Using a pot to cut fan voltage?

Thread Starter


Joined Feb 14, 2007
(A rehash of an earlier, lost, posting)

I am trying to quieten the fan in my 550W PSU by using an old radio-spare 50ohm potentiometer. It is a solution I sort of understand although a neater looking one - - was mentioned in a search of All About Circuits.

My questions are these:
a. Is this better done by making a voltage divider circuit - a la Vol1-DC Chap 6 - or a straight rheostat? Any different dangers/gotchas to accommodate?
b. ..and this where I admit to being a total newb at electronics... is putting a 50 ohm resistance across the power in as in the voltage divider not likely to 'short' the power somewhat and cook the pot? I would rather be told than learn by damaging what I have as it seems to be hard to get a 5W pot these days from Maplins et al.

Many thanks in advance,

Thread Starter


Joined Feb 14, 2007
Consider what might happen if the PSU cooks due to reduced cooling. I wouldn't take the chance.
Ouch, indeed, sir!

My quest for (reasonable, not fanatic-pleasing) quietness has this exercise as its starting-point. If it has a worthwhile effect on this disappointing new PSU's decibel level then I'll
either get a new PSU, :mad:
or try and find a way to make the fan respond to the actual cooling needs of the PSU (the ' project I mentioned, perhaps?), :confused:
or just use the pot to find the speed that balances noise with thermal safety. :p

I look forward to any answers to the original questions or advice re this little fan project.

Thanks again,
Hello Hilarious,

I suspect you have a relatively physically light and "cheap" power supply. I bought one of these once as a replacement when I could not locate a better one in time. You also find them in new computers and computer cases. These types skimp on heatsinks and more robust internal switching circuitry to save a few cents. They make up for the extra spewing heat, yither and yon with an obnoxiously loud fan.
If you keep your PC in an cool environment, you can get away with lowering the fan speed a little. Don't use the pot as it will most likely be destroyed. This is an easy way:

First, search Google for terms like: switchmode power supply safety repair capacitors. You MUST understand the dangers before proceeding.

Secondly, buy 6-10 rectifier diodes, these are dirt-cheap and you can find them in wrecked power supplies. Every diode will reduce the voltage by around 0.7V. If you don't know how to connect them, search the net.

Third, get heatshrink, or spaghetti tubing to encase them; any short circuits inside a PC power supply will be disastrous.

Fourth, get a small switch to bridge the diodes, supplying full power to the fan if things get hot.

You must solder these components. Experiment with the amount of diodes that will get the sound to a tolerable level, but still keep your PC from overheating. Remember though, cooling is more important than noise reduction.

First, search Google for terms like: switchmode power supply safety repair capacitors. You MUST understand the dangers before proceeding.

Fifth, post an update on this board when you're done. Good luck.


Joined Apr 25, 2006
A much better way to control a DC fan is with PWM. (Pulse Width Modulation.)
Some of my supplies (more expensive ones,) have a small circuit with a thermistor, and PWM controller for the fan. You might want to google PWM. You could do it with a 555 timer, or a small microcontroller.