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From: Jan Panteltje <alien@comet.invalid>
Newsgroups: sci.electronics.design
Subject: Re: cpu cooler fan
Date: Sat, 30 Nov 2024 07:24:43 GMT
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On a sunny day (Sat, 30 Nov 2024 04:03:57 -0000 (UTC)) it happened "Don"
<g@crcomp.net> wrote in <20241129a@crcomp.net>:

>bitrex wrote:
>> 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.
>
>Figure 2 of an AD App Note shows a typical schematic for a circuit
>embedded in a 4-wire DC fan. The embedded circuit contains three
>N-channel JFETS, a Hall sensor, and a couple of resistors.
>
>It's a brain teaser to deduce reversal repercussions. When the Tach's
>temporarily disconnected the PWM periodically pinches all of the JFETs
>off. Someone smarter needs to untangle what happens when 12 VDC is
>re-connected to the Tach output.
>
>    Why and How to Control Fan Speed for Cooling Electronic Equipment
>
>    ... In addition to the power, ground, and tach signal, 4-wire fans
>    have a PWM input, which is used to control the speed of the fan.
>    Instead of switching the power to the entire fan on and off, only
>    the power to the drive coils is switched, making the tach
>    information available continuously. Switching the coils on and off
>    generates some commutation noise. Driving the coils at rates
>    greater than 20 kHz moves the noise outside of the audible range,
>    so typical PWM fan-drive signals use a rather high frequency
>    (>20 kHz). Another advantage of 4-wire fans is that the fan speed
>    can be controlled at speeds as low as 10% of the fan’s full speed.
>    Figure 2 shows the differences between 3-wire and 4-wire fan
>    circuits. ...
>
><https://www.analog.com/en/resources/analog-dialogue/articles/how-to-control-fan-speed.html>

Nice, thank you!