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Date: Wed, 27 Nov 2024 00:35:09 -0500
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Subject: Re: cpu cooler fan
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On 11/26/2024 7:17 PM, john larkin wrote:
> A cpu cooler has a 4-pin connector for the fan.
> 
> Does anybody know if the fan will blow up if the connector is plugged
> in backwards?
> 


I did a quick test on a spare fan...so if the pins are reversed we get 
+12 from the mobo to the RPM output signal from the fan, 12 volt input 
for the fan connected to the RPM signal input on the mobo, PWM output 
from the mobo to fan ground, and motherboard ground to PWM input on the fan.

MOBO PWM OUT <--> FAN GND
MOBO +12 OUT <---> FAN RPM OUT
MOBO RPM IN <--> FAN +12 IN
MOBO GND <--> FAN PWM IN

I think Rpm in to fan +12 in is nothing, like connecting a high 
impedance digital input to the input of voltage regulator, maybe. For 
the +12 out to fan RPM out...all RPM outputs are open collector AFAIK so 
if the fan isn't powered and isn't spinning I don't think there's any 
harm there, I don't see any current flow when fan PWM in is grounded and 
I put +12 there, anyway.

However reversing the PWM and GND pins looks to cause to a pretty good 
short, 85 mV at 10 mA current limit. The PWM drive is probably current 
limited, and/or the mobo may detect that and shut down I dunno.