Whats the output of the third pin from a PC case fan?

[bonestorm]

case fan driver

Does anyone know the output of the third pin (white wire) from a PC case fan? Is it a PWM or voltage or a square wave that is proportional to the RPM of the fan?

 

[lvjrf]

Re: Computer Case Fan

Its a square wave to control the speed of the fan

 

[bonestorm]

Re: Computer Case Fan

Do you mean that the motherboard outputs a square wave to control the fan? Or do you mean that the fan outputs a square wave to indicate speed of the fan?

 

[FoxyRick]

Re: Computer Case Fan

The fan outputs pulses to indicate speed. I think it is 2 pulses per revolution from an open collector output.

I don't know if they are squarewave, they don't need to be, just pulses.

I used to modify fans without a third wire by adding a diode and two resistors to give this output. That was in the days before they all had them.

FoxyRick.

 

[SPATAN]

Re: Computer Case Fan

It use for check speed fan. If speed slow then shutdown

 

[minaflying]

Computer Case Fan

I have designed a cpu fan driver IC,the output waveform is just a square wave ，it only indicate the speed of the fan,the cpu and motherboard don't control fan's speed by this pin.