Variable Resistor Basics: Potentiometers and Rheostats Explained

Variable resistors are commonly used for volume control and other adjustments in radios and televisions. They are often referred to as “pots” (short for potentiometers). For high-power applications, fixed wire-wound resistors are typically used, while for low-power applications, fixed carbon resistors with high resistance values are preferred.

As shown in the figure, a variable resistor usually has three terminals: two outer ones and one in the center. A wiper makes contact with a resistive track, and this wiper is connected to the center terminal. Rotating the spindle moves the wiper along the track, changing the resistance between the center terminal and the end terminals.

Slide-type variable resistors have a straight resistive track. These come in different types, notably linear and logarithmic (log).

• Linear Type: In a linear variable resistor, the change in resistance is directly proportional to the angle of rotation.
• Log Type: In a logarithmic variable resistor, the change in resistance at one end of the track is less than at the other end for equal angular positions. This is useful for audio applications, where our perception of loudness is logarithmic.

Variable resistors are available with typical maximum resistance values such as 10 kΩ, 50 kΩ, 100 kΩ, 500 kΩ, and 1 MΩ. These devices, often called potentiometers, are tuned to the desired resistance value using a screwdriver or similar tool.

Functions of Variable Resistors

Variable resistors are used for several functions:

• As a Rheostat: To control the current in an electronic circuit.

• As a Voltage Divider: To obtain any voltage from zero to the maximum of the supply voltage. This is achieved by tapping off a portion of the voltage across the resistor. If $V_{in}$ is the input voltage and $R_1$ and $R_2$ are the resistances above and below the wiper, respectively, then the output voltage $V_{out}$ is given by:

  $V_{out} = V_{in} \cdot \frac{R_2}{R_1 + R_2}$

Variable Resistor Manufacturers

Here are some well-known manufacturers of variable resistors, also referred to as trimmers, potentiometers, and rheostats:

1. Vishay
2. Hindustan Electronic Components
3. Jameco Electronics
4. Mouser Electronics
5. Ohmite
6. CTS
7. NTE Electronics Inc.
8. Murata Electronics
9. ACP
10. C & K Components
11. Nobel
12. Omite
13. TOCOS America
14. P3 America