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Hard hack to auto speed control a 3-pin fan?

E

ettin

Junior Member
I installed a gigantic passive CPU heatsink in my case, and now that I don't have a CPU fan, the four pin CPU fan header on the motherboard is free.

I have a giant case exhaust fan that pulls a lot of air over the CPU heatsink, and it's actually kinda loud when running at full speed. I would love to use the motherboard CPU fan speed regulation to slow down that fan when the CPU is cool, but the fan has only three pins/wires. So here's my question: Is there a hack that I could do to make that case fan slow down and speed up as a CPU fan would? What does that fourth pin do? Does it send out a varying voltage? That would be very convenient. Anyway, can someone think of a circuit which would allow this four-pin header to control a standard three-wire case fan?
 
Some motherboards (gigabyte for instance) let you select in the BIOS whether fan control is done via PWM or voltage regulation. So check it out.

p.s. PWM is used for control on the 4-pin fans, voltage on 3-pin.
 
It looks like I can't control the fan control mode in the BIOS, although that was a good idea! It looks like I'm stuck with PWM. But here's the thing: Can I just take the red wire from the fan that normally goes to PIN 2 (+12V), and hook it up to PIN 4 instead? Does the fourth pin just send out pulses of +12V (whereas PIN2 is just uninterrupted +12V)? If so, just a simple rewiring job should work. What do you think?

But if this is how it is, I wonder why Intel fans even have four wires, if the voltage drop is really between pins 1 and 4. I mean, what does the Intel CPU fan do with the third pin?

I came across this - the official PWM spec sheet:

http://www.formfactors.org/developer/specs/4_Wire_PWM_Spec.pdf
 
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Pin 4 is a control signal and basically has no power? It's not going to drive a fan.

I would 7v mod your exit fan. I.e. use a molex connector and run the yellow 12v grounded to the 5v red. So from fan to molex respectively Red->Yellow Black->Red.
 
Yeah, I know about the 7V trick and I've used it on many fans, but I was hoping to get this specific fan to vary its speed with the needs of the CPU. It's easy to build a switch that will bump up to 12V when you throw it, but I don't trust myself to do this manually.

I guess I was picturing pin 4 as being intermittent +12V, because I don't really know much about circuits. Honestly, I have no idea about what the control signal does for the fan. Does it basically tell it to shut off for certain pulsed periods? I suspect that you're right. Something I read in the spec game me the impression that it operates at a voltage much lower than +12. Oh well!
 
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