Buying Capacitors - what to think about

Buying capacitors for a project or circuit is not as straightforward as you may think - read our key hints and tips to help avoid the pitfalls and get the right capacitor for your job.

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Buying capacitors may not be as straightforward as it may seem at first sight. Even though the value may be correct, if you buy the wrong type of capacitor there is a good chance a circuit may not work. Buy the right type of capacitor and there will be no problem.

When setting out to buy capacitors there are several hints and tips that can be followed to help make sure that the right capacitor is bought.

Selection of leaded and surface mount capacitors.

Buying a capacitor - considerations

Some of the key selection priorities to be considered when buying a capacitor are listed below.

  • Dielectric type:   One of the first considerations to look at when buying a capacitor is the dielectric type. The dielectric governs many of the properties of the capacitor including size and package, but more importantly the electrical properties like the frequency range, series resistance and possible the inductance as well. Capacitors like electrolytic capacitors have a limited frequency range but can be obtained in large values. They can also have a low equivalent series resistance ESR. Ceramic capacitors can have a much higher frequency range but generally have much smaller values. Other types too may have the required properties.
  • Working voltage:   It is important to ensure that any capacitor that is bought and used has a sufficient working voltage. The working voltage should never be exceeded so if transients or other increases are expected, then the capacitor should be able to accommodate these. When a capacitor is chosen for any particular application, many companies who design electronic circuits suggest the normal voltage should only ever reach 50 or 60% of the stated working voltage of the capacitor. Not only does this give a good margin, but it also means that the capacitor is not unduly stressed operating to close to the maximum, and this improves the overall reliability.
  • Package:   The package is another important factor when buying capacitors. There is obviously a choice to be made between traditional leaded components and surface mount varieties. Normally the required type must be chosen for the type of assembly methods to be used. Leaded for through hole mounting and other similar applications, and surface mount for automated manufacture. Surface mount components can also be very good for RF applications (ceramic capacitors only) as they normally have much lower levels of inductance, etc.
  • Ripple current:   When buying capacitors for high current applications like the use in power supplies for smoothing, etc, it is necessary to consider the ripple current. It is not normal to consider this for capacitors used for timing, as well as for normal coupling and decoupling applications.
  • Cost:   Cost is obviously a major consideration for most applications. It is necessary to secure high quality components for the minimum cost. As a result it is worth looking around to ensure the optimum deals. For many hobby applications, some of the supplier assessments are unlikely to be applicable and many low cost options are ideal.

Buying capacitors - hints, tips and useful facts

Some hints, tips and facts that might be useful when buying capacitors:

  • Don’t use electrolytics where values are critical:   The value tolerance of electrolytic capacitors is very poor - don’t use them for any applications where the value is critical. Typically their tolerance may be of the order of +50% and -20%. This means that for any value critical applications, like timing circuits, they should not be used for the timing circuit itself. Normally they are ideal for coupling, decoupling and smoothing.
  • Check on the type of ceramic capacitor:   Ceramic capacitors can use a host of different types of ceramic and they have very different properties. This is particularly apparent for surface mount capacitors where there are many types in use. Check the performance, although normally all types are good for coupling and decoupling.
  • Don’t use electrolytics where leakage might be an issue:   Although aluminium electrolytic capacitors are very good, they need to be used in situations where leakage is not an issue. The technology used by aluminium electrolytic capacitors means that they exhibit a small level of leakage and this can be an issue when they are used in some circuits. The op-amp circuit below is a good example:
    Single supply rail non-inverting operational amplifier circuit

    Single supply rail non-inverting operational amplifier circuit.
    The electrolytic capacitor should not be used for C3 as this is a high impedance point and even the small amount of leakage of the electrolytic will cause the circuit output to run into the supply rail.
  • Don’t stress tantalums:   Tantalum capacitors are really great - huge capacitance for the volume, but they definitely do not like being subject to over-voltage, being reverse biased or passing much current. They very soon tell you if they don’t like the way you are treating them by exploding! Always run tantalums below their rated voltage. If they are used within their ratings they work very well.

Buying the right capacitor can take a little while to achieve, but there are many options that can enable many types of capacitor to be bought quite cheaply and securing fast delivery times as well.

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