Capacitor types and their uses

- an overview, information and tutorial about the different capacitor types and the uses these different types of electronic capacitors may be suited to as electronic components within electronic circuits.

Electronic capacitors are one of the most widely used electronic components. These electronic capacitors only allow alternating or changing signals to pass through them, and as a result they find applications in many different areas of electronic circuit design. There are a wide variety of types of capacitor including electrolytic, ceramic, tantalum, plastic, sliver mica, and many more. Each capacitor type has its own advantages and disadvantages can be used in different applications.

Capacitor construction

In essence the construction of an electronic capacitor is very simple, although in practice a lot of research and development has been put into capacitor technology. The basic electronics components consist of two plates that are insulated from one another. In between them there is an insulating medium known as the dielectric. The value of the electronic capacitor is dependent upon the area of the plates, the distance between them and the dielectric constant of the material or dielectric between them. The greater the area of the plates, the closer they are together and the greater the value of the dielectric constant the greater the value of capacitance.

Today, electronic capacitors are able to provide relatively high levels of capacitance within components that occupy a small volume. This is achieved in a number of ways. One is to have several sets of plates, and another is to place the plates very close to one another, having a thin layer of dielectric placed between them. In addition to this special insulating dielectric materials have been developed to enable high levels of capacitance to be achieved.

The method of construction of these electronic components is also important. In some capacitors the plates may be flat, and normally these capacitors will have rectangular, or more exactly cuboid shapes. Some will be tubular and in these capacitors the plates will be wound round on each other. The reasons for these types of construction are normally dependent upon the way in which the capacitors must be manufactured. The final stage in the construction of an electronic capacitor is to place it in a protective casing. In some instances it may be dipped in an insulating coating, in others it may be contained within a metal can.

Some capacitors types are what are termed polar or polarised. When this is the case the electronic capacitor has a positive and a negative connection and it must be placed in circuit so that the voltage across it is in a particular sense. If the voltage is incorrectly placed across the component then it may be damaged. Fortunately many capacitors, and in particular low value ones are non-polar and can be placed in circuit either way round.

Although there is a large variety that are available the most commonly used are ceramic, plastic film types, electrolytic and tantalum. These names refer to the type of dielectric that is used within the capacitor.

Ceramic

Ceramic capacitors are normally used for radio frequency and some audio applications. Ceramic capacitors range in value from figures as low as a few picofarads to around 0.1 microfarads. In view of their wide range and suitability for RF applications they are used for coupling and decoupling applications in particular. Here these ceramic capacitors are by far the most commonly used type being cheap and reliable and their loss factor is particularly low although this is dependent on the exact dielectric in use. Their stability and tolerance is not nearly as good as silver mica types, but their cost is much less.

There are a number of dielectrics that can be used with ceramic capacitors. For low values a dielectric designated "COG" is normally used. This has the lowest dielectric constant but gives the highest stability and lowest loss. Where higher values are required in a given size, a dielectric with a higher dielectric constant must be used. Types with designations X7R and for higher values, Z5U are used, however their stability and loss are not as good as the capacitors made with COG dielectric.

Silver Mica

Silver mica capacitors are not as widely used these days as they used to be. However these electronic components 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.

Values are normally in the range between a few picofarads up to two or possibly three thousand picofarads.

For this type of capacitor the silver electrodes are plated directly on to the mica dielectric. Again several layers are used to achieve the required capacitance. Wires for the connections are added and then the whole assembly is encapsulated.

Plastic film capacitors

There is a number of different types of plastic film capacitors. Polycarbonate capacitors, polyester capacitors and polystyrene capacitors are some of the most common. Each of these electronic components 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. 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.

Electrolytic capacitors

Electrolytic capacitors are the most popular type for values greater than about 1 microfarad. Electrolytic capacitors are constructed using a thin film of oxide on an aluminium foil. An electrolyte is used to make contact with the other plate. The two plates are wound around on one another and then placed into a can that is often aluminium.

Electrolytic capacitors are polarised, and care should be taken to ensure they are placed in circuit the correct way round. If they are connected incorrectly they can be damaged, and in some extreme instances they can explode. Great care should also be taken not to exceed the rated working voltage of the electrolytic capacitor. Normally they should be operated well below this value. Also in power supply applications significant amounts of current may be drawn from them. Accordingly electrolytic capacitors intended for these applications have a ripple current rating which should also not be exceeded. If it is, then the electronic component may become excessively hot and fail. It is also worth noting that these components have a limited life. It is often as little as 1000 hours at the maximum rating. This may be considerably extended if the component is run well below its maximum rating.

Electrolytic capacitors have 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.

These are normally contained in a tubular aluminium can, each end being marked to show its polarity.

Tantalum capacitors

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 electrolytic capacitors and instead of using a film of oxide on aluminium they us a film of oxide on tantalum. Tantalum capacitors 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, tantalum capacitors 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.

Table of capacitor types and capacitor uses and applications

The most suitable way to summarise the various types of capacitor and the applications for which these electronic capacitors are suited is in a table.

Application	Suitable types	Reasons
Power supply smoothing	Aluminium electrolytic	High capacity, high ripple current
Audio frequency coupling	Aluminium electrolytic< Tantalum Polyester / polycarbonate	High capacitance High capacitance, small size Cheap, but values not as high as electrolytics
RF coupling	Ceramic COG Ceramic X7R   Polystyrene	Small, cheap, low loss Small cheap, but higher loss than COG Very low loss, but larger than ceramic
RF decoupling	Ceramic COG   Ceramic X7R	Small, low loss. Values limited to around 1000 pF Small, low loss, higher values available than for COG types
Tuned circuits	Silver mica Ceramic COG	Close tolerance, low loss Close tolerance, low loss, although not as good as silver mica

Summary

There is a huge number of different capacitor types and they are one of the most widely used electronic components. While different capacitors may have the same value, each different type of capacitor has its own properties and this will make this particular electronic capacitor suitable for a particular application. If the wrong type of capacitor is used, then it can make a circuit function incorrectly. As a result, choosing an electronic capacitor for a circuit means making more than the value calculations. Choosing the correct capacitor type is equally important.