Variable & Trimmer Capacitors
1 min read
Quick Answer
A variable capacitor changes its capacitance mechanically — rotating one set of plates into another, or tightening the spacing between them. Tuning types sweep from a few picofarads to a few hundred for adjusting resonant circuits by hand; trimmers are their set-once cousins, adjusted with a screwdriver during calibration and then left alone.
The moving-plate construction is the whole story. Capacitance is proportional to the overlapping plate area and inversely proportional to the gap, so a shaft that rotates a stack of half-moon rotor plates between fixed stator plates sweeps capacitance smoothly from minimum to maximum — the mechanism behind every old radio's tuning dial, where the capacitor and a coil set the resonant frequency together. Trimmer capacitors shrink the same idea to a few millimetres: a screw compresses or rotates the plates once, during factory alignment or service, and a dab of paint holds the setting. Two habits matter in practice. Adjust trimmers with a plastic tool, because a metal screwdriver's own capacitance shifts the reading while you touch it. And treat the marked range as the usable range — the extremes of travel drift more with temperature than the middle.