Adjustable (trimmer) capacitor · Exterior view

Adjustment rotor (screwdriver slot)Ceramic body · 5–30 pFPlastic basePins

Small colored ceramic body, a slotted rotating disc on top, two or three legs
Body ≈ 5–7 mm, screwdriver slot on top

Capacitors

Adjustable (trimmer) capacitor

In simple terms

What is this?

A very small capacitor adjusted with a screwdriver; you change how much the two plates overlap by turning it.

What is it for?

In radios and oscillators it raises and lowers the capacitance to fine-tune the frequency.

Where do you see it?

Inside old radio and walkie-talkie boards.

How do you spot it?

A small body a few millimetres across with a screwdriver slot on top; two pins.

Caution:

Turn it slowly and gently, it breaks if you force it; the frequency changes as you adjust.

Technical definition

A small capacitor whose capacitance can be changed by turning it with a screwdriver. The overlap area of a rotating electrode with a fixed electrode changes, which changes the capacitance. Values are in the pF range and it is used for fine tuning at radio frequency.

Range≈2–60 pF

AdjustmentScrewdriver

PolarityNone

Schematic symbol Same in IEC and ANSI

IEC 60617: the drawing style used in Türkiye and Europe.

Pins 2

1Fixed electrode (stator)Connects to the circuit line being adjusted1

2Rotating electrode (rotor)Connected to adjustment screw; usually grounded2

Features typical

Capacitance range: ≈2–60 pF (depends on model)
Dielectric: Ceramic · plastic film · air
Adjustment: Half a turn from minimum to maximum
Polarity: None

Common mistakes

Adjusting with a metal screwdriverA metal screwdriver and the nearness of your hand change the capacitance; when you pull your hand away, the setting drifts. Use a plastic trimming screwdriver.

Connecting the rotor to the signal lineIf the rotating terminal connected to the screw goes to ground, the setting doesn't drift when a hand comes near.

Test with a multimeter

Test whether this part is good with the multimeter on the measurement screen.

Take a measurement

Where it is used

Fine-tuning a station on a radio receiverFrequency trimming in a crystal oscillatorRF filter tuning