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How to determine which potentiometer I need for this project? Answered

Hi all, 

I am building a laptop cooling pad with external power supply, and I would like to include a potentiometer so I could control the speed of the fan rotation. However, I am not sure about the resistance of the potentiometer.

I am using the adapter from an old HP printer. Here are the values: +16V, 500mA

And I am using the Cooler Master SickleFlow 120 fan. 12VDC, 0.35A


Use a voltage regulator, like an LM317, with a pot, to control the speed. You will struggle to get the fan to start at low speeds if it has just a resistor to control it.

Thanks, I'll do that. And which pot should I choose?

If you use a site like this


You can plug numbers in for the second resistor.

I'd use a 1K fixed resistor, in series with a 1K pot, for the lower R2 resistor. That will go to 12V on the high end, and 6 volts on the low end.

You can try a 2.2K pot, but I wouldn't recommend running the fan down to stall.

Are you absolutely sure that a 1K R1 and a 1K R2 pot will produce the voltage you mentioned? When I calculated the values using the online calculator, I got that using a 1K resistor as R1 and 1K potentiometer as R2 will only produce 1.25 to 2.5V.

Seems to me that I should use the default value for R1, which is 240 ohms, and a 2.2K potentiometer in order to get the voltage from 1.25 to 12V.

That's why I said

"I'd use a 1K fixed resistor, in series with a 1K pot, for the lower R2 resistor"

I DIDN'T change the 240 Ohm

Oh, now I get it :) Thanks again, I appreciate it!