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Bruce Trump

Potentiometers (pots) can be used as position sensors or to adjust circuits for proper operation. They function best as an adjustable voltage divider. They can also be used as an adjustable resistor but with some potential pitfalls. Do you know the difference?

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Anonymous

  • In many circuits, I see the pot used in 'adjustable resistor mode', but only using the wiper and one endpoint. In theory, this is an adjustable resistor, and connecting the wiper to the other endpoint doesn't make a difference as this connection is shoring the 'unused' part of the pot resistive material.

    But in real world application, the wiper has a wiper resistance and this resistance is not static. Moving the wiper may result in sudden changes of the wiper resistance due to surface oxidation, mechanical 'jumping' of the wiper and other influences. A typical situation is the 'craclling' noise when you change the volume of an old radio. (really expensive audio pots, about $25 and up, are oil-filled to avoid the oxidation part) Connecting the wiper to the second endpoint of the pot limits the maximum impedance to the total pot value, even if the wiper is temporarily lifting and has no connection at all.

    In ratiometric mode, the absolute value of the wiper resistance is unimportant as long as the total impedance of the pot is significantly below the input impedance of the following circuit. However, there is no protection against mechanical jumping anymore (resulting pot impedance -> oo)

    The featured article mainly focuses on the temperature stability and accuracy, but not on the dsynamic effects when achtually using the pot for 'runtime adjsutments'.

    p.s.: in Germany, we're used to differentiate between potentiometers and trim(m)-potentiometers. The latter ones are used for calibration (one-time usage) and usually require a screwdriver or such for being moved, while the first ones have a handle of some sort and are meant to be used by the user. These are the ones where precision and stability is of lesser concern than a stable wiper resistance during wiper movement.