Potentiometers

A potentiometer, often called a pot, is a resistor whose division you can change by turning a knob or moving a slider. Volume knobs, dimmers and many controls on electronic devices are potentiometers. This page builds directly on the voltage divider, so it is worth reading that first.

How it is built

Inside there is a strip of resistive material, called the track, with a pin at each end. A third pin, the wiper, is connected to a contact that slides along the track when you turn the knob. A 10 kΩ potentiometer has 10 kΩ between the two outer pins, always. What changes is how that 10 kΩ is split on either side of the wiper.

Move the knob in the tool and watch both uses at the same time.

As an adjustable voltage divider

Connect the two outer pins across the supply, one to plus and one to ground. The track is now a voltage divider, with the wiper as its output. With the knob at one end the wiper sees 0 V, at the other end it sees the full supply, and anywhere in between it sees the matching fraction. On 5 V, a quarter of a turn gives 1.25 V.

This is how a microcontroller reads a knob: it measures the voltage on the wiper. The same rule about loads applies as with any divider. The wiper output is meant for a signal, not for powering something.

As a variable resistor

If you use only one outer pin and the wiper, the potentiometer becomes a variable resistor. Turning the knob changes the resistance between those two pins from almost 0 Ω up to the full value. Used this way, it can adjust the current in a circuit, for example to dim an LED.

There is one important detail. With the knob turned all the way, the resistance between those two pins is close to 0 Ω, and nothing would limit the current. That is why the dimmer in the tool also has a fixed 220 Ω resistor in series. It sets the maximum current at a safe level, and the potentiometer can only reduce it from there. How to choose that fixed resistor is explained in the LED guide.

Choosing a potentiometer

• Value. 10 kΩ is the most common choice for reading a knob with a microcontroller. For a dimmer, the value decides how dim the lowest setting gets.
• Taper. A linear pot changes evenly as you turn it. A logarithmic one changes slowly at first and faster later, which suits volume controls because our hearing works that way. Pots are usually marked lin or log, but the letter codes for them differ between manufacturers, so check the datasheet.
• Power rating. Most small potentiometers are rated well under 1 W. Calculate the power in the part of the track that carries current, as described on the power page.
• Trimmers. Small potentiometers meant to be set once with a screwdriver are called trimmers. They are used to calibrate circuits.