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I am driving a two side by side fans (original Xbox 360) from 20v, 4A source via a 12v regulator and a 10k trim potentiometer. The goal is simply to adjust voltage between 3v-12v, nothing I havent done before but shortly after setting the fans to the speed I want (say 6v), they begin slow down, around 4v. Then I adjust the pot to speed them up and then sometimes they will randomly go full speed ahead.

The pot doesnt get warm, the regulator output voltage is stable 11.96v. I have noticed that if the pot outputs 7v with the fans unplugged, the voltage sags to below 5v when plugged in.

The fans pull roughly 320mA @ 12v and 170mA @ 5v in this setup so 3.85W maximum. Ive read a lot of trimpots have a max rating of 3.00W but this happens at power levels below that?

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You cannot use a 10K pot to control fan speed that way.

If the pot is set to 50% (top half is 5K), and the fan draws 170 mA, you would have 850 Volts across the top half of the pot, and the pot would be dissipating 144 Watts!!!

You should use an LM317 or similar adjustable regulator, and use the pot as one of the adjustment resistors (even then, you'll want a much lower resistance pot.)

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  • \$\begingroup\$ Whhaaat?! I cant quite comprehend that. I used to control those fans in my old xbox with a 1k or 10k pot (cant remember) flawlessly in series with its motherboard connector. Why am I not experiencing any heat or total failures and would you mind explaining the 850v thing? Thanks \$\endgroup\$
    – sryan2580
    Commented Jun 8, 2018 at 0:41
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    \$\begingroup\$ @IUsedToBeFunny: By Ohm's Law (E = IR), if you have 170 mA flowing through 5K Ohms, you will have 850 volts across the resistor (unless I've made a serious mistake in my calculation.) \$\endgroup\$
    – Peter Bennett
    Commented Jun 8, 2018 at 4:24
  • \$\begingroup\$ @PeterBennett Care to explain why you assumed there is "170 mA flowing through 5K" in the first place and why you'd also assume that a 12 volt regulator would output 850 volts? \$\endgroup\$
    – voneiden
    Commented Jan 12, 2021 at 0:05
  • \$\begingroup\$ @voneiden: the OP said the fan draws 170 mA at 5 volts. I am saying that in order to get 170 mA through a 5K resistor, you would have to apply 850 volts according to Ohm's Law (V = I x R) \$\endgroup\$
    – Peter Bennett
    Commented Jan 12, 2021 at 0:16
  • \$\begingroup\$ OK, it's a perfectly valid hypothetical scenario, but I think we can quite safely assume that the fan was not actually drawing 170 mA in series with a 5K resistor if we have a regulated 12 volt supply. If you think there's an error in the initial values OP provided, then I'd argue it's much easier for the reader if you point out that clearly. \$\endgroup\$
    – voneiden
    Commented Jan 13, 2021 at 8:25

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