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Confused about PWM and DC mode for different fans in my Asus mb BIOS

Hi,

I'm using Maximus GENE VII and my mb BIOS or AI Suite provides both PWM and DC mode for all the fans. Some ppl say DC for chasis fan (3 pin) and PWM for cpu fan (4 pin). I wonder whether PWM is better than DC. At lest from the BIOS, I see PWM mode is a bit better than DC mode by providing slashing lines.

Now my settings are:

2 top fan, 4 pin - CPU FAN & CPU OPT. From my search, I find CPU OPT simply replicate the CPU FAN's setting. If I'm wrong please correct me.

1 back fan, 4 pin - chasis 3. I wonder if I can use PWM mode on this one. PWM mode seems more intelligent than DC mode

2 front fan, 3 pin - chasis 1 & 2. I also wonder if I can use PWM mode on these two. Do I have to use 3-4pin adapters? I remember I have some from my Noctua fans' packages.

Also, when I make changes to my BIOS fan setting, it screw up my AI Suites fan setting... The AI Suite does not get the profile from my BIOS. Wonder if I should remove it or just make it not being a startup program.
13 answers Last reply Best Answer
More about confused pwm mode fans asus bios
  1. PWM = Pulse Width Modulation, meaning the power pulses to the fan saving electricity and lowering the Electro Magnetic field that coil winding's, located in all fans and electric motors, produce.

    DC in this case means direct current where the fans receive a constant flow of electricity to remain spinning. No money saving there...

    PWM fans have 4 pin connectors due to the necessary electronics on in the fan. 1 wire Power, 1 wire Negative, 1 wire Tachometer, 1 wire PWM control.
    DC fans have 3 pin connectors. 1 wire Power +, 1 wire Negative -, 1 wire Tachometer.

    Which one is best? PWM is better in most scenario's You can hook a 3 Pin fan to a 4 Pin header and not have issues. Many boards have the ability to control both on there own.
  2. bgunner said:
    PWM = Pulse Width Modulation, meaning the power pulses to the fan saving electricity and lowering the Electro Magnetic field that coil winding's, located in all fans and electric motors, produce.

    DC in this case means direct current where the fans receive a constant flow of electricity to remain spinning. No money saving there...

    PWM fans have 4 pin connectors due to the necessary electronics on in the fan. 1 wire Power, 1 wire Negative, 1 wire Tachometer, 1 wire PWM control.
    DC fans have 3 pin connectors. 1 wire Power +, 1 wire Negative -, 1 wire Tachometer.

    Which one is best? PWM is better in most scenario's You can hook a 3 Pin fan to a 4 Pin header and not have issues. Many boards have the ability to control both on there own.


    Thank you. But I'm still a bit confused. 4 pin fan has 1 more pin for PWM control. Then you said 3 pin fan can still be hooked to a 4 pin header on mb. Are you saying many board can still control 3 pin fan with PWM mode? I'm asking about the tech behind that, just to confirm. If so, I don't need to use any 3-4 pin adapters.
  3. The main reason that PWM fans are better than DC fans is that they will spin at lower speeds. So if you can get enough airflow at these low RPM's then it's advantageous to have PWM fans. However if you need to maintain higher RPMs to get decent airflow, then you might as well go with DC fans since they are generally cheaper.

    Also just FYI, PWM fans can be operated in DC mode and they will still have speed control. The other way around though (DC fans in PWM mode) will just run a full speed with no speed control.
  4. Best answer
    Many can still control a 3 pin fan on a 4 pin header even with PWM enabled. Those boards when they do not receive the PWM signal from the fan automatically switch to DC control limiting the voltage that is constantly supplied changing the speed of the fan..

    Being a Asus Maximus board it should adjust the fans on its own. The way to tell is watch the tachometer if it goes up and down with temperature then it will adjust with the settings you have set if not change them.

    There is no need to adapt the fan pins. IF you have a PWM fan it can plug in to a 3 pin header and work and be adjusted. It just wont use the PWM function.
  5. The wiring behind PWM and DC is similar, the ground and tach are on the same pins. The main difference is the voltage pin on DC fans increases or decreases the voltage (0 - 12V) to vary the speed, on PWM fans it's a constant 12V (for motherboards). The other obvious difference is the addition of the 4th pin which is the PWM signal. Essentially how this works is that you get a pulse signal which goes from 0V to 12V. When the signal is at 0V, the fan stops (or slows down since the fans momentum keeps it spinning between pulses), when at 12V the spins. To vary the speed of the fan, the positive pulse width is varied. The narrower the 12V pulse is the slower the fan spins, the wider the pulse is the faster the fan turns. If the signal is absent, it's the same as 0V all the time which causes the fan to stop. This is why if you have your motherboard set to DC mode for a PWM fan, it will not run or run at a minimum RPM.

    I hope this helps you to understand how it works.
  6. Pulse Width Modulation means that there's additional electronics in the fan and in the motherboard to turn the fan on and off really quickly. The longer the on bursts are, the fast the fan spins. The voltage on the fan is kept constant. For a 3 pin fan, the voltage is increased or decrease to control the speed of the fan. This is similar to the dimmer switch on a light fixture. The motor designs for the PWM and the 3 pin fans are different to accommodate the control method. Generally, PWM fans generate less noise and/or support higher air flow.
  7. techgeek said:
    The main reason that PWM fans are better than DC fans is that they will spin at lower speeds. So if you can get enough airflow at these low RPM's then it's advantageous to have PWM fans. However if you need to maintain higher RPMs to get decent airflow, then you might as well go with DC fans since they are generally cheaper.

    Also just FYI, PWM fans can be operated in DC mode and they will still have speed control. The other way around though (DC fans in PWM mode) will just run a full speed with no speed control.


