Capacitor Uses and Applications

- it is particularly important to select the right capacitor type for any given application - understanding the key requirements for any given capacitor application or capacitor use will ensure the circuit operates correctly.


Capacitor Tutorial Includes:

Capacitor uses     Capacitor types     Electrolytic capacitor     Ceramic capacitor     Tantalum capacitor     Film capacitors     Specifications & parameters     Conversion table    


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 type 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 use for the right capacitor is all part of the design process for a circuit. Using the wrong type of 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 capacitor types 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.

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 types, 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.

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.

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 types 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 Supercapacitors 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.

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.



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