1 Reply Latest reply on Jun 30, 2010 1:06 PM by Steve17

    DH57DD fan control

    Steve17

      The DH57DD product manual implies that PWM and non-PWM fans can be controlled by the motherboard.

       

      That seems to be the case for the front fan.  If I plug in a PWM (4 pin) fan it will supply a PWM signal and it puts 12 volts on the power pin (pin2).  If I plug in a non-PWM (3 pin) fan it supplies no PWM signal and it puts 5.85 volts on the power pin.  This is when my low power CPU is idling.  I guess that voltage will increase if things get hot.

       

      How does the motherboard know if there is a  PWM or non-PWM fan plugged in?

       

      However the rear fan header will only drive a non-PWM fan.  It supplies no PWM signal, and it puts 5.85 volts on the power pin.

       

      The CPU fan header always drives the fans in PWM mode.  It puts a strong PWM signal on the PWM pin and 12 volts on the power pin.  A non-PWM fan will always run at full speed.

       

      Is there a BIOS or motherboard problem?  I have the latest BIOS version 537.   I'm particularly interested in knowing how the motherboard tells if the plugged in fan is PWM or not.

       

      Thanks,

      Steve

        • 1. Re: DH57DD fan control
          Steve17

          I figured out how the mobo knows if a PWM fan is plugged in.  PWM fans have a pullup resistor to 5 volts (or 3.3 volts) on the PWM pin (pin4).  Max current sourced when the pin is grounded is 5 ma.  I use a 10k ohm resistor connected to 3.3 volts.

           

          I still wonder why the mobo always drives the rear fan in non-PWM mode and always drives the CPU fan in PWM mode.