    It seems you are right. I've just used my Fan Xpert on all my fans, and my front ones are not recognized by it anymore. I don't why. Perhaps it is because I set them controlled in PWM mode in my BIOS. It's weird. Originally, they are set in DC mode in BIOS, and I remember my Fan Xpert recognized them and offer PWM mode for me to adjust.
  8. Scottray said:
    Pulse Width Modulation means that there's additional electronics in the fan and in the motherboard to turn the fan on and off really quickly. The longer the on bursts are, the fast the fan spins. The voltage on the fan is kept constant. For a 3 pin fan, the voltage is increased or decrease to control the speed of the fan. This is similar to the dimmer switch on a light fixture. The motor designs for the PWM and the 3 pin fans are different to accommodate the control method. Generally, PWM fans generate less noise and/or support higher air flow.


    finix429 said:
    techgeek said:
    The main reason that PWM fans are better than DC fans is that they will spin at lower speeds. So if you can get enough airflow at these low RPM's then it's advantageous to have PWM fans. However if you need to maintain higher RPMs to get decent airflow, then you might as well go with DC fans since they are generally cheaper.

    Also just FYI, PWM fans can be operated in DC mode and they will still have speed control. The other way around though (DC fans in PWM mode) will just run a full speed with no speed control.


    It seems you are right. I've just used my Fan Xpert on all my fans, and my front ones are not recognized by it anymore. I don't why. Perhaps it is because I set them controlled in PWM mode in my BIOS. It's weird. Originally, they are set in DC mode in BIOS, and I remember my Fan Xpert recognized them and offer PWM mode for me to adjust.


    You should get a RPM reading from any fan connected as long as it's turning. The tach signal doesn't change regardless of control mode. If you don't see an RPM signal and the fan is turning, there is a few things that can cause this. Either the fans you have don't support a tach reading (unlikely), something is wrong with the tach signals of the fans, or something wrong with the motherboard reading the tach signal.
  9. techgeek said:
    Scottray said:
    Pulse Width Modulation means that there's additional electronics in the fan and in the motherboard to turn the fan on and off really quickly. The longer the on bursts are, the fast the fan spins. The voltage on the fan is kept constant. For a 3 pin fan, the voltage is increased or decrease to control the speed of the fan. This is similar to the dimmer switch on a light fixture. The motor designs for the PWM and the 3 pin fans are different to accommodate the control method. Generally, PWM fans generate less noise and/or support higher air flow.


    finix429 said:
    techgeek said:
    The main reason that PWM fans are better than DC fans is that they will spin at lower speeds. So if you can get enough airflow at these low RPM's then it's advantageous to have PWM fans. However if you need to maintain higher RPMs to get decent airflow, then you might as well go with DC fans since they are generally cheaper.

    Also just FYI, PWM fans can be operated in DC mode and they will still have speed control. The other way around though (DC fans in PWM mode) will just run a full speed with no speed control.


    It seems you are right. I've just used my Fan Xpert on all my fans, and my front ones are not recognized by it anymore. I don't why. Perhaps it is because I set them controlled in PWM mode in my BIOS. It's weird. Originally, they are set in DC mode in BIOS, and I remember my Fan Xpert recognized them and offer PWM mode for me to adjust.


    You should get a RPM reading from any fan connected as long as it's turning. The tach signal doesn't change regardless of control mode. If you don't see an RPM signal and the fan is turning, there is a few things that can cause this. Either the fans you have don't support a tach reading (unlikely), something is wrong with the tach signals of the fans, or something wrong with the motherboard reading the tach signal.


    Thank you for explain these for me. I now start to understand the difference between 3 pin and 4 pin fans. I do get RPM readings from fan tuning. But from the report I see for my front one, the control range is either N/A or 100 - 100%. I wonder whether it is because I set BIOS to read them in PWM mode and it screw things up...
  10. bgunner said:
    Many can still control a 3 pin fan on a 4 pin header even with PWM enabled. Those boards when they do not receive the PWM signal from the fan automatically switch to DC control limiting the voltage that is constantly supplied changing the speed of the fan..

    Being a Asus Maximus board it should adjust the fans on its own. The way to tell is watch the tachometer if it goes up and down with temperature then it will adjust with the settings you have set if not change them.

    There is no need to adapt the fan pins. IF you have a PWM fan it can plug in to a 3 pin header and work and be adjusted. It just wont use the PWM function.


    Thank you. So mb can control 3 pin in PWM mode by sending varying voltage to them. But I seems screw my fan's setting up. When I set every fan be controlled in PWM mode through my BIOS. Then in AI Suite, the fan Xpert shows my 3 pin ones cannot be tuned. The control range from my fan tuning report is mostly N/A and I cannot control them. Is it because my BIOS setting?
  11. If you are using 3 pin fans (DC fans) in PWM mode, they will spin at full speed. That is because in PWM mode, the voltage pin is at 12V all the time.
  12. techgeek said:
    If you are using 3 pin fans (DC fans) in PWM mode, they will spin at full speed. That is because in PWM mode, the voltage pin is at 12V all the time.


    I've found where the problem happens. I've just set my BIOS to control my 3 pin fans in DC mode. Then in AI Suite Fan Xpert, I can control them in PWM mode. bgunner is right in this part. The mb is probably sending variable voltage to 3 pin fans and control them in PWM mode.

    You are also right in the BIOS part. If I only control my fan using PWM mode in BIOS. Then those 3 pin fan will only run in full speed. Both you guys are right in each part, I really wanna pick two solutions... I feel like bgunner's answer helps me more to find the problem. I'm really sorry for this :/ Hope you don't mind :)
  13. No problem, glad you got it sorted out. That is what is important in the end.
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