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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Capacitor Tutorial Includes:
Capacitor uses Capacitor types Electrolytic capacitor Ceramic capacitor Ceramic vs electrolytic Tantalum capacitor Film capacitors Silver mica capacitor Super capacitor Surface mount capacitors Specifications & parameters How to buy capacitors - hints & tips Capacitor codes & markings Conversion table

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.

Buying the right capacitor is not just about buying the right type of capacitor with the right value and right dielectric for the particular electronic circuit design, but it may be necessary to get them from the right electronic component distributor that can provide the required supply scheduling at the right price and from the right sources.

Selection of leaded and surface mount capacitors.

In fact there are more considerations when buying capacitors, or indeed any other forms of electronic component, than just buying the cheapest items from anywhere, although it depends upon the application and whether it is a purchase for a hobby electronic design, or for full scale production and manufacturing.

Buying a capacitor - specifications

Just as with other electronic components, with capacitors as well it is necessary to look carefully at the specifications.

There are many specifications that need to be considered, and there are some major parameters that will be important for all electronic circuits, whereas others will be particularly important for a smaller number of electronic circuit designs.

We have included some of the more important specifications to consider below:

Capacitance value: The most obvious point is the actual value of the capacitor. This will typically be measured in µF (10^-6 Farads), nF (10^-9 Farads) or pF (10^-12 Farads). That said, some super capacitors are now on the market with values around a Farad or more.

The choice of the electronic component value will be drive by the electronic circuit design, but ensuring that the value falls into the E series standard values will make the component easier to source and cheaper.

There is also benefit in trying to ensure that E-series values are used and the number of different values and hence the number of different electronic component types is reduced as this will ease production and simplify buying etc. Larger quantities of fewer electronic components are much easier to buy than smaller quantities of more types of electronic components.
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.

Tolerance: The tolerance of capacitors can vary widely dependent upon the type of capacitor. Electrolytic capacitors can typically be +80% / -20%, whereas other types can be ±1% and others can have other levels of tolerance. Ot is necessary to carefully consider what tolerance os required for the circuit to operate satisfactorily.

It's worth taking a look at what the capacitors are to be used for They can be placed into a few broad categories that will enable the choice to be made.

Usage	Details
Power supply smoothing	The precise value is not critical. Electrolytic with +80%/-20% are quite adequate
Supply line decoupling	Again the exact value is not critical. Ceramic capacitors of values around 0.1µF are often used. Tolerance levels of ±10% of even ±20% are quite acceptable. Electrolytic as above can be used for reducing lower frequencies
General inter-stage coupling	Again, values are generally not excessively tight, and tolerance levels, possibly ± 10% or better can be used. Sometimes there may be low frequency break points that might be more critical, but this can be discovered by understanding the electronic circuit design and the surrounding electronic components.
Oscillator & filters	The capacitors used as the frequency governing elements in oscillators and filters often need to be accurately specified. Often tolerances of ±1% may be used, but the exact tolerance can be assessed from an understanding of the electronic circuit design and the surrounding electronic components.

These are very broad guidelines to the tolerance that might be expected in different areas of electronic circuits. The main requirement is to understand the electronic circuit design and the electronic components around the capacitor to see what tolerance might be needed.

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.

The performance and specification parameters for the capacitor should be mathed to the particular electronic circuit design so that the optimum performance is obtained.

These and any other relevant parameters for the capacitor should be checked to ensure that the right electronic component is being selected for the particular position in the circuit.

Buying capacitors - hints, tips and useful facts

There are many considerations when buying capacitors and the electronic circuit designs where they are to be used govern many of the requirements. There are a few types of capacitor that are well suited to some electronic designs, or places within circuits, but should not be used in others.

Here are some hints, tips and facts that might be useful when buying capacitors for particular electronic circuit designs:

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.

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.

Electronic components distributor / supplier choice

Buying a capacitor is often not just about buying the cheapest component that fits the bill, although this can sometimes be the way to go, especially for hobbyist applications and some electronic circuit design laboratories where components may be needed cheaply and quickly.

Especially for commercial organisations where electronic manufacture is undertaken, there may be other issues to consider and the choice of electronic component distributor is more crucial.

Timescales: It is often necessary to consider the timescales when selecting who to buy capacitors and other electronic components from. Some electronic component distributors may be able to deliver very quickly or on a required date. and this is an advantage in many instances. Having the right product when it is required can mean that the amount of inventory held on the electronics manufacturing site can be reduced.

Also having the electronic components reliably scheduled to arrive when they are needed can reduce the difficulties of handling shortages in the manufacturing process.
Cost: Cost is a major consideration when buying electronic components. Particularly for private individuals this may be one of the most important considerations. Normally quality is good, but this needs to be taken into account especially when prices are low. Normally though quality is very good.
Wide availability: In some instances a wide availability of stock is needed and this may be a reason for selecting a particular electronic components distributor or stockist.
Ship to stock: For large scale electronics manufacturing, it is possible to set up a facility known as ship to stock. By entering relationship with an electronic components distributor, they can be aware of the requirements and when stock runs low at the production facility, then new components can be shipped directly into the production line within minimum of formality.
Traceable stock: For many professional applications it is necessary to have electronic components, and in this case capacitors that are from a guaranteed source and not counterfeit. If counterfeit stock entered the supply chain then it could have an impact on performance, reliability and a number of other factors. In addition to this, there can be penalties of stock is shown to be counterfeit, particularly when manufacturing equipment for government organisations. Using an established, authorised electronic components distributor can ensure that the stock is reliable.

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.