Variable Resistors: Potentiometers, Rheostats, and Trimmers
A variable resistor is a resistor whose effective resistance can be adjusted. The most familiar forms are the potentiometer, rheostat, and trimmer. They all use the same basic idea: a movable contact called a wiper touches a resistive track at a selected point.
The three terminals of a potentiometer
The resistance measured from one end terminal to the other is the total resistance of the potentiometer. Moving the wiper changes how much of that track lies between the wiper and either end.
Potentiometer, rheostat, or trimmer?
| Name | Typical connection and purpose |
|---|---|
| Potentiometer | Usually uses all three terminals as an adjustable voltage divider. Common for volume, level, position, and user controls. |
| Rheostat | Uses the wiper and one end terminal as a two-terminal variable resistance. Traditional rheostats may be physically larger and intended for more current or power. |
| Trimmer / preset | A small potentiometer intended for occasional adjustment during setup, calibration, or service rather than constant operation by the user. |
Rotary, sliding, and multi-turn controls
- Rotary potentiometer: adjusted by turning a shaft or knob.
- Slide potentiometer: the wiper moves in a straight line; common in mixers and control panels.
- Multi-turn trimmer: requires several screw turns to move through its full range, allowing finer adjustment.
- Dual-gang or multi-gang pot: two or more potentiometers are moved by one shaft, useful for stereo audio and matched controls.
Linear and audio taper
The word taper describes how resistance changes as the shaft or slider moves. In a linear taper, equal mechanical movements produce approximately equal changes in resistance. In an audio or logarithmic taper, the change is deliberately non-linear to better match the way a volume control is expected to feel to the listener.
Important specifications
| Specification | Why it matters |
|---|---|
| Total resistance | The resistance from one end of the track to the other. |
| Taper | How resistance changes with shaft or slider position. |
| Power rating | How much heat the resistive element can safely dissipate. |
| Mechanical travel | Rotation angle, slider distance, or number of turns. |
| Wiper current | The movable contact also has an electrical current limit. |
| Life / cycles | The track and wiper wear mechanically with use. |
| Noise / contact quality | A dirty or worn wiper may create crackling, dropouts, or unstable adjustment. |
Mechanical wear is part of the electrical story
Unlike a fixed resistor, a conventional potentiometer contains a moving contact. Dust, oxidation, wear, vibration, and repeated movement can affect the electrical connection. That is why an old audio volume control may become noisy when it is turned.
Next: Using Variable Resistors shows the two most important connections and several practical circuit habits.