Fixed capacitors usually consist of sheets of metal foil between which is sandwiched the dielectric, or alternatively the metal, such as aluminium, is deposited onto both sides of the dielectric. The characteristics and quality of the capacitor depends mainly on the dielectric, which may be paper, chemically impregnated paper, plastics mica or ceramic.
Variable capacitors are usually meter plates with air as the dielectric. The variation is achieved by varying the area of overlap of the plates.
Preset capacitors may use air, mica or ceramics as the dielectric.
Electrolytic capacitors use the metal oxide as the dielectric which is formed directly on the metal plates. High values of capacitance can be achieved here with small physical size. Most electrolytic capacitors must be connected into circuit with the correct polarity or damage (possibly including explosion) may result.
SAFE WORKING VOLTAGE
The safe working voltage is the maximum d.c. voltage that can safely be applied to a capacitor without causing the dielectric to break down.
When breakdown occurs, the electric field is strong enough to ‘tear' electrons free from their orbits. A current then flows with the production of a large amount of heat. The dielectric is commonly burned through rendering the capacitor unserviceable.
Higher voltage require thicker dielectrics, but this reduces capacitance. Thus, a given value of capacitance requires a larger capacitor (greater plate area ‘a') for greater voltage working.