4 pin fan header =/= PWM

[LB777]

So after some research I just discovered that ALL my fan headers (except the cpu one) on my Asus P9X79 Pro, which are 4 pin headers, don't actually have PWM.

 

I don't know if everyone already knew this and I'm just showing my ignorance but the fact is, you have to look in the manual to see if a header has PWM or not. There isn't anything on the box, on Asus' website or anywhere else that could make you think it's not PWM.

 

What's even more strange is that the 4th pin that isn't used for PWM is only a 5 Volts pin, meaning that any fan will run at its full speed no matter if it has PWM or not. So what's the point ? A 3 pin header would have done the exact same thing.

 

You might say well what's the problem ? A 3 pin fan can perfectly be controlled by voltage and not PWM... Yes that's true but it's way less efficient when going into very low rpm. I bought Noctua PWM fans and if I let them regulate via voltage they won't turn on until 60% of their full speed which is quite useless... might aswell put them in full constantly...

 

So yeah that's my little rant about Asus motherboards (and others because I think it's the same thing for Gigabyte ones...), I feel like I've been fooled since one of the reason I bought the board was all the 4 pin headers... but oh well I guess that's life ;)

 

Sorry for this long annoying post, hope it might help some people though.

[Leonard]

I already knew this but and i have never looked into why they use 4 pin for pwm when you can use 3pin but i would like to know though.

[MODZERO]

It's an old thread but Asus are still pulling this crap. On my IMPACT VII I have a PWM DDC pump attached to the 4-pin Fan header on the small daughter board it was perfect placement. I even cut my PWM cable on the pump very short to keep it looking clean... just to find out it's not PWM. It's not mentioned in the manual and THIS thread is first I've found on the subject. The system is done, and far to much effort to re-route now. Learnt my lesson.

 

J,

[Queek]

So after some research I just discovered that ALL my fan headers (except the cpu one) on my Asus P9X79 Pro, which are 4 pin headers, don't actually have PWM.

 

I don't know if everyone already knew this and I'm just showing my ignorance but the fact is, you have to look in the manual to see if a header has PWM or not. There isn't anything on the box, on Asus' website or anywhere else that could make you think it's not PWM.

 

What's even more strange is that the 4th pin that isn't used for PWM is only a 5 Volts pin, meaning that any fan will run at its full speed no matter if it has PWM or not. So what's the point ? A 3 pin header would have done the exact same thing.

 

You might say well what's the problem ? A 3 pin fan can perfectly be controlled by voltage and not PWM... Yes that's true but it's way less efficient when going into very low rpm. I bought Noctua PWM fans and if I let them regulate via voltage they won't turn on until 60% of their full speed which is quite useless... might aswell put them in full constantly...

 

So yeah that's my little rant about Asus motherboards (and others because I think it's the same thing for Gigabyte ones...), I feel like I've been fooled since one of the reason I bought the board was all the 4 pin headers... but oh well I guess that's life

 

Sorry for this long annoying post, hope it might help some people though.

Correct me if I'm wrong, but I thought that PWM was a frequency thing, not a voltage thing.  The duty cycle dictates the speed of the fan, which is based on the rpm sensor (third pin).  

How did you come to determine that your 4th pin is only 5V constant voltage (the implication of your post)?  Doesn't the 4th pin provide a square wave at 5V (@25 kHz if wikipedia is to be believed).  

[Kc7vwc]

My wife's Asus m5a97 board had this issue too. Cpu fan was good, one 3pin header, and two 4pins that could do basic controls, but the variance is only 200-300 rpm between silent and performance settings.