Common Commercial Capacitors
Air capacitors use air as the insulator. Most variable capacitors are of this type. Variable capacitors are most often made of two sets of parallel aluminum plates that are interleaved. One set of plates is fixed, while the other can be rotated. Rotation changes the effective area of the plates, thereby varying the capacitance.
Oil or liquid dielectric capacitors consist of rigid metal plates immersed in oil or some other liquid insulator. The entire unit is sealed in a leakproof container.
A mica capacitor consists of alternate layers of mica and aluminum foil in a plastic case. Such capacitors are compact, durable, and stable; they are used in precision work.
Ceramic Capacitors. One type is a hollow cylinder of a ceramic material, forming the insulator; the plates are thin films of metal deposited on the cylinder's inner and outer surface. Another type is a block containing many plates interleaved with a ceramic material. Both types are sealed in plastic to protect them from damage and moisture. Ceramic capacitors are much used in situations involving very high frequencies, as in television sets.
Paper Capacitors. Two metal foils are separated by a layer of paper or polyester film. Another layer of paper or film is placed on the outside of one of the pieces of foil. This sandwich is rolled up, impregnated with oil, and sealed in a moisture-tight container. Paper capacitors are widely used.
Electrolytic Capacitors. One of the conductors consists of a metal—usually tantalum or aluminum—covered by a thin oxide film. The oxide film serves as the insulator separating the metal from an electrolyte or some other nonmetallic conductor. The most common type of electrolytic capacitor is a block of tantalum with numerous interconnected oxide-lined pores containing the nonmetallic conductor. Electrolytic capacitors provide a relatively large amount of capacitance for their size.