Resistor values for permanent speed control....

PrinceXizor · Post by **PrinceXizor** » Wed Nov 19, 2008 8:15 pm

I've got a low-power/low-heat system (Celeron 400) that I mounted a 120mm fan over the processor. I'd like to permanently wire in a resistor to reduce the fan speed to a very low RPM, any recommendations? I'd rather not have to buy a fan controller for this application.

Thanks!

Strid · Post by **Strid** » Wed Nov 19, 2008 11:27 pm

Put the fan connectors in the 5 V lines of a molex plug! Can't help you with resistor values, though - maybe someone else has tried that.

Cheers!

PrinceXizor · Post by **PrinceXizor** » Thu Nov 20, 2008 6:39 am

I know what you're saying but I can't wrap my head around an easy way to do that! Maybe it's too early in the morning

A little more info on that would help me out. Right now the 120mm fan is being fed by a 4 pin pass through with the two 12v wires going out to the fan.

Thanks!

QuietOC · Post by **QuietOC** » Thu Nov 20, 2008 6:58 am

PrinceXizor wrote:A little more info on that would help me out. Right now the 120mm fan is being fed by a 4 pin pass through with the two 12v wires going out to the fan.

You can pop both the 12V and 5V leads out of the 4-pin Molex connector with a small screwdriver and then swap them.

Nick Geraedts · Post by **Nick Geraedts** » Thu Nov 20, 2008 8:37 am

Just make sure that the fan will actually start at 5V. Otherwise you might have to get a little more fancy with your homebrew circuit design.

Strid · Post by **Strid** » Thu Nov 20, 2008 8:49 am

Nick Geraedts wrote:Just make sure that the fan will actually start at 5V. Otherwise you might have to get a little more fancy with your homebrew circuit design.

You can always do the 7 V trick then.

Well, I will try to guide you through the 5 V mod. I don't have a PWM (4 wires) with me, so I can't remember the colors. But two of the wires carry +12 V and ground respectively coming from the motherboard header. Another one is "speed control" and last one is RPM sensor. From the 4 wired fan plug, pull out the +12 and ground wire. You need to push down a small metal tap and then they'll pull out with minimum force.
Find a molex plug, and the bare wires you just had will fit right in the holes. Yellow and the black next to yellow is +12 V and ground, while red and the adjacent black is +5 V and another ground.
If you want 7 V, connect the fan over the yellow +12 V to the red +5 V.

Hope this helps!

PrinceXizor · Post by **PrinceXizor** » Fri Nov 21, 2008 7:48 am

I'm following you....

Let me clarify, it's a 2 wire fan. 1 Red and 1 Blk go up to a traditional 4 pin molex connector with extra molex attached to it. It's designed to receive a 4-pin from a PSU with the 2-lead going to the fan while the second connector plugs into whatever you would originally have used the lead from the PSU for (like a CD-ROM or whatever).

I hope that clarifies it for you. It's one of the generic 120mm from SVC they ran on sale many years ago.

Cheers!

zoatebix · Post by **zoatebix** » Fri Nov 21, 2008 10:26 am

I did a little bit of resistor math in this thread. All that's required is reading the fan's label and doing some algebra.

That said, I'd recommend doing what others in this thread have recommended: re-wiring your fan to draw 5 volts directly from your power supply. Use the diagrams from this SPCR article for reference. You'll use tiny tweezers or a precision screwdriver to... Actually,
this article on eXoid has much better photos than any description I could write.