacitors are often used in circuits where current levels are relatively high. Also under some circumstances and current sourced from them needs to have a low source impedance, for example when the capacitor is being used in a power supply circuit as a reservoir capacitor. Under these conditions it is necessary to consult the manufacturers datasheets to discover whether the electrolytic capacitor chosen will meet the requirements for the circuit. If the ESR is high, then it will not be able to deliver the required amount of current in the circuit, without a voltage drop resulting from the ESR which will be seen as a source resistance.
Frequency response: One of the problems with electrolytic capacitors is that they have a limited frequency response. It is found that their ESR rises with frequency and this generally limits their use to frequencies below about 100 kHz. This is particularly true for large capacitors, and even the smaller electrolytic capacitors should not be relied upon at high frequencies. To gain exact details it is necessary to consult the manufacturers data for a given part.
Leakage: Although electrolytic capacitors have much higher levels of capacitance for a given volume than most other capacitor technologies, they can also have a higher level of leakage. This is not a problem for most applications, such as when they are used in power supplies. However under some circumstances they are not suitable. For example they should not be used around the input circuitry of an operational amplifier. Here even a small amount of leakage can cause problems because of the high input impedance levels of the op-amp. It is also worth noting that the levels of leakage are considerably higher in the reverse direction.
Ripple current: When using electrolytic capacitors in high current applications such as the reservoir capacitor of a power supply, it is necessary to consider the ripple current it is likely to experience. Capacitors have a maximum ripple current they can supply. Above this they can become too hot which will reduce their life. In extreme cases it can cause the capacitor to fail. Accordingly it is necessary to calculate the expected ripple current and check that it is within the manufacturers maximum ratings.
Electrolytic SMD capacitors
Electrolytic capacitors are now being used increasingly in designs that are manufactured using surface mount technology, SMT. Their very high levels of capacitance combined with their low cost make them particularly useful in many areas. Originally they were not used in particularly high quantities because they were not able to withstand some of the soldering processes. Now improved capacitor design along with the use of reflow techniques instead of wave soldering enables electrolytic capacitors to be used more widely in surface mount format.
Often surface mount device, SMD versions of electrolytic capacitors are marked with the value and working voltage. There are two basic methods used. One is to include their value in microfarads (µF), and another is to use a code. Using the first method a marking of 33 6V would indicate a 33 µF capacitor with a working voltage of 6 volts. An alternative code system employs a letter followed by three figures. The letter indicates the working voltage as defined in the table below and the three figures indicate the capacitance on picofarads. As with many other marking systems the first two figures give the significant figures and the third, the multiplier. In this case a marking of G106 would indicate a working voltage of 4 volts and a capacitance of 10 times 10^6 picofarads. This works out to be 10µF
SMD ELECTROLYTIC CAPACITOR VOLTAGE CODES
LETTER
VOLTAGE
e
2.5
G
4
J
6.3
A
10
C
16
D
20
E
25
V
35
H
50
Electrolytic capacitor markings
There is a variety of different markings that are used for electrolytic capacitors to indicate their value, working voltage and possibly other parameters. Often the values are written directly on the can as there is space, but factors like tolerance, and sometimes working voltage may be coded.
The coding or marking system used will depend upon the type of capacitor, whether it is leaded or SMD and also the manufacturer, magnitude of the value, size of the component, etc..
Aluminium electrolytic capacitor lifetime
Aluminium electrolytic capacitors do degrade with time. Many electrolytics have a vent for allowing excess gasses to escape. This escape can result in the electrolyte drying out and the performance of the capacitor falling.
Also if aluminium electrolytic capacitors are left for a few years, then the oxide layer on the anode can dissipate. When this happens the capacitor needs to be repolarised. This can be done by applying a current limited voltage to the capacitor. Initially the leakage current across the capacitor will be relatively high and then it will fall as the oxide layer forms.
It is also wise to take precautions to prolong the life of the capacitor. There are four golden tips to maximise the life of an aluminium electrolytic capacitor:
Run within its voltage limits: It is always wise to run any component with a good margin below the maximum ratings. Many companies state in their design rules that for electrolytic capacitors, they should only be run at about 50% of their maximum ratings to ensure optimum reliability. If the maximum limits are exceeded then leakage current levels will rise and there is the possibility of localised breakdown leading to an explosive failure of the component.
