cpu fan controlled by mobo 4 pin plug?

71monaro · Post by **71monaro** » Sun Nov 05, 2006 3:45 pm

My question is, how do I use a 3 wire fan (as most good ones are) off my 4 pin mobo, with mobo controlling speed and sensing correct rpm? I dont think possible. Then why arent more fans 4 wire (and I dont mean just 4pin molex)My Nexus fan supplier says the realsilent fans are available in 3 or 4 pin but I am assuming when they say 4 pin it is a molex plug, not mobo 4pin? Iread the recent thread titled 4 pin fan. and seemed to be no answer besides buying a JMC fan.
It seems (electrically) an easy mod to add the permanent 12v wire to speed sense circuitry, but has anyone succeeded?

pyogenes · Post by **pyogenes** » Sun Nov 05, 2006 5:39 pm

With a few exceptions, you can plug a 3-pin fan into a 4-pin CPU fan header. The little ridges on the connector will prevent you from plugging it in wrong. Many (but not all) motherboards will be able to control the speed of a 3-pin fan. What MB do you have?

1st and 2nd pins = power and ground
3rd pin = rpm signal
4th pin = PWM signal from MB to fan

71monaro · Post by **71monaro** » Sun Nov 05, 2006 6:22 pm

I have purchased an Asus p5B and an XP-120. Still gathering parts before build. I have a Scythe 120x25mm or a Nexus 120x38mm(aluminium frame) both 3 wire fans.

When plugging onto this mobo and not using the PWM lead, can the ASUS Q-fan control the 'power' out linearly with cpu temp?

Does it vary the Chasis fan connects linearly with other thermal properties measured?

orihara · Post by **orihara** » Sun Nov 05, 2006 6:52 pm

As far as I can tell, the P5B is unable to control the speed of a 3-pin fan on the CPU header. It can control headers CHA1-3, but they all recieve the same output.

Chocolinx · Post by **Chocolinx** » Sun Nov 05, 2006 7:07 pm

Yeah for my P5B I just connected my Nexus to a CHA fan connector to control the fan speed on the CPU. On Speedfan 4.31 you can control the speed of the fans. Speed04 controls the speed for all fans at the same time CHA 1 through 3.

You'll have to go to your bios and put Ignore on your CPU RPM so that you don't get an error.

71monaro · Post by **71monaro** » Sun Nov 05, 2006 7:31 pm

If you are turing off rpm speed sense of the cpu fan by the mobo, then why cant you still use the PWM output of the cpufan header to control the cpu fan? I thought the only problem, was the pwm causes havic with the speed sense, so the 4th wire gives a constant 12v to the fans speed sense circuitry. leaving the pwm to control the fan speed only.

cmthomson · Post by **cmthomson** » Mon Nov 06, 2006 5:18 pm

71monaro wrote:If you are turing off rpm speed sense of the cpu fan by the mobo, then why cant you still use the PWM output of the cpufan header to control the cpu fan? I thought the only problem, was the pwm causes havic with the speed sense, so the 4th wire gives a constant 12v to the fans speed sense circuitry. leaving the pwm to control the fan speed only.

No, it's the other way around. On a 4-pin header, the base 3 pins are ground, 12V and sense, and the fourth is a PWM control line. The PWM signal gates the 12V inside the fan, rather than on the motherboard.

On a 3-pin header, the 12V pin is modulated, either with PWM or DC, depending on the motherboard. With some fans and some motherboards, using PWM on a 3-pin header messes up sensing. But not on all of them... Flip side, if DC is used, if the voltage is too low, the fan may not start...

71monaro · Post by **71monaro** » Tue Nov 07, 2006 1:30 am

The PWM signal gates the 12V inside the fan, rather than on the motherboard.

So what your saying is the PWM wire from the MOBO carrys some varying signal that is used by the fan to control its pwm circuit inside a special fan.(that has a PMW circuit inside it). It is eaither a trigger pulse or a varying DC signal, neither of which can directly run a fan.

Is this correct?

Someone must have the circuit for taking that control signal and turning it into a 12 amplitude PWM signal.

Will the mobo still need a speed sense signal to correctly control the cpu fan speed dynamiclly with cpu temperature?

cmthomson · Post by **cmthomson** » Tue Nov 07, 2006 4:03 pm

71monaro wrote:
The PWM signal gates the 12V inside the fan, rather than on the motherboard.
So what your saying is the PWM wire from the MOBO carrys some varying signal that is used by the fan to control its pwm circuit inside a special fan.(that has a PMW circuit inside it). It is eaither a trigger pulse or a varying DC signal, neither of which can directly run a fan.

That is correct. The fourth pin carries a low-power variable-duty-cycle square-wave control (PWM) signal that is used inside the fan to switch the higher-power 12V line. The tach uses the unmodulated 12V line to produce a clean signal.

Someone must have the circuit for taking that control signal and turning it into a 12 amplitude PWM signal.

Not much of a circuit. Just a transistor with the gate connected to the PWM wire.

Will the mobo still need a speed sense signal to correctly control the cpu fan speed dynamiclly with cpu temperature?

The speed control and speed sensing are independent. In fact, all control software I'm aware of uses a temperature sensor to dial the effective fan voltage up and down (more pedantically to vary the PWM duty cycle), and only uses the tach signal to raise an alarm if the fan isn't spinning.

71monaro · Post by **71monaro** » Wed Nov 08, 2006 12:12 am

[quote="cmthomson"]The tach uses the unmodulated 12V line to produce a clean signal.
[quote]

Do you think it would be possible to add this wire to the tach generator in the fan?.

Or it may be easier to find the coils of the motor, disconect from there current source and add a wire as a fan feed. This wire then modulated as mentioned externally.

Any experts on the fan internals confirm this?