CAPACITOR

A capacitor is a passive electronic component that stores energy in the form of an electrostatic field. In its simplest form, a capacitor consists of two conducting plates separated by an insulating material called the dielectric. The capacitance is directly proportional to the surface areas of the plates, and is inversely proportional to the separation between the plates. Capacitance also depends on the dielectric constant of the substance separating the plates.
The standard unit of capacitance is the farad, abbreviated F. This is a large unit; more common units are the microfarad, abbreviated µF (1 µF = 10-6 F) and the picofarad, abbreviated pF (1 pF = 10-12 F).
Capacitors can be fabricated onto integrated circuit (IC) chips. They are commonly used in conjunction with transistors in dynamic random access memory (DRAM). The capacitors help maintain the contents of memory. Because of their tiny physical size, these components have low capacitance. They must be recharged thousands of times per second or the DRAM will lose its data.
Large capacitors are used in the power supplies of electronic equipment of all types, including computers and their peripherals. In these systems, the capacitors smooth out the rectified utility AC, providing pure, battery-like DC.

How capacitor work

In a way, a capacitor is a little like a battery. Although they work in completely different ways, capacitors and batteries both store electrical energy. If you have read How Batteries Work, then you know that a battery has two terminals. Inside the battery, chemical reactions produce electrons on one terminal and absorb electrons at the other terminal.
A capacitor is a much simpler device, and it cannot produce new electrons -- it only stores them. In this article, you'll learn exactly what a capacitor is and how it's used in electronics.

Capacitor types

Capacitor applications in electronic circuits
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 polarised. 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 capacitors

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

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.

Air-spaced:

Air-spaced are used mainly as variable capacitors for tuning. Air-spaced capacitors consist of a set of fixed plates, with a set of moving plates, mounted on a spindle, that exactly mesh with the fixed plates. The moving plates are controlled by a knob or by a screwdriver adjustment. Values from 5 to 500 pF and voltage rating up to 500 V for receivers, and several thousand volt for transmitters are available. There are special types for some applications.

Electrolytic:

In electrolytic type of capacitor the dielectric is a very thin layer of aluminium oxide formed on the plates by a conducting chemical compound when a DC potential is applied. The surface of the plates, which are made of aluminium foil, may be etched to increase the surface area and hence the capacitance value. The large surface area and the very thin dielectric means that a very large capacitance value can be obtained. Another similar type uses tantalum oxide as the dielectric. Tantalums, range from 0.01 to 3000 µF with voltages up to 100 volts, while aluminium types range from 0.1 µF to nearly 1 F with a voltage range of from 3 to 700 volts. The higher the value of capacitance, the lower is the voltage range. In both types a leakage current is essential to maintain the dielectric, and they are generally used on a DC voltage for smoothing and de-coupling. The polarising voltage must also be high enough to keep the leakage current flowing, otherwise the capacitance value will be reduced.