pots and voltage (or/also current)

[Danich_ivanov]

I'm working on a fuzz pedal right now, and there are plenty of cool things i can do with it by adding pots, and all the locations i would put pots in sit at around 5-7 volts. However not that long ago i had an accident in which 500k pot burned down, it was within long tail pair context, which leaves me concerned. So i'm wandering, what are the peak conditions, under which average pots will operate normally, and what to watch out for?

[ElectricDruid]

The track on a pot can fairly easily be damaged by too much current flowing, as you've discovered. The Taiwan Alpha 16mm pots are rated at 60mW, for example.

5V across a 1K pot would give a current of I=V/R = 5/1K = 5mA. 5mA current flowing at 5V is 25mW, so would be safe. 9V across the same pot would fry it: 9/1K=9mA, 9mA*9V = 81mW, which is significantly more than the rated 60mW.

Larger pot values get safer and safer, given our typically low voltages.

Where things tend to go wrong is where the pot is wired up as a variable resistor rather than a potential divider. E.g. the voltage is applied across one end and the wiper. In this case, as the pot is turned, the resistance drops and the current goes up. When the pot reaches the end of the track the resistance will be very low and the current will be very high and the pot will get fried. It's always a good idea to add a fixed resistor in series with a variable resistor like this to avoid this problem and to make sure you have a known "minimum" value at the end of the pot's travel.

HTH,
Tom

[PRR]

> When the pot reaches the end of the track the resistance will be very low and the current will be very high and the pot will get fried.

ALSO: if only 1% of the pot is "in use", the current is high, AND all that heat happens in 1% of the complete pot.

If the pot is rated 1/4 Watt, 0.25W, a 1% slice can only handle 0.002,5 Watts. (Probably some more since the 99% is cold, but still a teensy fraction of whole-pot rating.)

100K 0.25W, turned-down to 1%, is 1K 0.003W. Then 1.7 Volts will smoke the 1% of the pot.

[PRR]

Here is a "pot" big enough to see. This is rated 1,000 Watts.... over the WHOLE winding-track. With the wiper set as shown, about 20%, it can handle 200W. If the wiper is set to 1%, it can handle 10 Watts (maybe 20W since there is a lot of metal at the end).


Our dinky pots are the same except smaller. (Also using carbon-track instead of heavy wire, cheap board instead of ceramic former, $1 instead of $1,000....)

[Danich_ivanov]

The track on a pot can fairly easily be damaged by too much current flowing, as you've discovered. The Taiwan Alpha 16mm pots are rated at 60mW, for example.

5V across a 1K pot would give a current of I=V/R = 5/1K = 5mA. 5mA current flowing at 5V is 25mW, so would be safe. 9V across the same pot would fry it: 9/1K=9mA, 9mA*9V = 81mW, which is significantly more than the rated 60mW.

Larger pot values get safer and safer, given our typically low voltages.

Where things tend to go wrong is where the pot is wired up as a variable resistor rather than a potential divider. E.g. the voltage is applied across one end and the wiper. In this case, as the pot is turned, the resistance drops and the current goes up. When the pot reaches the end of the track the resistance will be very low and the current will be very high and the pot will get fried. It's always a good idea to add a fixed resistor in series with a variable resistor like this to avoid this problem and to make sure you have a known "minimum" value at the end of the pot's travel.

HTH,
Tom

Gotcha, thanks man!