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Can a PWM fan be turned on and off through the control pin? Or can it only adjust speed? Is it just a fet that turns the 12V on and off?

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  • \$\begingroup\$ Post a link to the fan's datasheet, please. \$\endgroup\$ Feb 5 '16 at 18:43
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    \$\begingroup\$ Which is the 'control' pin? According to Intel's spec. for PWM computer fans: If you provide 0 volts to the 'PWM' pin the fan will not spin. If you provide 5 volts to the 'PWM' pin the fan will spin at 100% of it's rated speed. A ~25kHz square wave (of 0 to 5V) will cause the fan to spin at a speed linearly proportional to the duty cycle, although performance is 'undetermined' below a 20% duty cycle. \$\endgroup\$ Feb 5 '16 at 18:48
  • \$\begingroup\$ Okay, this is how I was hoping it would work (0V on the PWM pin would be off, and 5V on), but it doesn't seem to be working that way. It is the Arctic F9 PWM arctic.ac/us_en/arctic-f9-pwm-co.html \$\endgroup\$
    – seven68
    Feb 5 '16 at 18:51
  • \$\begingroup\$ I thought maybe I had a ground issue since the fan is powered with a wall wort (12 V) and my control / pwm / 0V,5V signal was from an arduino board, but that is not the cause. I am trying to turn it on and off with a digital out from the arduino. Changing the voltage on the PWM/control pin does nothing to change the fan speed. Any suggestions? I could just use a fet, but I thought a pwm fan would make this easier. \$\endgroup\$
    – seven68
    Feb 5 '16 at 19:47
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    \$\begingroup\$ It depends on the fan. I've worked with fans that, by spec, will not go slower than 30% of full speed, even at 0% PWM. In this case, to actually stop the fan, you'd need to cut the +12V. Read the datasheet! \$\endgroup\$
    – DoxyLover
    Feb 5 '16 at 20:03
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Before 4-pin fans became common, people had to build their own fan controllers to change the voltage applied to 3-pin fans. Some folks used rheostats, just burning off the excess power as heat, others chopped the incoming 12V, making a rudimentary form of buck converter.

The 4-pin fans simply integrated the PWM control circuit inside the fan -- this may be implemented in many ways, all dependent on the fan model. Dead simple brushed DC motors probably have a single FET in there that is roughly chopping the voltage. If there's a fan built with a BLDC motor, it's probably using the PWM signal to drive the BLDC motor controller.

enter image description here

Your fan's datasheet seems to show that at PWMs below roughly 10%, it will not spin (for a 12V supply). If it is actually spinning, I would measure the voltage on the PWM line -- perhaps something else is driving onto it.

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  • \$\begingroup\$ Based on this, and the next answer, I would expect 0V on the PWM fan (0% duty cycle) to turn the fan off, and 5V (100% duty cycle) would turn the fan on to its max speed. HOWEVER, this is not the case. Connecting the PWM pin (blue wire) to either the +12 V (yellow wire) or 0 V (black wire) does nothing to change the speed of the fan. Is there some subtlety about the 25 kHz frequency that I don't understand? There must be something to that, right? \$\endgroup\$
    – seven68
    Feb 6 '16 at 16:48
  • \$\begingroup\$ So I got the arduino running the PWM pin at 25 kHz, and I am able to change the fan speed. However, the fan cannot be completely stopped... in fact as the average voltage drops (duty cycle lowers) below a critical value of about 1 V, the fan spins up to max speed again. So yeah, there's clearly some other crazy stuff happening in the fan circuitry. The fan's behavior does not seem to follow these RPM vs PWM% curves shown here either. I guess I'll just use a fet to turn the 12 V on and off. \$\endgroup\$
    – seven68
    Feb 6 '16 at 18:13
  • \$\begingroup\$ I feel like connecting the PWM pin directly to 12V would potentially destroy the control circuitry inside -- isn't PWM for those fans a 0-5V signal? \$\endgroup\$ Feb 6 '16 at 18:45
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There are three possible implementations according to Intel's specs.

Type A: enter image description here

Type B: enter image description here

Type C: enter image description here

If your fan happens to be a Type A it (a bit like Friedman & Rosenman's human 'type A' personality category) will not shut down from the PWM input.

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