Keep within its current rating: In many applications an electrolytic capacitor will be required to provide high levels of ripple current. This is to be expected in applications like being used as a smoothing capacitor in a power supply. Ii is imperative to ensure that the capacitor can withstand the current being required from it. Check that the capacitor is operating within its current limits and is not becoming too warm in operation.
Never reverse bias the capacitor: When run under a reverse bias, the leakage levels will be very much higher than in the forward direction. Again this can lead to catastrophic breakdown and failure.
Keep temperatures down: Heat shortens the life of any aluminium electrolytic capacitor. A good rule of thumb is that every 10°C over 85°C will halve the life expectancy of the component.
Even though aluminium electrolytic capacitors have a life expectancy, this can be raised towards its maximum if these rules are followed and it is operated well within its ratings.
Reforming aluminium electrolytic capacitors
It may be necessary to re-form electrolytic capacitors that have not been used for six months or more. The electrolytic action tends to remove the oxide layer from the anode and this needs to be re-formed. Under these circumstances it is not wise to apply the full voltage as the leakage current will be high and may lead to large amounts of heat being dissipated in the capacitor which can in some instances bring about its destruction.
To reform the capacitor, the normal method is to apply the working voltage for the capacitor through a resistor of around 1.5 k ohms, or possibly less for lower voltage capacitors. (NB ensure that it has sufficient power rating to handle the capacitor in question). This should be applied for an hour or more until the leakage current drops to an acceptable value and the voltage directly on the capacitor reaches the applied value, i.e. virtually no current is flowing through the resistor. This voltage should then be continued to be applied for a further hour. The capacitor can then be slowly discharged through a suitable resistor so that the retained charge does not cause damage. Once reformed take care when using the capacitor to make sure that it has been fully reformed and is able to function correctly.
The ceramic capacitor gains its name from the fact that it uses ceramic materials for its dielectric.
Although the ceramic capacitor has been used for many years as a leaded device, it is a surface mount device where its properties enable the very small capacitor sizes to be achieved while still retaining high levels of performance. As a result, countless billions of these ceramic capacitors are used each year.
Ceramic capacitor basics
The ceramic dielectric used in these capacitors gives the many properties including a low loss factor, and a reasonable level of stability, but this depends upon the exact type of ceramic used.
Ceramic dielectrics do not give as high a level of capacitance per unit volume as some types of capacitor and as a result ceramic capacitors typically range in value from a few picofarads up to values around 0.1 µF.
Leaded ceramic capacitor selection
For leaded components, disc ceramic capacitors are widely used. This type of ceramic capacitor is extensively for applications like decoupling and coupling applications. More highly specified capacitors, especially used in surface mount types of capacitor often have specific types of ceramic dielectric specified.
Ceramic capacitor types
There are many different types of ceramic that can be used as the dielectric in ceramic capacitors.
The more commonly seen types include:
COG: Normally used for low values of capacitance. It has a low dielectric constant, but gives a high level of stability.
X7R: Used for higher capacitance levels as it has a much higher dielectric constant than COG, but a lower stability.
Z5U: Used for even higher values of capacitance, but has a lower stability than either COG or X7R.
Whilst these types are commonly seen, very many other types of ceramic dielectric are available. To help with their use, different classes have been developed to help simplify the situation.
CERAMIC CAPACITOR DIELECTRIC SUMMARY
CLASS
DESCRIPTION
COMMON TYPES
Class 1
These ceramic capacitors offer a high level of stability and exhibit low loss levels and they are ideal for use in resonant circuits.
NP0, P100, N33, N75, etc.
Class 2
Class 2 ceramic capacitors offer high volumetric efficiency, i.e. large capacitance for a given volume for smoothing, by-pass, coupling and decoupling applications.
X7R, X5R, Y5V, Z5U, etc.
Class 3
Class 3 ceramic capacitors offer higher volumetric efficiency than the class 2 ceramic capacitors, but their temperature stability is not nearly so good. A typical performance for the change of capacitance with temperature is −22% to +56% over a range of 10°C to 55°C.
Only available as leaded components. No longer standardised.
These ceramic capacitor classes are standardised by international bodies including the IEC, International Electrotechnical Commission and the EIA, Electronic Industries Alliance.
A vast number of ceramic chip capacitors are used everyday. They are used on most surface mount boards in everything from televisions to mobile phones and heating timers to automotive systems.
The surface mount capacitor uses what is called a multilayer chip capacitor, MLCC construction. Effectively, the MLCC consists of a number of individual capacitors that are stacked together in parallel with the overall contact being made via the component terminal surfaces.
