What are Trimmer and variable capacitors?

Trimmer and variable capacitors can adjust their capacitance within a narrow range. They cover capacitance values from a few picofarads to a few nanofarads, with voltage ratings from tens of volts to several kilovolts. They're often used in calibration, tuning, and matching tasks. But they're more expensive and can be misadjusted, so fixed-value capacitors are usually a better choice.

Trimmed and variable capacitors provide a range of variable capacitance. The difference between the two is in the design intent. "Trimmed" capacitors are adjusted only a few times. "Variable" capacitors require periodic adjustment. "Variable" capacitors need to be adjusted often. They are made in different ways, but usually they are electrostatic capacitors that can be tuned by changing the surface area between the electrodes, the distance between them, or both.

Common usages & Applications

Trimmer and variable capacitors are used for tuning and matching in RF circuits. Radio receivers that show the selected tuning frequency by moving a mechanical indicator across a scale usually have a mechanical link between the indicator and the variable capacitors used in the tuning circuit. Most receivers are older or cheaper, but some still use trimmer capacitors. Variable capacitors are rare because newer designs are cheaper and easier to make.