Six general types of capacitors are the most widely used:
The electrolytic capacitor is marked with + and - and has polarity that must be observed when it is connected in a circuit. The other types do not need a polarity marking. Below shows an electrolytic capacitor that may be found in air conditioning, refrigeration, and heating applications. It is used as ac motor run and start capacitors below:
Air capacitors have air for a dielectric. They are usually variable capacitors used in the tuning circuits of radios.
Aluminum foil is used as the plate material in mica capacitors. Between the aluminum foil plates is a thin sheet of mica. Sometimes the mica is sprayed with a conducting paint. The paint then forms the plate on one side of the mica. Mica capacitors are usually sealed in Bakelite or some type of plastic.
Aluminum is also used as the plate material in paper capacitors. However, the plates are separated by a paper dielectric. The materials (paper and aluminum) are rolled into a cylindrical shape. A wire is connected to alternate ends of the foil and it is encased in plastic.
Ceramic dielectric materials make high-voltage capacitors. They have very little change in capacitance due to temperature changes. These small capacitors usually consist of a ceramic disc coated on both sides with silver. They are made in values from 1 picofarad up to 0.05 microfarad. Breakdown voltages of ceramic capacitors run as high as 10,000 volts and more.
Oil-filled capacitors.Oil-filled capacitors are paper capacitors encased in oil. They are sometimes referred to as bathtub capacitors. The main advantages of these capacitors are sturdy construction and high voltage breakdown ratings. They are used in places where grease and oil are likely to be encountered.