ABSTRACT
This chapter discusses fixed-value, variable-value, polarised and trimmer capacitors by their circuit symbol and explains the factors that determine the physical size of a capacitor. Plastic-film capacitors are general-purpose capacitors which use thin dielectric films made from materials such as polycarbonate, polyester and polystyrene. Aluminium electrolytic capacitors are manufactured from two layers of aluminium foil, with a spacer soaked in an electrolytic gel, such as manganese dioxide, rolled and inserted into a cylindrical enclosure. The positive plate is formed by depositing a thin layer of oxide on one of the aluminium foils. An electrolytic capacitor is a polarised capacitor which uses an electrolyte, usually in a paste or gel form, in order to achieve a larger capacitance than most other capacitor types. Variable-value capacitors use air as a dielectric. There are two ways of making a variable-value capacitor: by varying the area of overlap of the capacitor's plates, or by varying the distance between the plates.