SMD ceramic capacitor selection
Ceramic capacitor summary
The table below gives a summary of some of the salient features of ceramic capacitors.
CERAMIC CAPACITOR SUMMARY
PARAMETER
DETAILS
Typical capacitance ranges
10 pF to 0.1µF (100nF)
Rated voltage availability
From around 2V upwards - some specialised ones can have voltages of 1kV and more.
Advantages
Cheap to manufacture
Good high frequency performance
Good stability dependent upon actual ceramic dielectric
Available in both leaded and SMD packages
Disadvantages
Cannot reach high capacitance levels of polarised types
Tantalum capacitors enable very high levels of capacitance to be provided within small packages.
Although they do not have the current capacity and they are not as electrically robust as electrolytic capacitors, their size and performance mean that they are widely used in many applications.
Tantalum capacitors are also widely used in their surface mount formats because they are much cheaper than their aluminium electrolytic relations and they can withstand the soldering process better.
Instead of using a film of oxide on aluminium they us a film of oxide on tantalum. They do not normally have high working voltages, 35V is normally the maximum, and some even have values of only a volt or so.
Like electrolytic capacitors, tantalums are also polarised and they are very intolerant of being reverse biased, often exploding when placed under stress. However their small size makes them very attractive for many applications.
Leaded tantalum capacitors
Tantalum basics
Tantalum capacitors are a specific form of electrolytic capacitor. Unlike the more familiar aluminium electrolytic capacitor, tantalum ones are much smaller and offer a very high level of capacitance for a given volume and weight. They also posses a low ESR (equivalent series resistance) than aluminium electrolytics along with a higher operating temperature capability and lower leakage
The tantalum capacitor consists of a small pellet of tantalum which acts as the anode for the capacitor. This is covered by a layer of oxide which acts as the dielectric for the capacitor and in turn this is surrounded by a conductive cathode. The use of tantalum in the capacitor allows for a very thin oxide layer to be used.
The thin oxide layer means that much higher capacitance levels can be achieved than if some other type of dielectric was used., and it also offers excellent stability over time.
Tantalum capacitor failure modes
One of the disadvantages of having a very thin oxide layer as the dielectric is that it is not particularly robust. As a result, care has to be taken when using tantalum capacitors.
Tantalum capacitors are reliable provided they are operated within their specification limits. Many reliability standards recommend operating them at a maximum of 50% or 60% of their rated working voltage to give a good margin. If this is done then they operate reliably and provide good service.
Tantalum capacitors are not tolerant of abuse. If they are reverse biased or their working voltage is exceeded ten they can fail in a dramatic way. At best they can emit a little smoke, but they can also fail explosively as well.
Care must be taken to ensure this does not happen as it can lead to equipment failure or even fires in some instances.
Leaded tantalum capacitors
Leaded tantalum capacitors typically come in a small and encapsulated in epoxy package to prevent damage.
In view of their shape they are sometimes referred to as tantalum bead capacitors.
Leaded tantalum capacitor
The capacitor markings are normally written directly onto the encapsulation as figures, although a colour coding system was popular at one time and some capacitors may still be seen using this system.
Leaded tantalum capacitor markings
SMD tantalum capacitors
Surface mount tantalum capacitors are widely used in modern electronics equipment. When designed with sufficient margins they provide reliable service and enable high values of capacitance to be obtained within the small package sizes needed for modern equipment.
Selection of SMD tantalum capacitors
Aluminium electrolytics were not initially available in surface mount packages as they were not able to withstand the temperatures needed in soldering. As a result tantalum capacitors which were able to withstand the soldering process were almost the only choice for high value capacitors in assemblies using surface mount technology. Now that SMD electrolytics are available, tantalum is still the capacitor of choice for SMD as they offer an excellent cost, size and performance parameters.
SMD tantalum capacitors come in a variety of sizes. Typically they conform to standard sizes defined by the EIA, Electronic Industries Alliance.
