PWM Fans - Are they working correctly?

Discussion in 'Troubleshooting Help' started by ausduckz, Nov 8, 2017.

  1. ausduckz

    ausduckz Member

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    I'm currently piecing together a new second hand build. It's my first system build since 09/10 when I built my current X58 system.

    Back then, I never used PWM fans, but for this new build, I'm using the NH-D15 which comes with PWM fans. While testing, I plugged the fan into the CPU Fan header, and when I entered the BIOS, noticed that it was spinning very low RPM, around 400 or so. The CPU temp said it was around 21C.

    On some occasions, when I booted up, the POST would halt, and I would get a CPU fan error, but I'm sure that's due to the fan running at low RPM. I did find an option in the BIOS to disable low RPM monitoring or something like that.

    So my question. How are PWM fans designed to work, and is it normal for them to run at this low RPM?

    Thanks.
     
  2. theSeekerr

    theSeekerr Member

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    Your motherboard is generating the PWM signal that runs the fans at that speed. Somewhere there should be a setting for minimum PWM duty cycle (if you've got a decent motherboard, there will be a setting per fan-header) or the ability to specify a fan-speed vs temperature curve.
     
  3. terrastrife

    terrastrife Member

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    Just set the profile to normal or turbo instead of silent.
     
  4. mike-s

    mike-s Member

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    There are two parts to this, the fan reporting the fan speed to the motherboard, and the motherboard's fan speed control.

    Three pin fans have both voltage (Red/black) as well as tachometer (usually yellow). The motor sends a pulse back along the tachometer pin to the motherboard every 1 or 2 revolutions (I cannot remember which) and the motherboard uses the pulse count to figure out and report on the fan speed.

    As to speed control there is the old method which is voltage regulation, whereby the motherboard would alter the voltage supplied to the fan to vary the fan speed and PWM control. PWM control means that the motherboard supplies a full 12 volts to the fan on the +/- pins and uses a fourth pin/wire to supply the above mentioned signal to the fan (It's somewhere around 22kHz or so). The fans are designed so that if they don't receive any signal on the PWM pin, they run at full speed. The PWM signal can dictate the fan speed in 8 bit/255 steps. The thing is, most fans have difficulty starting at below 40% or so and also have insufficient momentum at below around 25% power. That's why most PWM fans go to full speed upon startup and then spin down to whatever speed has been set.
     
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