energy. The simplest form of capacitor is two metal plates insulated from each other by some dielectric. Capacitors are the second most widely purchased passive components next to resistors. There are both fixed and variable capacitors for electronics, and their capacitance values vary from a few picofarads (pF) to thousands of microfarads (μF). The schematic symbol for a fixed capacitor is shown in Fig. 2 and that for a variable capacitor is shown in Fig. 3.
Fig. 1
Fig. 2
Fig. 3
Capacitors are classified as either electrostatic or electrolytic. Electrostatic capacitors have dielectrics that are either air or some solid insulating material such as plastic film, ceramic, glass, or mica. (Paper dielectric capacitors are no longer specified in electronics.)
Electrolytic capacitors are further classified as aluminum or tantalum because those metals form thin oxide film dielectrics by electrochemical processing. They can have wet-foil, wet-slug, or dry-slug anodes.
The capacitance value of fixed capacitors remains essentially unchanged except for small variations caused by temperature changes. By contrast, the capacitance value of variable capacitors can be set to any value within a preset range of values. Variable capacitors are usually used in RF circuits.