SURFACE MOUNT TANTALUM CAPACITOR SIZES
PACKAGE DESIGNATION
SIZE (MM)
EIA DESIGNATION
Size A
3.2 x 1.6 x 1.6
EIA 3216-18
Size B
3.5 x 2.8 x 1.9
EIA 3528-21
Size C
6.0 x 3.2 x 2.2
EIA 6032-28
Size D
7.3 x 4.3 x 2.4
EIA 7343-31
Size D
7.3 x 4.3 x 4.1
EIA 7343-43
SMD tantalum capacitor markings
The markings on SMD tantalum capacitors normally consist of three numbers. The first two form the significant figures, and the third is the multiplier. Values are in picofarads. Therefore the SMD tantalum capacitor shown below has a value of 47 x 105pF, which works out to be 4.7µF.
SMD tantalum capacitor markings
Sometimes values will be marked more directly as shown in the example below. The value is obvious from the markings.
SMD tantalum capacitor markings
Tantalum capacitor summary
The table below provides some of the salient features about tantalum capacitors to take into consideration when designing circuits or replacing old components.
TANTALUM CAPACITOR SUMMARY
PARAMETER
DETAILS
Typical capacitance ranges
1µF to 100µF
Rated voltage availability
From around 1.5V to 20V.
Advantages
High volumetric capacitance
High capacitance per unit weight
Reliable when used within electrical limits
Cost effective
Available in SMD format
Wide operating temperature range
Disadvantages
Must be used within its electrical limits otherwise it can fail catastrophically.
High voltages and very high capacitance levels not normally available.
Does not like voltage above limits
Does not like reverse bias
Low ripple current rating
Plastic film capacitors come in several forms. Essentially they use a metallized plastic film to enable the electrodes to the created and spaced apart from one another.
The different types of metallized plastic film capacitor provide different properties, each suited to slightly different applications.
The values of metallized plastic film capacitors may range anywhere from several picofarads to a few microfarads dependent upon the actual type. Normally they are non-polar. In general they are good general-purpose capacitors that may be used for a variety of purposes, although their high frequency performance is not usually as good as that of the ceramic types.
Film capacitor basics
Film capacitors are known under a variety of different names including, plastic film capacitors, film dielectric capacitors, or polymer film capacitors, and metallised film capacitors.
The basic concept of this type of capacitor is the insulating plastic film as the dielectric. This may sometimes be combined with paper as carrier of the electrodes.
The dielectric films are drawn to an extremely thin film - hence the name. This is then used as the dielectric material between the two electrodes or plates.
The dielectric and the electrode films or plates are wound together into a cylinder, or several layers are placed together and the external connections added.
A variety of different plastic films may be used. Each type has its own properties as detailed in the section below on dielectrics.
Polypropylene film capacitor
Film capacitor construction
There are two main formats for the construction of film capacitors. The actual construction type depends upon the dielectric material used and the requirements for the physical construction.
Film foil: This form of film capacitor has two metal foil electrodes that are separated by the plastic film. The terminals are typically connected to the end-faces of the electrodes by means of welding or soldering.
Metallised film: In this type of film capacitor the plastic film has a very thin layer of metallisation deposited onto the film. The thin metal layer is typically only 0.02 to 0.1µm thick. This is vacuum deposited onto the plastic film. My leaving a small area free of metallisation at either end, it is possible to connect the metallisation to one connection or the other of the film capacitor.
Some special types of construction may be used for film capacitors that are required for niche or specialist applications. These are not widely seen.
Self healing
One aspect of film capacitors is that of self-healing. The self healing or clearing occurs when defects caused by small pinholes, flaws in the film, or external voltage transients. Any arcing vapourises the thin metallisation of the film around the failure thereby removing the metallisation in the area of the flaw and removing any conductive material in the area. If there is no conductive material, the capacitor cannot short across between the plates and failure is removed.
Film capacitor dielectrics
Many of the dielectric are known by their common names and they also have abbreviations.
As there is a wide variety of different dielectrics for the different types of film capacitor, a summary of the different types is given below.
Polyester / Mylar PET: This type of plastic film capacitor is manufactured in both metallised wound versions and the film foil types. These film capacitors are low cost types and relatively small for their capacitance. They are generally used for general purpose electronic applications.. Their maximum temperature rating of 125°C enables them to be manufactured as surface mount components although in this format they are not nearly as widely used as ceramic types that have almost cornered them market for values below 1µF. Polyester film capacitorOne downside of this type of film capacitor is that it can introduce noise when used in applications where there is vibration. . . . . . Read more about polyester capacitors
Polycarbonate, PC: Although a very successful and useful form of film capacitor, this type is no longer made because the manufacturer of the film ceased production of it around the year 2000. The nearest replacement type is generally considered to be the PP, polypropylene dielectric.
