A problem with the constant-voltage solutions is that the fan needs relatively high voltage to spin up, but then only little to keep running, and I'd like to run it at very low speed. I'm now thinking about connecting 12V to the fan via a capacity (as in Batalov's suggestion) and 5V via a diode. Or I'll just use Batalov's suggestion.

Alex

[quote=Batalov;169651]Seriously, you may screw up both your 12V and 5V at the same time[/quote]

Probably best to listen to those that really know what they are talking about.

The cpu fans on my old dual celeron BP6 are powered through a simple circuitry consisting of a 3-way switch and 2 resistors. Input is 12V. I imagine it switches between parallell and serial resistors, and straight 12V. It is probably not specced for running as powerful fans as I have connected to it, because it appears to have overheated. I got it from a friend, apparantly it came with his lian-li case.

I think Batalov's solution is the best, but you're going to need a large capacitor to provide the large start-up current needed for enough time while the capacitor is charging. I'd start with at least an 18V 470uF capacitor and go up from there if necessary. If you have an old discrete sound card around that you can sacrifice, it probably has one.

The other way to do it is to use a switch IC. Something like the MAX4622 would work, which will operate on 0, 5V, and 12V and has guaranteed break before make. The time at 12V is fully adjustable by using an appropriate RC on the trigger input.

It's been done that way (using 12v and 5v) to slow fans down for some time now (2002?).
[url]http://www.ocforums.com/showthread.php?t=372297[/url]
[url]http://www.silentpcreview.com/article6-page1.html[/url]
[url]http://www.hardforum.com/showthread.php?t=1151998[/url]
[url]http://www.asktheramguy.com/v3/showthread.php?t=76932[/url]
[url]http://www.computerpoweruser.com/editorial/article.asp?article=articles%2Farchive%2Fc0803%2F25c03%2F25c03.asp[/url]

Google will give you many more hits with ' 7v fan ' :wink:
Xeon cpu fans were always very noisy, especially them 10,000 rpm one... slowing them down just a tad made it more tolerable.

Just do it. :smile: