Help me understand PWM vs DC modes?

daszani

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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?
 
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.
 

daszani

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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?

 

daszani

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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.

 
A major difference between the two fan systems is the METHOD used to control the fan motor speed. For older 3-pin fans this is done by changing the voltage supplied to the fan on Pin #2 (DC Mode). In the newer 4-pin system that voltage is always 12 VDC, and the fan receives a new PWM signal from Pin #4 (PWM Mode) that it uses internally to modify that power supply and change the motor speed. A 3-pin fan plugged into a 4-pin header that actually is using the new PWM Mode does not receive the PWM signal and has no chip to use it anyway, so it "sees" only the constant +12 VDC supply and runs full speed all the time. No matter how you try to set the speed of the fan in BIOS Setup or elsewhere, if you are trying to use PWM Mode to control a 3-pin fan it can NOT change the fan speed.

Now, in most AIO systems the intent is to have the pump run at full speed all the time. One way to do this is a "trick" to take advantage of this quirk of "mis-matching" fan and Mode types. If the pump (which is a 3-pin type whose speed is controlled only by changing its supply voltage) is plugged into a mobo fan header using PWM Mode for control, the pump will do as designed - run full speed all the time. In that case you should NOT set the pump's header to use DC mode because you do NOT want it to slow down. In your system with the pump plugged into the CHA_FAN1 header, make sure to configure that to use PWM Mode. I presume you have the two radiator fans already plugged into the CPU_FAN header via the Splitter supplied, and that is correct. This header should be in PWM Mode (I believe the manual says you don't actually have a choice about that one). That allows the CPU_FAN header, which is guided in its automatic control operations by a temperature sensor inside the CPU chip, to control CPU cooling automatically by changing the speed of the rad fans.

Finally, your case ventilation fans are all connected to the CHA_FAN2 header - I presume you have used a Splitter for that. This is where it gets a little tricky. Two of those fans are 4-pin PWM types, and one is 3-pin type. If you connect them all to a header using PWM Mode, then the two PWM fans' speeds WILL be under automatic control by the header, but the 3-pin fan will NOT - it will always run full speed. You have three choices here:
(a) leave it this way with the header configured to use PWM Mode, and tolerate the full speed operation of the 3-pin fan.
(b) change the configuration to use DC Mode. This will control the 3-pin fan properly. But 4-pin fans also can be controlled this way, even if it is not the ideal method. Then all three of those fans are speed controlled.
(c) replace the 3-pin fan with a 4-pin model and use PWM Mode on that header.

Lastly, comments on your question about fans 4, 5 and 6 according to your third-party tool. Those are NOT the speeds of fans - I don't know what that software is reporting with those labels. All fans generate a speed signal inside the motor and it is sent back to the mobo header on Pin #3 of the header, so that the header can count pulses and report fan speed. This is NOT used for speed control, by the way. BUT the header can only deal with a speed pulse signal coming from ONE fan - give it more, and it gets totally confused and gives wrong answers and error messages. So any Splitter or Hub will send back to the mobo host header only ONE fan's speed signal, and the other fans' signals will be ignored completely. NO speeds can be reported by anything from those "other" fans on that Splitter.
 

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