Turning off PWM fans

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qwertynosik

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Jan 4, 2017
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Hello! In BIOS I cant turn off my chassis fans - I have several fan modes like silent, normal, turbo etc. I choose Manual mode to adjust the fan speed curve depending on CPU temp, but I cant set first point less than 20% of full fan speed. I have 2 fan modes for each motherboard fan ouptuts- PWM and DC. In DC mode I can set first point at 0% fan speed, but in PWM mode I cant do this. Why? I have 4-pin PWM fans and I want to have fans fully stoped when CPU temp is lower than 40-50 C. Sorry for my English.
 
Solution
clutchc
Not impossible, just not chosen to be programed that way by the motherboard/BIOS manufacturer. You'd have to contact them for the 'why'.
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clutchc

clutchc

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Because PWM (pulse width modulation) fans always have +12v applied to the fan motor. It simply modulates the DC in pulses to vary speed. 3-wire fans (commonly called DC mode fans) vary the speed by varying the voltage. So it is possible (in theory) to shut them down by lowering the voltage enough that they won't spin.
 
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Q

qwertynosik

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Jan 4, 2017
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Ok, but why it is not possible to simply turn off second pin voltage? If there would be 0v instead of permanent +12v, the fan would be stopped. And then, if the CPU temp would be higher than 40 or 50 C, the fan will turn on and will working at 20% RPM or higher. All the time CPU temp is lower than 40C the second pin voltage is equal to 0v. Why it is impossible?
 
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