Capacitors

A capacitor is basically a device which stores an electric charge. Physically it consists of two metal plates or electrodes seperated by an insulating material or dielectric. Application of a dc voltage across the capacitor will produce a deficiency of electrons on the positive plate and excess of electrons on the negative plate - fig. 22. This differential accumulation of electrons represents an electric charge, which builds up to a certain level (depending on the voltage) and then remains at that level.

As far as dc is concerned, the insulator acts as a blocking device for current flow (although there will be a certain transient charging current which stops as soon as the capacitor is fully charged). In the case of ac being applied to the capacitor the charge built up during one half cycle becomes reversed on the second half of the cycle, so that effectively the capacitor conducts current through it as if the dielectric did not exist. Thus as far as ac is concerned, a capacitor is a coupling device.

There are scarcely any electronic circuits carrying ac which do not incorporate one or more capacitors, either for coupling or shaping the overall frequency response of the network. In the latter case a capacitor is associated with a resistor to form an RC combination. The charge/discharge phenomenon associated with capacitors may be used in other types of circuits (e.g. the photographic electronic flash is operated by the charge and subsequent discharge of a capacitor triggered at the appropriate moment).

Like resistors, capacitors may be designed to have fixed values or be variable in capacity. Fixed capacitors are the main building blocks of a circuit (together with resistors). Variable capacitors are mainly used for adjusting tuned circuits.

Categories:

Fixed Capacitors fall into two main catagories:
1. Non-polarized capacitors and
2. Polarized or electrolytic capacitors.
The main thing which determines the type of capacitor is the dielectric material used.

Non-polarized capacitors consist, basically, of metallic foil interleaves with sheets of solid dielctric material, or equivalent construction.The important thing is that the dielectric is 'readymade' before assembly. As a consequence it does not matter which plate is made positive or negative. The capacitor will work in just the same way, whichever way round it is connected in a circuit, hence the description 'non-polarized'. This is obviously convenient, but this form of construction does limit the amount of capacitance which can be accomodated in a single 'package' of reasonable physical size. Up to about 0.1 microfarad, the 'package' can be made quite small, but for capacitance values much above 1 microfarad, the physical size of a non-polarized capacitor tends to become excessively large in comparison with other components likely to be used in the same circuit.

This limitation does not apply in the case of an electrolytic capacitor. Here initial construction consists of two electrodes seperated by a thin film of electrolyte. As a final stage of manufacture a voltage is applied across the electrodes which has the effect of producing a very thin film of non-conducting metallic oxide on the surface of one plate to form the dielectric. The fact that capacitance of a capacitor increases the thinner the dielectric is made means that very much higher capacities can be produced in smaller physical sizes. The only disadvantage is that an electrolytic capacitor made in this way will have a polarity corresponding to the original polarity with which the dielectric was formed, this correct polarity being marked on the body of the capacitor. If connected the other way round in a circuit, the reversed polarity can destroy the dielectric film and permanently ruin the capacitor.

There is also one other characteristic which applies to an electrolytic capacitor. A certain amount of 'unused' electrolyte will remain after its initial 'forming'. This will act as a conductor and can make the capacitor quite 'leaky' as far as dc is concerned. This may or maynot be acceptable in particular circuits.

Non polarized Capacitor Types:

1. Paper dielectric capacitors:
   Generally recognizable by their tubular form, are the least expensive but generally bulky, value for value, compared with more modern types. Their other main limitation is that they are not suitable for use at frequencies much above 1 MHz, which virtually restricts their application to af circuits. They are generally available in capacities from 0.05 micro farad upto 1 or 2 micro farad, with working voltages from 200 to 1000 volts. Plastic impregnated paper dielectric capacitors may have much higher working voltages.

2. Ceramic Capacitors:
   They are now widely used in miniaturized af and rf circuits. They are relatively inexpensive and are available in a wide range of capacities from 1 pF to 1 micro F with high working voltages and also characterized by high leakage resistance. They are produced in both disc and tubular shapes; also as metallized ceramic plates.

3. Silver-mica capacitors:
   They are more expensive than ceramic capacitors but have excellent high frequency response and much smaller tolerances. So are generally regarded as superior for critical rf applications. They can be made with very high working voltages.

4. Polystyrene capacitors:
   They are made from metallic foil interleaved with polystyrene film, usually with a fused polystyrene enclosure to ensure high insulation resistance. They are noted for their low loses at high frequencies (i.e. low inductance and low series resistance). Good stability and reliability. Values may range from 10pF to 100,000 pF, but working voltages generally falls substantially with increasing capacity (e.g. as low as 60 volts for a 100,000 pF polystyrene capacitor).

5. Polycarbonate capacitors:
   They are usually produced in the form of rectangular slabs with wire end connections designed to plug into a printed circuit board. They offer high values of capacity (upto 1 micro F) in very small sizes, with the characteristics of low losses and low inductance. Like polystyrene capacitors, working voltages become more restricted with increasing capacity value.

6. Polyester film capacitors:
   They are also designed for use with printed circuit boards, with values from 0.01 micro F to 2.2 micro F. Value for value they are generally larger in physical size than polycarbonate capacitors. Their low inherent inductance makes them particularly suitable for coupling and decoupling applications. Values of polyester capacitors are indicated by a colour code consisting of five colour bands, reading from the top.

7. Mylar film capacitors:
   They can be regarded as a general purpose film type, usually available in values from 0.001 micro F to 0.22 micro F with a working voltage upto 100 volts dc.

Electrolytic Capacitors:

The other main type of electrolytic is the tantalum capacitor. This is produced both in cylindrical configuration with axial leads, or in tantalum bead configuration. Both (and the latter type particularly) can offer very high capacitance values in small physical sizes, within the range 0.1 to 0.01 micro F. Voltage ratings are generally low, e.g. from 35 volts down to less than 10 volts dc.

All electrolytic capacitors normally have their value marked on the body or case, together with a polarity marking (+ indicating the positive lead). Tantalum bead capacitors, however, are sometimes colour coded instead of marked with values.

Variable Capacitors:

There is also a general distinction between tuning capacitors used for frequent adjustment (e.g. to tune a radio receiver to a particular station) and trimmer capacitors used for initial adjustment of a tuned circuit. Tuning capacitors are larger, more robust in construction and generally of air-dielectric type. Trimmer capacitors are usually based on a mica or film dielectric with a smaller number of plates, capacity being adjusted by turning a central screw to vary the pressure between the plates and mica. Because they are smaller in size, however, a trimmer capacitor may sometimes be used as a tuning capacitor on a sub-miniature radio circuit, although special miniature tuning capaciotrs are made for radios designed to mount directly on a printed circuit board.

In the case of tuning capacitors the shape of the vanes determines the manner in which capacitance changes with spindle movement. These characteristics usually fall under one of the following descriptions:

1. Linear - Where each degree of spindle rotation produces an equal change in capacitance. This is the most usual type chosen for radio receivers.
2. Logarithmic - Where each degree of spindle movement produces a constant percentage change in frequency of a tuned circuit.
3. Even Frequency - Where each degree of spindle movement produces an equal change in frequency in a tuned circuit.
4. Square law - Where the change in capacitance is proportional to the square of the angle of spindle movement.