What Is a Variable / Trimmer Capacitor?
A variable capacitor (trimmer) is a capacitor whose capacitance can be adjusted by hand over a certain range. It's used where fine adjustment (calibration) is needed.
How Does It Work?
The capacitance is adjusted by mechanically changing the plates' overlap area or the distance between them. Fine adjustment is made with a small screw.
Types
| Type | Property |
|---|---|
| Ceramic trimmer | Small, cheap, on-board |
| Air variable | High quality, old radios |
Where Is It Used?
Frequency tuning in resonant and RF circuits, oscillator calibration and precise timing adjustment.
How Is It Adjusted?
Adjustment is done with a plastic trimmer tool; using a metal screwdriver corrupts the reading because of the capacitance of the hand and the tool. With the circuit running, it's brought to the desired value against a reference (e.g. the output frequency); if it might drift with vibration, it's fixed with lacquer/adhesive.
Common Mistakes
- Over-forcing it: the adjustment screw is fragile; too much torque damages the plates.
- Metal screwdriver effect: during adjustment the hand/tool capacitance interferes; a trimmer tool is preferred.
- Vibration: the setting can drift over time; fixed in critical spots.
Frequently Asked Questions
What is a trimmer capacitor for?+
What is the adjustment range?+
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