What is a Variable Resistor?

An adjustable variable resistor is a resistor whose electrical resistance can be set mechanically or electronically. The set resistance can control current, divide a voltage or set a circuit parameter under Ohm’s Law. This article focuses on adjustable resistors with a resistive element and wiper.

In a mechanical unit, the wiper moves along a resistive track and changes the active track length between terminals. The measured value also includes wiper and contact resistance, so the minimum setting may be above zero. Thermistors and photoresistors are variable resistors in a wider sense, but temperature or light changes their resistance without a movable wiper.

A potentiometer normally has three terminals: two end terminals across the resistive element and a movable wiper terminal. Its external connections determine whether it operates as a rheostat or a potentiometer.

Rheostat mode uses two electrical nodes: the wiper and one end of the track. The resistance between them varies with wiper position and controls current within the component’s voltage, current and power limits. Designers often connect the wiper to the used end terminal so an intermittent wiper contact leaves the full track resistance instead of an open circuit. The data sheet must permit that connection.

A potentiometer uses all three terminals as a voltage divider. The two ends connect across a voltage source, and the wiper supplies the divided output. With negligible wiper load, Vout = Vin × Rlower/(Rupper + Rlower). A finite load sits in parallel with part of the track and changes this ratio, so a buffer may be needed.

A preset or trimmer is a small potentiometer used for production calibration or service adjustment. Single-turn and multi-turn versions use a screw or rotor. They are normally adjusted infrequently, and their allowed cycles, contact-resistance variation and adjustment tool are specified in the data sheet.

Types and Characteristics of Variable Resistors

Variable resistors differ by resistive material, movement, electrical taper, total resistance, tolerance, power rating and expected life. Selection also covers maximum working voltage, wiper current, temperature range, sealing, noise, resolution and mounting.

Common track materials include carbon composition, conductive plastic, cermet and wirewound elements. Carbon parts are common in low-cost controls. Cermet parts provide stable trimming over a wide resistance range. Wirewound parts can support precision or higher power, but winding creates discrete resistance steps and adds inductance. No material is always the best choice for either high or low resistance.

Movement can be rotary, linear slide, single-turn or multi-turn. A rotary shaft moves the wiper around an arc. A slider moves it along a straight track. Multi-turn gearing provides finer mechanical adjustment. Movement shape does not define the electrical taper.

Adjustment can be mechanical or digital. A mechanical unit moves a physical wiper. A digital unit selects taps on an internal resistor string with electronic switches, so resistance changes in discrete codes. Digital potentiometers also have terminal-voltage, wiper-current, package-power and power-up-state limits. Their analog terminals usually must remain between the permitted supply rails.

Total resistance is measured between the two end terminals. Tolerance states how far that value may differ from nominal. Taper describes how the wiper ratio changes with travel. Other specifications include end resistance, wiper resistance, contact-resistance variation, independent linearity, rotational life and temperature coefficient.

A linear-taper potentiometer gives an approximately proportional resistance ratio with travel. For an unloaded 10 kΩ unit whose independent linearity and end resistance are neglected, the midpoint is close to 5 kΩ on each side. Manufacturing tolerance affects total resistance more than the divider ratio in many applications.

A logarithmic or audio taper uses a nonlinear ratio. Its midpoint is defined by the maker’s taper curve and depends on direction, so it cannot be inferred from the 0 Ω to 10 kΩ range alone. Reverse-log and custom tapers are also available.

Audio gain controls often use an audio taper to give a smoother perceived loudness change across knob travel. The correct law still depends on source level, amplifier gain, loading and whether the control attenuates voltage or sits in a feedback network.

Body markings can show nominal end-to-end resistance, taper and tolerance. A 10K LIN marking usually denotes a nominal 10 kilo-ohms linear-taper part, but marking schemes vary. Confirm the part number and data sheet before replacement.

Applications of Variable Resistors

Variable resistors set analog levels, calibration points and user controls. They handle the full controlled current only when their electrical and thermal ratings permit it. Common applications include:

• Audio control: Potentiometers set volume, tone, balance, bass and treble in mixers, amplifiers, instruments and some headphones or speakers. The wiper usually supplies a low-power signal or sets amplifier gain. Audio taper is common for level controls, while linear or other laws suit balance and tone networks. Wiper loading and channel tracking affect accuracy.
• Television: Older television sets used panel potentiometers for brightness, contrast and picture position. Current displays more often use digital controls, encoders and stored settings. Trimmers may still set an analog calibration point, but the circuit design determines the taper. CRT and LCD controls cannot be assigned one universal potentiometer law.
• Motion control: A potentiometer commonly supplies a low-power speed, direction or position command to an electronic controller. Direct series rheostats can vary current in limited low-power or specialist circuits, but they waste power and must be rated for the load. Motor drives normally use power semiconductors for efficient control.
• Transducers: A position potentiometer converts shaft angle or linear travel into a voltage ratio. A thermistor changes resistance with temperature, and a photoresistor changes resistance with light. These are resistive sensors rather than wiper-adjusted potentiometers. Most microphones use capacitive, piezoelectric, electrodynamic or semiconductor transduction, so a microphone is not generally a variable resistor.
• Computation: Analog computers use potentiometers to set coefficients, initial conditions and reference levels. A wiper can scale an input to an operational amplifier or comparator. A summing amplifier then combines weighted signals. The resistor network and op-amp input currents determine accuracy.
• Home Electrical Appliances: Knobs in dimmers, thermostats and motor-speed controls may turn a potentiometer that sends a low-power command to switching electronics. The potentiometer does not normally carry the lamp, heater or motor load. High-power rheostats exist for rated applications, but they dissipate heat and require power derating. A thermostat combines a temperature sensor with a switching or modulating controller.

For every application, calculate terminal voltage, element dissipation and wiper current at all settings. In rheostat mode, concentrating current in a short active section can exceed the local track rating even when total-element power appears acceptable. Check end-stop operation, fault conditions, temperature derating and mechanical life.

Conclusion

A three-terminal potentiometer can operate as a loaded voltage divider or as a two-node rheostat. Track material, movement and taper describe different properties. Total resistance alone does not define accuracy or safety; wiper resistance, linearity, power, voltage, current, temperature and life ratings also matter.

Potentiometers set audio gain, calibration values, motion-control commands and analog-computing coefficients. Resistive sensors such as thermistors and photoresistors vary because of a physical input rather than wiper movement. Modern appliance controls normally use a potentiometer only as a command device for power electronics.

Select the part from the full data sheet and circuit conditions. Verify both end positions, maximum wiper load, track dissipation, signal voltage, taper error and behavior after contact wear. For a digital potentiometer, also verify supply sequencing, terminal-voltage limits, code resolution and its specified power-up state.