Trimmer capacitors are miniature variable capacitors. Trimmer capacitors are mounted directly on the printed circuit board with a provision to vary their value using a small screwdriver. The trimmer capacitors are set during production or servicing of the electrical equipment and are not meant to be varied by the end user.

There are many parameters of an electrical circuit, such as rise and fall times and latencies that depend on the oscillator frequency generated by the trimmer capacitor. If there is a drift in these parameter values over time, then the recalibration can be done by varying the trimmer capacitor value.

There are two types of trimmer capacitors based on the dielectric used in it. The dielectric used could be air or ceramic. Trimmer capacitors can be made of semi-circular metal plates. One of the plates is fixed while the other can be rotated using a screwdriver. The capacitance changes when the shaft is rotated thereby increasing or reducing the amount of overlap between the two plates. To improve the precision of the capacitance values, simple gear mechanisms can be used that allow multiple turns between the lowest and highest settings.

The value of trimmer capacitors is less than 100 pico farads. Trimmer capacitors are commonly used in various RF, VHF and microwave frequency band circuits. The other applications include oscillators, tuners, crystal oscillators, filters, and communications equipment such as mobile radios, aerospace transmitters and receivers, signal splitters and CATV amplifiers.