How to use potentiometer well?

Author: Bruce Trump, TI expert

Translation: David Zhao, Signal Chain Engineer at TI

Potentiometers can act as position sensors and can also be used to adjust circuits. Potentiometers are best used as voltage dividers. Potentiometers can also act as variable resistors, but there are some potential drawbacks. Do you know the difference between the two functions?

When used as a voltage divider, the absolute resistance of the potentiometer does not affect the output voltage. The output voltage is proportional to the input voltage. Commonly used potentiometers have poor resistance accuracy and poor temperature coefficient. However, as long as the potentiometer resistance is uniform, 30% of the voltage will be divided at the 30% position regardless of the resistance accuracy or temperature coefficient. Assuming the wiper is connected to a high impedance circuit, the contact resistance of the wiper will not affect the output voltage. The wiper contact resistance is the resistance at the contact point between the wiper and the resistor element.

As shown in Figure 2, when the potentiometer acts as a variable resistor, its resistance accuracy and temperature coefficient will affect the circuit. The wiper contact resistance will affect the resistance of the circuit, and the wiper contact resistance value will change with position, temperature, vibration and time.

Sometimes a variable resistor is necessary, and this can be designed by using a potentiometer in proportional mode. The two circuits shown in Figure 3 have similar functions. Figure 3a uses P1 as a variable resistor to produce an inverting voltage in the output voltage of U1 that is linear with the potentiometer position. Since P1 has a wide range of resistance, gain adjustment is achieved through R3. In addition, the linearity and repeatability of the output voltage versus the potentiometer position are affected by the contact resistance of the wiper.

In Figure 3b, operational amplifier U2 is introduced to make the potentiometer work in proportional mode, thus eliminating the defects in Figure 3a. Operational amplifier U2 provides high input impedance to reduce the influence of contact resistance. As the position of the potentiometer changes, the output voltage of operational amplifier U2 changes accurately from 0V to Vref. An accurate voltage range is provided to operational amplifier U3, so that only a fixed resistance value needs to be set for R5 and R6, and no gain adjustment of the circuit is required.

To make the potentiometer work in ratiometric mode, does your circuit need an additional op amp or increased complexity? This is a tricky decision for you to make. Sometimes it is unavoidable to use a potentiometer as a rheostat. But by understanding the hazards and sensitivities, you can choose a potentiometer to meet the required performance. And you will know to pay special attention to possible problems when testing and certifying your circuit.