Whether serving as front panel controls or as trimpots on circuit boards, mechanical potentiometers can develop intermittent slider connections. And if the slider connection becomes an open circuit, whatever that slider is connected to could exhibit unpredictable behaviour.

EDN slider 01 (cr) Figure 1: An intermittent slider will lead to an indeterminate output.

It would be prudent if in anticipation of such a part failure, that one include a shunt resistance from the slider connection to a safe and predictable voltage, in all probability, to ground.

EDN slider 02 (cr) Figure 2: Adding a shunt resistance keeps the output predictable in case of slider failure.

Suppose that the potentiometer's resistance was 10K. The added shunt resistance from the slider to ground might be chosen as 1 Meg. That shunt resistance would be large enough for the resistor to not materially affect the potentiometer's normal function but small enough to pull the next device's input connection to a safe and controlled level.

First published by EDN.