Ive got a generic 50 mm 12v fan to provide airflow over hd. (Trying to reduce running temp)

It has small white 3 pin idc type connectir.
If it follows colour convention the red is 12 v, black is 0v.

But what is yellow wire connected to?
I camt find data sheets anywhere.

Pretty sure it is speed control by pwm, does it go to 0v to turn on fan or to 12v?

http://pcbheaven.com/wikipages/How_PC_Fans_Work/

Tells you what you want to know...

If I can read properly..
3 pin is just a tacho feed - ie a check that the fan is spinning, or PC slows the CPU/system down to protect itself

4 pin is what you need (or maybe not).
3rd pin tacho as above, monitoring the fan rpm in real time
4th pin is a pulse width across the supply to switch the power on/off - effective controlling the fan speed. low pulse frequency, slow fan, high pulse/continuous = fast fan.

So its not a simple as just giving it 12v on the spare pin. Needs an IC to control it all.

[Edited on 9/12/15 by MattD]

Excellent info, thx.

the third wire is usually to feedback the current speed of the fan, (they have a hall effect sensor)
if you just wire it 0v black, 12v to red and ignore the yellow wire it will run at full speed

or you could just run it in parallel with the processor fan, (again without the yellow wire) it will then run at a similar rate to that

the pwm is on the 12v feed.

edit for clarity.

[Edited on 9/12/15 by gremlin1234]