Variable Resistors (Potentiometers)
Potentiometers, or pots, allow you to adjust resistance continuously. Pots are three-terminal devices, meaning that they provide three places to connect to the outside world. Between the two outermost terminals is a fixed resistance — the maximum value of the pot.
Between the center terminal and either end terminal is an amount of resistance that varies depending on the position of a rotatable shaft or other control mechanism on the outside of the pot.
Inside a potentiometer is a resistance track with connections at both ends and a wiper that moves along the track. Each end of the resistance track is electrically connected to one of the two end terminals on the outside of the pot, which is why the resistance between the two end terminals is fixed and equal to the maximum value of the pot.
The wiper inside the pot is electrically connected to the center terminal and mechanically connected to a shaft, slide, or screw, depending on the type of potentiometer. As you move the wiper, the resistance between the center terminal and one end terminal varies from 0 (zero) up to the maximum value, while the resistance between the center terminal and the other end terminal varies from the maximum value down to 0 (zero). Not surprisingly, the sum of the two variable resistances always equals the fixed maximum resistance (that is, the resistance between the two end terminals).
Most often, potentiometers are marked with their maximum value, and they don’t always include the little ohm symbol.
Potentiometers are available in various packages known as dial pots, slide pots, and trim pots:
Dial pots contain rotary resistance tracks and are controlled by turning a shaft or knob. Commonly used in electronics projects, dial pots are designed to be mounted through a hole cut in a case that houses a circuit, with the knob accessible from outside the case. Dial pots are popular for adjusting volume in sound circuits.
Slide pots contain a linear resistance track and are controlled by moving a slide along the track. You see them on stereo equipment (for instance, faders) and some dimmer switches.
Trim pots (also known as preset pots) are smaller, are designed to be mounted on a circuit board, and provide a screw for adjusting resistance. They are typically used to fine-tune a circuit — for instance, to set the sensitivity of a light-sensitive circuit — rather than to allow for variations (such as volume adjustments) during the operation of a circuit.
If you use a potentiometer in a circuit, bear in mind that if the wiper is dialed down all the way, you have zero resistance, and you aren’t limiting current with this device. It’s common practice to insert a fixed resistor in series with a potentiometer as a safety net to limit current. You just choose a value for the fixed resistor so it works with your variable resistor to produce the range of resistance you need.
Note that the range on the potentiometer is approximate. If the potentiometer lacks markings, use a multimeter (set to ohms) to figure out the component’s value. You can also use a multimeter to measure the variable resistance between the center terminal and either end terminal.
The circuit symbol commonly used to represent a potentiometer consists of a zigzag pattern representing the resistance and an arrow representing the wiper.