Trimmer potentiometer
A trimmer (or 'trimpot') is a small variable resistor adjusted by screwdriver rather than by a panel knob. Designers use it for one-time calibration: trim a voltage reference, null an op-amp offset, set a contrast level, or balance a sensor bridge.
A movable wiper divides a resistive track into two adjustable sections. Cermet means a ceramic-metal resistive material, not conductive plastic. A multi-turn mechanism spreads adjustment over several screw rotations. More turns make manual adjustment finer, but do not by themselves guarantee a particular circuit accuracy or long-term stability.
In plain terms
A tiny calibration knob for the technician: set the circuit's balance, then check that it stays within the required tolerance.
Why designers use it
- Calibrate the gain or offset of an instrumentation amplifier on the bench.
- Set the contrast voltage on a character LCD without burning a digital pin.
- Null the input-offset of a precision op-amp at production test.
- Balance a Wheatstone bridge for a strain gauge or pressure sensor.
- Set a comparator threshold for a low-volume prototype before swapping in a fixed divider.
Best for
- Bench calibration
- LCD contrast
- Offset null
- Bridge balance
Key specifications
- Example family: Bourns 3296 (Figures below apply to the cited family.)
- Effective travel: 25 turns nominal
- Rotational life: 200 cycles (Manufacturer resistance-change test conditions apply.)
- Power rating: 0.5 W at 70 °C (Derates to zero at 125 °C; voltage and track-use limits also apply.)
- Temperature coefficient: ±100 ppm/°C
When not to use it
- For a frequently operated front-panel control unless the selected part is rated for that use.
- Where its power, working voltage, environmental or adjustment-life ratings are insufficient.
- Where an uncontrolled adjustment could make the circuit unsafe; add appropriate limits and safeguards.
Common mistakes
- Confusing total resistance tolerance with how finely the wiper can be adjusted.
- Ignoring wiper/contact resistance and the loading imposed by the surrounding circuit.
- Exceeding the rating of the active part of the resistive track.
- Assuming a multi-turn trimmer cannot drift or wear.
Where you will find it
- A calibration circuit can use a trimmer to adjust a divider ratio. Choose its range and resolution for the surrounding circuit rather than assuming that more turns guarantee a particular voltage accuracy.
- A multi-turn adjustment spreads wiper travel over several screw rotations, helping make small manual changes.
A short history
The Bourns 3296 is a documented example of a sealed, multi-turn cermet trimming potentiometer. Its datasheet specifies nominal 25-turn travel, adjustment characteristics, temperature coefficient and rotational life separately. Those are different properties: a finely adjustable setting is not automatically immune to temperature changes or ageing.
Good to know
- Trimpot is a Bourns trademark.
- The 3296 datasheet gives separate figures for voltage adjustability, resistance adjustability and rotational life. They do not describe the same limitation.