Trimpot drift

Jay Kickliter

Is it common for a trimpot to drift over a short period of time, say 30 minutes?

I'm using a 200 k trimpot to drop 120 V RMS to a more manageable ~3 V p-p. But I found I have to calibrate it when the circuit is warm, because the the output drifts around 30 % from cold to hot, dropping more voltage when warm. The pot isn't getting warm, just the transformer next to it, and it definitely isn't hot. What causes this? The screw doesn't appear to be moving. A quick search on the internet led me to believe they can drift over a long period of time, but I didn't see anything to indicate that it would happen so fast.

EDIT: in hindsight I realize I should have dropped most of the voltage with a single resistor, but I'm still curious about the drift.

PJAllen

metal-film precision resistor/s

erco

Careful soldier! Many trimpots are only rated 0.5W power dissipation. If you are really putting 120 VAC across the pot, just a few mA can exceed that rating and get you into trouble. Even if it's not getting hot, it seems like PJ's metal film resistors would be a safer bet.

Edit: After re-reading your pot value of 200K, you should be OK current-wise. Voltage rating OK on your unit? Some are only 50V.

Jay Kickliter

Disregard. The problem doesn't appear to be with the trimpot, since the voltage jumps even if I cycle power to the board for just a second. Weird.

LoopyByteloose

The fact still remains that Ohm's Law is only one half of the engineering solution. If you don't consider the limitations of the components power (in Watts) it will always get too hot, may begin to drift in value, and more than likely go up in smoke.

Variable resistors have the additional hazard that one end is actually Zero ohms. Twisting the knob or screw too far may result in a disaster unless other resistance in series is limiting the circuit.

Erik Friesen

If you use a another larger resistor to do the heavy lifting, and lower the trimpot value, as you mentioned, your trimpot drift will affect the voltage very little.

kwinn

To expand on what Loopy posted, you have to take the voltage, current, and power dissipation into account. Even if the power dissipated by a pot or resistor is well below it's rating you can have internal arcing if the voltage across the resistor is too high.

Jay Kickliter

kwinn wrote: »

To expand on what Loopy posted, you have to take the voltage, current, and power dissipation into account. Even if the power dissipated by a pot or resistor is well below it's rating you can have internal arcing if the voltage across the resistor is too high.

I think may have been it. Perhaps a voltage spike every time the circuit was unplugged. I cut a trace and added 300 k resistor in series, and the problem seems to have gone away.

Thanks everybody