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CPU Cooling Fan

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#1 dobber1978


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Posted 23 February 2009 - 12:28 PM


So my motherboard died on me and I replaced it over the weekend,

I reused all my old hardware, HD, CPU, Fans, etc...

My question comes with the CPU fan, my old MB and Fan only had 3 pins, my new board has 4 pins,

I plugged the 3-pin fan into the 4-pin MB connector the only way it would go and it works, it is alot louder now, seems to be spinning a little faster as well (where I think all the extra noise if coming from)

What is the 4th pin for on my MB fan connector?


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#2 dc3


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Posted 23 February 2009 - 12:38 PM

Connector pinout Pin Function Wire color
1 GND Black
2 12V Yellow
3 Sense (tach.) Green
4 Control (PWM) Blue

Signal GND is ground and 12V is voltage supply for fan.

Signal Sense (tachometer) provides two pulses per revolution of fan. Output is opened collector and main board must have pull-up resistor to 12V.

Signal Control (PWM) is input for PWM pulses. Base frequency is 25kHz and it is acceptable from 21kHz to 28kHz. Input has TTL level and includes pull-up resistor to 5V or to 3.3V in new constructions. Signal is not inverted and 100% PWM means maximal revolutions of fan. Motherboard has open-collector type output. This construction guarantee, that with disconnected PWM signal will runs fan with maximum revolutions.


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