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Help me understand PWM vs DC modes?

I have an ASUS H97 Plus mobo, cooled with the Cooler Master Masterliquid 120.

As far as I understand, PWM is superior for longevity and if you have 4 pins, you should be using PWM.

In the bios, (my pump is plugged into Chasis 1 slot) when I swap from DC to PWM, the LED gets significantly brighter and the liquid starts making a flowing/bubbling noise. However, when it's on DC, the pump is silent.

I also have 3 chasis fans (all connected to Chasis 2 slot):
2x Corsair SP120 PWM (1 with LED and 1 without)
1x NZXT stock fan that comes with the NZXT S340 case

When I switch from DC to PWM, the LED also becomes significantly brighter and the fans sound like they are running at full speed, despite being able to set a lower fan curve than I am with DC, they still run louder.

(From my mobo, the minimum fan speed for PWM is 20% and the minimum for DC is 60%)

From OpenHardwareMonitor, it says Fans 4, 5 and 6 (I assume those are the 3 case fans) are running at 100%.

Lastly, is it recommended that I leave my pump off till the CPU hits a certain temperature? My GPU (stock air cooled) is turned off till the core hits 40c. Should I do the same with my cpu?
6 answers Last reply Best Answer
More about understand pwm modes
  1. Best answer
    PWM (Pulse Width Modulation) is way where speed is adjusted by varying frequency and with of electrical pulses into a motor.
    DC (Direct Current) motor speed is adjusted by changing voltage available to it (typically 9 to 12VDC). With lower (than 12VDC) voltage. motor has less power so it has to turn slower because of air resistance.

    For longevity it doesn't matter at all but with PWM you can have finer speed control, that's all.
    This: "Lastly, is it recommended that I leave my pump off till the CPU hits a certain temperature" is completely false and quite opposite, pump on those coolers should be made to run full speed all the time. Only radiator fan(s) should be adjusted automatically according to CPU temps and case fans according to temps inside the case mostly measured at chipset and VRM.
    In most cases, CPU fan(s) is/are required to run at least on minimum rpm or computer wouldn't even BOOT.
  2. CountMike said:
    PWM (Pulse Width Modulation) is way where speed is adjusted by varying frequency and with of electrical pulses into a motor.
    DC (Direct Current) motor speed is adjusted by changing voltage available to it (typically 9 to 12VDC). With lower (than 12VDC) voltage. motor has less power so it has to turn slower because of air resistance.

    For longevity it doesn't matter at all but with PWM you can have finer speed control, that's all.
    This: "Lastly, is it recommended that I leave my pump off till the CPU hits a certain temperature" is completely false and quite opposite, pump on those coolers should be made to run full speed all the time. Only radiator fan(s) should be adjusted automatically according to CPU temps and case fans according to temps inside the case mostly measured at chipset and VRM.
    In most cases, CPU fan(s) is/are required to run at least on minimum rpm or computer wouldn't even BOOT.


    Thanks for the reply! When I turn the pump to full speed in either PWM or DC mode, I can hear the bubbling/flowing noise quite audibly. It's not extremely loud, but it's there. Is there something I'm missing or is my pump faulty?
  3. Where is your pump in relation to radiator ?
  4. It's a pump, its pumping liquid.
    Noise is absolutely normal.

    People buy aio's on the assumption theyre quieter but completely forget about pump noise.
  5. Do you mean the positioning of the pump?

  6. madmatt30 said:
    It's a pump, its pumping liquid.
    Noise is absolutely normal.

    People buy aio's on the assumption theyre quieter but completely forget about pump noise.


    I see, that may be the case. It's kind of like a bubbling noise. I'm usually quite ok with the gentle hum of fans, but the bubbling noise is pretty irritating. It's not loud, just irritating. Guess I'll have to learn to love it.
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