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BobingAbout

4pin CPU Fan header

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the CPU fan on my system makes up for about 90% of the noise, i installed the backplate speed controler that came with it and that made it quite, but i don't have any windows temperature measurement tools for this motherboard, so, i have no idea when it is a good time to turn the speed up...

 

ok, so i know its PWM, but, do you have any specs on the voltages given out?

 

i know this board has 3 settings, so, could it be something like:

off: constant 0V

slow: pulsing between 0v and 12v

full: constant 12v

if so, i can easily make something that can handle this, however, i doubt the 12v from the speed control line can power a fan, so, it would require some kind of buffer, this would work with either a transistor, an op-amp, or because it is digital, even a CMOS chip. (using a CMOS chip and a capacitor, this would give me 0v, 12v, or about 6v output(assuming a 1:1 mark to space ratio on the pulses), off, full and slow.)

 

however, i've looked and can't seem to find any specs, thats why i asked here.

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Hi

 

I purchased a CPU cooler for my daughter's PC that had a 92mm fan and that came with a 4pin 'plug' for the fan as standard.

 

As my daughter's board is the DFI Expert board I had to replace the fan with a standard 3pin fan.

 

Interesting information though so it will be interesting to see where this new 4 pin arrangement leads everyone in the future

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@radodrill, thanx for the enlightening info bro, appreciated.:)

Glad I could help.

the CPU fan on my system makes up for about 90% of the noise, i installed the backplate speed controler that came with it and that made it quite, but i don't have any windows temperature measurement tools for this motherboard, so, i have no idea when it is a good time to turn the speed up...

 

ok, so i know its PWM, but, do you have any specs on the voltages given out?

 

i know this board has 3 settings, so, could it be something like:

off: constant 0V

slow: pulsing between 0v and 12v

full: constant 12v

if so, i can easily make something that can handle this, however, i doubt the 12v from the speed control line can power a fan, so, it would require some kind of buffer, this would work with either a transistor, an op-amp, or because it is digital, even a CMOS chip. (using a CMOS chip and a capacitor, this would give me 0v, 12v, or about 6v output(assuming a 1:1 mark to space ratio on the pulses), off, full and slow.)

 

however, i've looked and can't seem to find any specs, thats why i asked here.

The fan header has the following pinout; 1=Gnd, 2=+12V, 3=Sense, 4=Control.

 

The control line would be a low current voltage oscillating between low and high voltage (probably ~0V and ~12V). This would be a square wave with period T=tl+th; where both tl and th are continuously variable. Connecting pin 2 to the collector (of a transistor) and pin 4 to the base would provide an oscillating +12V signal (at the emitter) to drive the fan. The fan speed is controlled by modulating tl and th; th~=T (tl~=0) results in the fan spinning full speed , tl~=T (th~=0) turns off the fan, and any other tl+th=T produces a slower than max fan speed. {the same general principle is applied in DCC model railroads; except that the supply voltage and control signal come across the same carrier by means of superposition (low frequency AC source and high frequency AC control signal)}

Here is an example of using an LM555 timer to control a Fan. By using a transistor and a capacitor in parallel with the fan (across the emitter of the transistor and the Gnd line) you should have a constant current voltage regulator for a 3pin fan.

 

Legacy (3pin) fans use similar circuitry within the MB to reduce the voltage on the +12V line as a means of controlling the speed; however, using PWM to rapidly switch on/off the fan motor is a more precise and reliable solution. Here and Here are some introductions to PWM; or Google for "PWM", "Pulse Width Modulation", "PWM Control", etc.

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i took some measurements, it apears that below the "off" temp, it uses PWM and results in a 20kHz+ signal, RMS 1.3V, in the mid range, or the "full" range, it apears to output 5V DC.

You would want to connect an oscilloscope across the control and Gnd lines so you could actually see what kind of a signal the MB is outputting.

 

I don't know if any special software is required to utilize the feature offered by the control line of the 4pin header

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Awesome PWM explanation, cleared up a lot for me!

 

~Ibrahim~

Glad to hear it; I tried to provide it in Lehmann's terms, but if you would like more details on a simple PWM circuit using a timer (with some info on the signal produced/calculations) I could provide it to you. {used in a EE lab I took to flash an LED}

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You would want to connect an oscilloscope across the control and Gnd lines so you could actually see what kind of a signal the MB is outputting.

 

I don't know if any special software is required to utilize the feature offered by the control line of the 4pin header

 

i can't fit an oscilloscope in my backpack, so i borrowed a rather expensive digital multimeter which includes RMS voltage measurements and a frequency counter.

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i can't fit an oscilloscope in my backpack, so i borrowed a rather expensive digital multimeter which includes RMS voltage measurements and a frequency counter.

 

For a PWM control signal the voltage (Peak and RMS) is not very critical (unless it is directly powering a motor; usually not the case); the frequency counter will only provide the period of the PWM signal but not the time periods for which the signal is high/low (th and tl).

 

i took some measurements, it apears that below the "off" temp, it uses PWM and results in a 20kHz+ signal, RMS 1.3V, in the mid range, or the "full" range, it apears to output 5V DC.

 

PWM signals are usually high frequency to make it easier to filter out for certain applications (i.e. DCC control of model railroads) where the supply and signal are delivered over the same carriers. The "full range" 5VDC output indicates that the Vh=5V and th~=T~=20kHz, tl~=0; knowing that the "mid range" provides an output Vrms=1.3V does not tell any info about Tl and Th, only that the it is an oscillating signal.

 

If you can get ahold of an oscilloscope please use it to get more details of the characteristics of the signal.

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1.3V RMS, assuming 5V high and 0V low squarewave, presumes that it is low about 3 times longer than it is high.

 

since there are only 3 speed settings, off, low and on, having 2 settings at constant 5v is confusing.

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