How do I determine if I can safely substitute potentiometers?

Question

I've got a number of audio projects on the workbench, and of course, each of them requires different potentiometers. Instead of waiting for another box o' parts to arrive, can I substitute? What are the rules of thumb for that? While my first inkling is that it'd be the same as a regular resistor (e.g, 5-10% is generally close enough), I've been second guessing and, well, have confused myself in that thought experiment. I've never ~noticed~ tolerance ratings on pots, but that doesn't mean they're not there.

So within what ranges can I safely substitute? 10k for 20k or vice versa? What'd happen if I subbed a 500ohm for a 100k? Looking forward to some guidance!

Answer 1

The acceptable range of pot value substitution would depend on the rest of your design:

In audio applications, volume control pots could, for instance, affect the input impedance (or not) or other performance parameters. If the pot is being used as a simple voltage divider, then you would not want to decrease values too far i.e. increase the current draw across the pot too much compared to design.

At the same time, increasing the resistance too much would introduce an additional noise source, and may decrease the designed linearity of the divider - such as if the input impedance of whatever is reading that divided voltage, is of comparable magnitude to the pot's resistance. That's like throwing in a third resistance in parallel to one of the arms of the voltage divider, it's no longer a simple voltage divider.

Another parameter especially significant in audio designs is whether the pot is expected to have a linear, log or other more exotic taper (rate of change of resistance with wiper rotation / movement). Substituting one type for another will almost definitely introduce odd behavior.

As a general rule, one would substitute for a quick experiment, but not for a production run, without revalidating the design for the new value. Such substitutions should retain the taper of the original pot. On value range, it is a personal preference: Some would say do not go over twice or below half the design values, perhaps that too is pushing things to the edge.

Finally, you won't know till you try - and sometimes letting the grey smoke out of your circuit is a good indicator that the substitution was too far beyond acceptable range.

To answer your 500 Ohm for 100 K substitution query: Don't. That's a good candidate for smoke effects.

Answer 2

TO choose a POT value. consider the impedance from stray noise and the loading effects.

• If applied to low impedance sources like op amp out then tap into high impedance. There is a wide tolerance of choices. So common values depend on loading factors, so if it doesn't matter 1K,10K,100K.

• If it taps into a circuit that depends on impedance matching for minimal input offset voltage from input current, then the selection range is smaller.

• if tuning range permits fine tuning with fixed series resistors, obviously value is chosen by design.
• If driving low impedances then pot must be lower but watch for power dissipation I^2R
• If wanting very high impedances POT beware of stray EMI and use twisted pair or coax for shielding and consider stray hum use RF beads.

Quality of POTs varies widely

• Thin film resistive surfaces Pots can wear out quickly and become open circuit instantly if you transfer a > 100mW, but are cheapest.
• Wire wound are more reliable but inductive so limited to audio ranges

• Cermet is best material for general purpose. (ceramic metallized)

• already mentioned are linear, audio(log like) and inverse audio tapers , but there is also sine-cosine quadrature and many more varieties of ganged pots, with/without detents, sleeve or roller bearing, shaft diameter etc etc.