Non-ceramic capacitors

It is desirable to have, as a background to a discussion of ceramic capacitors, some knowledge of alternative dielectrics and the capacitor structures used with them. The principal types are described below and their capacitance ranges indicated in Fig. 5.8. 

Essentially polymer-film capacitors comprise dielectric films (polymer or paper or both together) interleaved with aluminium electrodes, either as aluminium foil or, more commonly, in the form of a layer evaporated directly on the dielectric, and rolled together. They are sealed in an aluminium can or in epoxy resin. Because the dielectric films and evaporated electrodes have thicknesses of only a few microns and about 0.025 /mi respectively, volumetric efficiencies can be high. The dielectric films are polystyrene, polypropylene, polyester, polycarbonate or paper paper dielectrics are always impregnated with an insulating liquid. 

The use of metallized electrodes on certain polymers provides the capacitor with self-healing properties during manufacture and throughout its working life. If breakdown should occur at a fault in the dielectric layer, the local heating causes the deposited aluminium in the vicinity to evaporate, leaving an area bare of metal around the hole produced by the breakdown. 

Polystyrene capacitors have exceptionally low tan S values ( 10 q, making them well suited for frequency-selective circuits in telecommunications equipment. Polymer capacitors are widely used for power-factor correction in fluorescent lighting units, and in start/run circuitry for medium-type electric motors used in washing machines, tumble-dryers and copying machines for example. They are also used in filter circuits to suppress radio frequencies transmitted along main leads. Such interference noise may originate from mechanical switches, furnace controllers and switch mode power supplies it not only spoils radio and television reception but can also cause serious faults in data-processing and computer equipment. 

Aluminium electrolytic capacitors are exploited in a range of applications and their relatively low cost makes them attractive for printed circuits for car radios, stereo equipment, pocket calculators, digital clocks, etc. Also, the very high value capacitors are used in large photo-flash equipment and for voltage smoothing. 

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