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3pin fan on mobo 4pin header

Discussion in 'Electronics & Electrics' started by warren1133725, Nov 7, 2006.

  1. warren1133725

    warren1133725 Member

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    Nov 3, 2005
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    Location:
    South Australia
    Trying to correctly run a 3pin cpu fan off my Asus p5b mobo 4 pin cpufan header and have it use all the ASUS Q-fan features.

    From what Ive gathered the 4 wires are earth, permanent 12v, speed(rpm) sense wire and a pwm control wire. (not in that order)
    Correct me if Im wrong but I think the PWM wire from the MOBO carrys some varying signal that is used by the fan to control its pwm circuit inside a special fan.(that has a PMW circuit inside it). It is either a trigger pulse or a varying DC signal, neither of which can directly run a fan.
    These require special fan like the CNPS9500AT, not the majority of after market quiet fans that are 3 pin 12v, earth and speed sense.

    Someone must have the circuit for taking that control signal and turning it into a varying supply to control the speed of a 3 pin fan. (PWM or linear)

    The problem then if you use PWM it will play havoc with the speed sense, so
    will the mobo still need a speed sense signal to correctly control the cpu fan speed dynamiclly with cpu temperature?
    If so can you patch into the speed sense circuitry of a fan to supply it permanent 12v so fan gives out correct speed sense signal?
     
  2. gords

    gords Oh deer!

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    Don't cross-post. One thread in the Electronics forum is sufficient.
     
  3. OP
    OP
    warren1133725

    warren1133725 Member

    Joined:
    Nov 3, 2005
    Messages:
    71
    Location:
    South Australia
    Ive had confirmed that the PWM wire from the mobo is a low power square wave that is used by the fan to trigger its own 12v pwm square wave that runs the fan.

    It is possible to take that mobo signal and put it through a switching transistor to run a 3 wire fan correctly of the mobo with varying speed with cpu temp.

    The problem will be with the speed sense tach in the fan which needs a clean unmodulated 12v.

    Has anyone been able to separate this part inside fan with an added extra wire?
     

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