Variable capacitor - Basic electronics circuits

Trimmers consist of single-turn variable capacitors, using one fixed set and one rotating set of plates, with a solid insulator, usually polypropylene, separating the plates and providing a considerably greater capacitance than would be expected in a small bulk.

Such variable capacitors, once used in radio receivers, are now available only for replacement purposes and can be quite difficult to find. The reason is the almost universal adoption of electronic tuning methods, making use of voltage-variable capacitors, the semiconductor varicap diodes. The only remnants of the old-style variable capacitors are trimmers.

Capacitance variation depends on the number of plates and size of trimmer, and ranges from 1.4 pF to 10.0 pF and from 5.5 pF to 65 pF.