This type of film capacitor was manufactured as wound types as well as film / foil. The capacitors have a low dissipation factor and they are relatively temperature stable, often as low as ±80ppm over the entire temperature range. As a result they are often used in timing circuits, filters and other precision analogue applications.
This type of film capacitor introduces moderate level of loss which can increase with frequency. It has a very high insulation resistance. . . . . . Read more about polycarbonate capacitors
Polystyrene: These film capacitors are also sold under the trade name Styroflex. They used to be known as being cheap general purpose capacitors with high stability and low dissipation and leakage.
The films could not be made any thinner than 10µm and this limited the capacitance levels that could be achieved. The temperature ratings were also low with a maximum of 85°C operating temperature. As a result they are no longer widely used, normally being replaced by polyester types. Polystyrene film capacitorThese film capacitors tend to be very low loss but bulky. They also have a temperature coefficient of around -150 ppm / °C. . . . . . Read more about polystyrene capacitors
Polythene Sulphide, PPS: These film capacitors are only manufactured as the metallised film types. They offer a very low temperature variation over the temperature range, typically ± 1.5%. The dissipation factor is quite small as the frequency dependence.
These film capacitors are well suited to applications where frequency stability is paramount and in applications where high temperatures may be encountered. They may also be found in surface mount formats, although costs tend to be much higher than the ceramic counterparts.
Polyethylene naphthalate, PEN: These film capacitors are only produced as metallised film types. They belong to the polyester family but provide a better high temperature resilience and performance. As a result they are more suited to SMD applications where the soldering process takes temperatures much higher than they do for leaded versions.
The temperature and frequency dependence figures for PEN capacitors are very similar to those of the PET polyester versions. However as a result of the smaller relative permittivity, the physical capacitor size is larger for a given capacitance.
A special high voltage PEN dielectric is available and this is ideal for many high voltage and high temperature applications.
Overall PEN film capacitors are used for non-critical filtering coupling and decoupling in electronic circuits where temperature dependencies are not important.
Polytetrafluoroethylene, PTFE: PTFE film capacitors are manufactured in both metallised film and film / foil variants. One of the key features of this form of film capacitor is its very high temperature resistance - it can withstand temperatures of 200°C and above. There are difficulties in manufacturing the PTFE film to a sufficiently tight tolerance. As a result these capacitors tend to be expensive, and they are manufactured by a limited number of manufacturers. In view of this the capacitors tend to be reserved for specialist applications.
These are the main types of film capacitor that will be seen in most applications. There are other types, but these are comparatively rarely seen.
Metal film capacitor summary
The table below provides some of the salient features about some of the more widely used metallised plastic film capacitors that can be taken into consideration when designing circuits or replacing old components.
METAL FILM CAPACITOR SUMMARY
PARAMETER
PET
PEN
PPS
PP
Relative permittivity @ 1 kHz
3.3
3.0
3.0
2.2
Minimum film thickness (µm)
0.75
0.1
1.2
2
Moisture absorption (%)
Low
0.4
0.05
<0.1
Dielectric strength (V/µm)
580
~500
470
650
Capacitor DC voltage range (V)
50 - 100
10 - 250
10 - 100
40 - 2000
Capacitance range
100pf – 22µF
100pF – 1µF
100pF – 0.47µF
100pF – 10 µF
Dissipation factor a 1 kHz
50 - 200
40 - 80
2 - 15
0.5 - 5
Dissipation factor a 10 kHz
100 - 150
50 - 150
2.5 - 25
2 - 10
Dissipation factor a 100 kHz
170 - 300
120 - 300
10 -6-
2 -25
Dissipation factor a 1 MHz
200 - 350
20 -70
5 - 40
There are many different types of metallized film and plastic film capacitors that are available. Even though they use a simialr form of technology, the variation of dielectrics means that they have a variety of performance parameters and careful selection of the type of dielectric is neceessary to provide the optimum performance.
Electrolytic capacitor summary
ALUMINIUM ELECTROLYTIC CAPACITOR SUMMARY
PARAMETER
DETAILS
Typical capacitance ranges
1µF to 47 000µF
Rated voltage availability
From around 2.5V upwards - some specialised ones can have voltages of 350V and more.
Advantages
High capacitance per volume compared to most other types, relatively cheap when compared to other types of similar value.
Disadvantages
High leakage currents, wide value tolerances, poor equivalent series resistance; limited lifetime.
