Potentiometers

Potentiometers are a common example for voltage dividers. Let's have a look at how they work and how they can be used.

Potentiometers

Potentiometers are pretty simple components. They have 3 pins, a resistive track and wiper. The wiper can moved to different positions on the resistive material. Usually the two outer pins are connected to the ends of resistive track, while the middle pin is connected to the wiper. In the picture below you can see the schematic symbol for a potentiometer. There are two different symbols depending on whether the American ANSI or international IEC style is used for the circuit diagram.

From the tutorial on conductivity and resistance we know, that the resistance increases proportional with the length of the resistive material. By moving the wiper we can therefore adjust the resistance between the wiper and both ends of the resistive track. The closer we get to one end, the less resistance we get. If we move the wiper further away from the end the resistance increases. The total resistance of the resistive track always remains the same. If we have a 10 kΩ potentiometer, the 10 kΩ refer to the resistance of the full resistive track.

Most potentiometers use a carbon film as resistive material. These potentiometers are low cost, but also less precise. Alternatively wire bound potentiometers can be used. They usually have a lower resistance value, a higher power rating and also a higher precision.

There are many use cases for potentiometers. A common one is the use as volume control knob on audio devices. Besides that potentiometers can also be used to enable user input to microcontroller applications. In the Arduino introduction series we used one to adjust the brightness level of a PWM-driven LED. Other use cases include position detection in servo motors and the use as a variable resistor in measuring or RC circuits. Let's first look into the two basic configurations in which potentiometers can be used.

Potentiometers as Variable Resistors

The first way in which potentiometers can be used, is the use as a variable resistor or rheostat. In this configuration only need two pins of the potentiometer: one end of the resistive track and the wiper. As already explained, you can then change the resistance between these two by moving the wiper.

A typical usage example for variable resistors in hobbyist projects are measuring circuits with resistive sensor. They can be used to adjust threshold values, to calibrate the measurement circuit and to adjust the measurement range. More complex usage scenarios can include adjustable RC circuits. Of course, a variable resistor can be also used for current limiting. Most potentiometers are not constructed for this purpose, however. The potentiometers you find in Arduino or similar beginner kits have resistances values around 1 - 1 MΩ. This is not a useful resistance range for current limiting in circuits. Keep in mind, that potentiometers have a maximum power rating too and that the resistance is practically zero, once the wiper touches the metal clamp at the end of the resistive track. The power rating is always given for the use of the whole resistive track. For linear potentiometers you can assume that the acceptable power is linearly dependent on the wiper position. If you use only 10% of the resistive track, the maximum power rating in this configuration is only 10% of the power rating given in the data sheet.

To symbolize a potentiometer used as variable resistor in schematics, you can use the potentiometer symbol with one of the ends left unconnected. Alternatively there is a specific symbol for rheostats:

In some circuits you can see an alternative form of wiring a potentiometer as variable resistance. The second end of the resistive track is shorted to the wiper pin:

As the wiper connection has a resistance that is magnitudes lower than the resistive material, this won't have a noticeable difference on the resistance value. The advantage is, however, that you don't have an open circuit if the wipers has no contact to the resistive material. In this case, we get the resistance of the full resistive track as worst-case resistance. Depending on the age of the potentiometer and the environmental circumstances in which it is used, the possibility that the wiper looses contact should be taken into account. Depending on your circuit, it is possible that an open-circuit-situation in one path will overload and damage other components. With this alternative wiring the behavior of your circuit might be unexpected, but the resistance value is still inside the operating range. In consequence, one should prefer this alternative wiring form.

Potentiometers as Voltage Dividers

The second and more common usage configuration of potentiometers, is the use as an adjustable voltage divider. In this configuration both ends of the resistive track are connected to different voltage levels and the wiper can be used to select an output voltage in between.

The voltage selection works, because at any given position of the wiper you have a certain amount of resistive material on both sides of the wiper. You can think of it as two resistors with the wiper in the middle. Together they are forming a voltage divider. The cool thing is that you can control the resistor values and thereby the output voltage by moving the wiper. The ability to adjust the voltage potential on the output using the wiper is what gives potentiometers their name.

It is possible to build potentiometers, which can be used with currents up to several amperes. The same rules that apply to voltage dividers apply to potentiometers, however. As with voltage dividers, there is always a certain current flowing through the resistive track of the potentiometer. For potentiometers with a low total resistance you loose a lot of energy as heat. If you use potentiometers with a high resistance, on the other hand, you won't have a good output voltage stability.

Potentiometers as a voltage divider is used in a lot of ares like volume adjustment, user input to microcontrollers or to create an adjustable threshold for comparators.

Potentiometer Types and Labeling

There are multiple different types of potentiometers available on the market. The first and most obvious difference is the build form: there are rotary as well as linear sliding potentiometers. Not so obvious is, that there are also different resistance tapers. Not all potentiometers are linear ones. The graph below shows the most common three resistance tapers for potentiometers. The most common and cheapest ones are linear potentiometers. They are straight forward to construct, because as explained the resistance increases proportional with the length of the resistive material in between the wiper and the ends of the resistive track. For audio applications there are however specialized logarithmic potentiometers. We as humans don't have a linear perception of sound. An increase of the audio level by 10 dB is perceived as doubling of loudness. To compensate this we need a more fine-grained volume adjustment for higher audio levels (low resistance) and bigger changes for low audio levels (high resistance). This compensation is achieved by using a logarithmic instead of a linear potentiometer. There is one caveat however, exactly logarithmic potentiometers are difficult to construct. The taper of the commonly used audio potentiometers differs from the optimal logarithmic curve shown in the graph, to make them easier and cheaper. Refer to their data sheets, if you need to know the exact resistance taper.

We now know about different types of potentiometers, let's continue with the labeling. To make it short: it's a mess. Printed on almost all potentiometers is their total resistances. For the potentiometers in the picture below they are 10 kΩ and 500 kΩ.

More difficult and sometimes nearly impossible is to find out the taper type. The right potentiometer uses a vendor specific code. The left one uses the American labeling in which B stands for a linear potentiometer. The problem is that there is not only the American code, but also an European one. These are shown in the table below. There are even more codes available and the bad news is, that they not only differ, but also overlap. To be sure which is one used, you need to know the preferred labeling style of the manufacturer. It's a lot easier to simply measure the resistance in the center position to determine the type.