Capacitors are one of the most widely used electronics components. Passing only alternating or changing signals, capacitors are used in a variety of applications. There are a wide variety of types of capacitor including electrolytic, ceramic, tantalum, plastic, sliver mica, and many more. Each type has its own advantages and disadvantages can be used in different applications.
In essence the construction of a capacitor is very simple, although in practice a lot of research and development has been put into capacitor technology. The basic component consists 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 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 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 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 a 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 are what are termed polar or polarized. When this is the case the 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. These 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 they 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. 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 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.
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, 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. 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
This type of capacitor is the most popular type for values greater than about 1 microfarad. It is contructed 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.
These 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. Care should also be taken not to exceed the rated working voltage. Normally they should be operated well below this value. Also in power supply applications significant amounts of current may be drawn from them. Accordingly capacitors intended for these applications have a ripple current rating which should also not be exceeded. If it is, then the 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.
These 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
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.
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.
Summary
The most suitable way to summarise the various types of capacitor and the applications for which they 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 |
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