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Resistors

Using Variable Resistors in Real Circuits

The same three-terminal potentiometer can be used in two very different ways: as an adjustable voltage divider, or as a two-terminal variable resistance. Understanding that distinction prevents a great deal of schematic confusion.

Connection 1 — potentiometer as a voltage divider
POTENTIOMETER AS A VOLTAGE DIVIDER +10 V VOUT End terminals take the supply; the wiper provides the adjustable output.

With 10 V across the end terminals, an ideal unloaded linear potentiometer can provide a wiper voltage ranging from near 0 V to near 10 V. Halfway along the resistive track would ideally give about 5 V.

This is why a potentiometer is so useful for volume controls, reference voltages, set points, joystick positions, and other continuously adjustable controls.
Connection 2 — used as a rheostat
POTENTIOMETER USED AS A RHEOSTAT terminal A terminal B wiper tied to terminal B
This drawing shows the actual three-terminal potentiometer connection used as a rheostat. The diagonal-arrow variable-resistor symbol shown on Variable Resistors is a shorter two-terminal schematic representation of an adjustable resistance.

Here only the resistance between terminal A and the wiper position is important. The drawing also shows a common practice: tie the wiper to the end terminal that is being used.

Why tie the wiper to an end?

If the wiper momentarily loses contact with the track because of dirt, wear, or vibration, the extra connection can make the failure less abrupt in some circuits. The exact fail behavior still depends on the circuit, so it is not a universal safety rule.

Do not forget the fixed resistance

A variable resistor is often used with one or more fixed resistors so that the adjustment cannot reach an unsafe extreme. For example, a fixed resistor may guarantee a minimum amount of series resistance even when the variable control is turned to zero.

Do not assume a small panel potentiometer can control a high-current load simply because its resistance value is suitable. Its resistive track and wiper have power and current limits. A transistor, MOSFET, regulator, or amplifier stage is often used to handle the actual load power while the potentiometer supplies only a control voltage.
Power rating gets trickier in rheostat service

The printed power rating normally applies to the entire resistive element under the manufacturer's stated conditions. When only a small portion of the track is used as a rheostat, that small section cannot necessarily dissipate the full rated power of the whole track. The manufacturer's derating information should be used for power applications.

Finding the terminals with an ohmmeter
  1. Disconnect power from the circuit.
  2. Measure between pairs of terminals.
  3. The pair whose resistance stays nearly constant while the shaft moves are the two end terminals.
  4. The remaining terminal is the wiper.
  5. Measure from the wiper to either end while turning the shaft; the resistance should change smoothly.
Never use an ohmmeter resistance range on a powered circuit.
Common real-world uses
Reading an old schematic

Some schematics draw the wiper as an arrow touching a resistor symbol. Others draw a three-terminal potentiometer in a more compact form. A rheostat may be drawn as a two-terminal resistor with a diagonal arrow. The important question is not only the symbol shape, but which terminals are actually connected.

A useful troubleshooting habit: before assuming a variable resistor is bad, measure the two end terminals first, then measure from the wiper to each end while moving the control. A sudden open circuit, large jump, or erratic reading can reveal a worn or dirty track or wiper.