Inside the potentiometer
Physical Computing, Jeff Feddersen

Potentiometers are very useful components for varying resistance in a circuit. Although potentiometers can be confusing at first, if you understand how they're put together, they make a lot of sense.

Figure 1: Inside the potentiometer, back view

A potentiometer has three terminals, referred to as A and B (the outside two) and W or the wiper (the middle terminal). Inside the potentiometer, A and B are connected by a length of resistive material. The wiper, which is connected to the potentiometer's shaft, has a moveable contact point that can touch the resistive material anywhere along its length. Note how this is reflected in the symbol for the potentiometer:

The total amount of resistance in the material determines the rating of the potentiometer: if its a 100K pot, there is 100,000 Ohms of resistance between A and B. Note that the resistance between A and B always remains the same.

However, because the wiper is moveable, there can be a variable amount of resistive material between A and W or B and W. The more resistive material there is, the greater the resistance; the less material, the less resistance. Consider the following situations:

Figure 2: dial is turned all the way counter clockwise
Here, all the resistive material is between B and W, and none of it between the A and W. Remember, a circuit is just a path for electricity; if we make the circuit with A and W, but not B, there will be no resistive material in the path, since B is a "dead end". Likewise, if we make a circuit with B and W, electricity will have to go through all the resistive material to get through the pot. Assuming a 100K linear pot (more about linear and audio pots later), if we measure resistance between A and W we get 0 Ohms, but between B and W we get 100K Ohms.


Figure 3: dial turned all the way clockwise
There the situation is reversed; all the resistive material is between A and W. AW = 100K Ohms, BW = 0 Ohms.

Figure 4: dial in the middle
Here, the dial is in the middle, and there is an equal amount of resistive material in the circuit AW and the circuit BW. Measuring either will give a resistance of 50K Ohms.

Some things to note about potentiometers

1: The resistance between A and B is always the same. Making a circuit between A and B of a 100 Ohm pot is just like putting a 100 Ohm fixed resister in the circuit.

2: When turning the knob, resistance between the wiper and one terminal goes up while resistance between the wiper and the other terminal goes down.

3. Because of this, A and B are more or less interchangeable. Swapping A and B will change which way you turn the dial on the potentiometer to get the effect you want.

4. A linear pot - or slider, like on a mixing board - works just like a rotational potentiometer. Its just takes a different form.

Voltage Division

By using all three terminals of the potentiometer we can make a voltage divider circuit, which is very useful for interfacing potentiometers to the BX-24, among other things. To make the voltage divider we connect one terminal of the potentiometer to +5 volts, and the other to ground, then measure the voltage at the wiper. The voltage at the wiper will vary between 5 and 0 volts as the dial is turned.

Linear and Audio Taper (logarithmic) potentiometers

There are two different families of potentiometers; those whose resistance varies directly with the rotation of the knob, and those whose resistance varies exponentially with the rotation of the knob. When the resistance varies directly with the position the pot is called linear - a given amount of rotation yields a consistent change in resistance. The resistance in a logarithmic or audio taper pot doubles over a given amount of rotation. This is similar to the way the Richter and Decibel scales work, and for good reason. "Log pots" are used extensively in audio circuits, because the human ear works on a logarithmic scale as well - a linear increase in the perceived volume of a sound requires an exponential increase of the energy in the sound. Thus the second name for logarithmic pots - audio taper.

Just what is this "resistive material"?

There are lots of different materials that are used inside a potentiometer to form the strip of resistive material. Mimms observes that a pencil line on paper can be used. Our potentiometers use a conductive plastic polymer - generally the least expensive. There are also thin films of metal and compounds of metal and ceramic ("cermet"), carbon, or wire-wound resistors that work in a stepwise fashion.