Potentiometers & Variable Resistors

What is a Potentiometer?

A potentiometer (pot) is a three-terminal variable resistor. It has a resistive strip between two outer terminals, and a movable contact called the wiper that slides along the strip. By turning the knob (or moving the slider), you change how much of the total resistance is between the wiper and each outer terminal.

Potentiometer vs Rheostat

The same physical component can be used two ways:

• •As a potentiometer (3 terminals) — Used as a voltage divider. All three terminals are connected. The wiper voltage varies between the voltage at each end.
• •As a rheostat (2 terminals) — Used as a variable resistor. Only the wiper and one outer terminal are connected. Turning the knob changes the resistance in the circuit.

The Voltage Divider Connection

When a potentiometer's outer terminals are connected across a voltage source, the wiper outputs a fraction of that voltage. If the total resistance is R and the wiper divides it into R_top and R_bottom, then V_out = V_in × R_bottom / (R_top + R_bottom). Turning the knob all the way to one end gives 0 V, the other end gives V_in.

Types of Potentiometers

• •Rotary potentiometer — The most common type. A knob turns 270° (or 300°). Used for volume controls, dimmers, and panel adjustments.
• •Linear slide potentiometer — A slider moves along a straight track. Found on mixing consoles and EQ controls.
• •Trimpot (trimmer) — A tiny potentiometer adjusted with a screwdriver, mounted directly on the circuit board. Used for one-time calibration, not frequent user adjustment.
• •Digital potentiometer — An IC that simulates a variable resistor using electronic switches. Controlled by a microcontroller via SPI or I²C.

Linear vs Logarithmic Taper

The 'taper' describes how resistance changes with knob position:

• •Linear taper (B) — Resistance changes proportionally with position. At 50 % rotation, resistance is 50 % of the total. Used for most technical applications.
• •Logarithmic taper (A) — Resistance changes more at one end than the other, following approximately a logarithmic curve. Used for audio volume controls because human hearing perceives loudness logarithmically.

Common Applications

• •Volume control — Logarithmic pot adjusts audio level naturally.
• •LED brightness — Rheostat in series with an LED changes current.
• •Sensor input — Trimpots set reference voltages for comparators or ADCs.
• •Motor speed — Rheostat provides variable voltage to simple DC motors (low power only).
• •Calibration — Trimpots fine-tune circuits during manufacturing.