Path: ...!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail
From: "Don" <g@crcomp.net>
Newsgroups: sci.electronics.design
Subject: Re: cpu cooler fan
Date: Sat, 30 Nov 2024 04:03:57 -0000 (UTC)
Organization: A noiseless patient Spider
Lines: 63
Message-ID: <20241129a@crcomp.net>
References: <r5pckjlmmqfvj0ahde5ds34snu8k0ia6sp@4ax.com> <6746af8c$0$212419$882e4bbb@reader.netnews.com>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit
Injection-Date: Sat, 30 Nov 2024 05:03:58 +0100 (CET)
Injection-Info: dont-email.me; posting-host="6b223988f703b8852950946a4ea7b3ac";
	logging-data="1597262"; mail-complaints-to="abuse@eternal-september.org";	posting-account="U2FsdGVkX1+dliuyoRrW/igqnDtld7kY"
Cancel-Lock: sha1:QI4HzQZ0m8XpHfH0mIh/JX5tKG8=
Bytes: 3715

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>

Danke,

-- 
Don, KB7RPU, https://www.qsl.net/kb7rpu
There was a young lady named Bright Whose speed was far faster than light;
She set out one day In a relative way And returned on the previous night.