A variable capacitor (variable capacitor, KPI) is a capacitor whose electrical capacitance can be changed mechanically or electrically, under the influence of a change in the voltage applied to the plates. Variable capacitors are used in oscillatory circuits and other frequency-dependent circuits to change their resonant frequency - for example, in the input and heterodyne circuits of radio receivers, in the correction circuits of the frequency response of amplifiers, generators , antenna devices. The capacitance of variable capacitors usually varies from units to several tens or hundreds of picofarads .
According to their purpose, variable capacitors are divided into those intended for frequent adjustment during operation (for example, for tuning the receiver or transmitter), and tuning (trimmers, in the Soviet literature until the 1950s were also called semi-variables), which are regulated relatively rarely, only during commissioning equipment. Trimmer capacitors are simpler in design (they do not need to use high-quality bearings, etc.) and usually have a narrower range of capacitance. Sometimes they are equipped with a device that allows you to lock the rotor after adjustment (for example, a collet clamp ).
By choosing the shape of the KPE plates, one can obtain various types of dependence of the capacitance on the angle of rotation of the rotor. The most common are direct-frequency (direct-wave) and direct-capacitive KPIs. For direct capacitance, the frequency of tuning of the circuit in which such a capacitor stands is proportional to the angle of rotation of the rotor; in the case of direct-frequency, the dependence is chosen such that, in proportion to the angle of rotation of the rotor, the resonant frequency of the oscillatory circuit in which the KPI is included changes.
KPE blocks are very common, consisting of two, three or more sections with the same or different range of capacities mounted on one shaft. They are used when it is necessary to ensure a coordinated restructuring of several circuits, for example, an input filter, an intermediate frequency filter, and a local oscillator in a radio receiver. Often several tuning capacitors are built into such a unit to precisely fit the capacities of individual sections.
- Mechanical KPIs:
- with air dielectric;
- with solid dielectric;
- Electric KPIs:
- varicondas .
- Azarch S. Kh. Variable Capacitors. - M.-L.: “Energy”, 1965
- Handbook of electric capacitors / M. N. Dyakonov, V. I. Karabanov, V. I. Prisnyakov and others; Under the total. ed. I.I. Chetvertkov and V.F.Smirnov. - M .: Radio and communications, 1983
- V.A. Lomanovich. Handbook of radio components (resistance and capacitors) - M.: DOSAAF Publishing House, 1966
- Electric capacitor - article from the Great Soviet Encyclopedia .