Question about 4-pin PWM computer fans - grounding pin 4?

Thread Starter

zirconx

Joined Mar 10, 2010
161
Hi, I'm wiring up some 12v fans for my RV fridge. These are 4-pin pwm computer fans. The 4th pin is the pwm input signal. I know the fan runs fine without this signal, but it runs at 100% and I would like it to run much slower. I found that grounding pin 4 causes the fan to spin around 10% of it's full speed. Is this a legitimate way to slow the fan down? Or will it damage anything? I suspect not, that if that were the case it would have been damaged right away.

I had read that grounding pin 4 causes the fan to stop, but I have tried this with two different fans from different companies and they both slowed down to around 10%. They seem to start up fine with pin 4 grounded also.

Thanks.
 

Thread Starter

zirconx

Joined Mar 10, 2010
161
I have not been able to find a spec sheet. Computer fans are cheap and all the ones I've seen come with minimal specs, sometimes not even CFM. Just watts and volts sometimes.

But there is plenty of information explaining 4-pin PWM fans, and how the PWM signal has a 5.25 max voltage, etc. But I haven't found anyone saying - "Hey here's a neat trick. Gound the PWM pin and you can run the fan at 10% of speed." Which just happens to be the speed I need for this application. So maybe I am that guy.
 
Intel owns the 4-wire PC fan specification and some fans go down to 0% fan speed, others are undefined at the low end, others go to a minimum speed only.
I guess it's all about the circuitry inside the fans (Type A,B,C) some are really cheap and can't start spinning unless they get a bump or nudge (higher PWM request) so they would be stuck at a low speed request and apparently damage the motor.
Older spec https://www.glkinst.com/cables/cable_pics/4_Wire_PWM_Spec.pdf
 
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