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 Post subject: Difference between PWR_FAN and SYS_FAN
 Post Posted: Sat Aug 30, 2014 7:26 pm 
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Scott I was recently cleaning out/dusting my pc. This is the motherboard. Anyway I decided to check the fan speed of the front fan in the case. I went into cmos setup and noticed that CPU Fan is running, Sys Fan 1 is running, but Sys Fan 2 is not. PWR Fan is.

I opened up the case and my front fan is connected to PWR FAN. It only has three leads the red, yellow, and black. PWR FAN has three pins, SYS FAN 1 has three pins, Sys FAN 2 has four. So I guess it was only logical that I plugged it into PWR FAN.

However my question remains, what is the difference between these?.


Last edited by FatGuy on Sat Aug 30, 2014 7:35 pm, edited 1 time in total.

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 Post subject: Re: Difference between PWR_FAN and SYS_FAN
 Post Posted: Sat Aug 30, 2014 7:29 pm 
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Front fan

Rear fan


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 Post subject: Re: Difference between PWR_FAN and SYS_FAN
 Post Posted: Sat Aug 30, 2014 8:09 pm 
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3-pin fan connections provide a tachometer signal so the motherboard can monitor the fan speed, 4-pin fan connections provide an additional Pulse-Width Modulation (PWM) signal so the motherboard can control the fan speed. Some motherboards can crudely control fan speeds even on 3-pin connections by varying the voltage. Note that you can plug 3-pin fans into 4-pin connectors (and vice versa), the only consequence is the PWM speed control will be disabled since there is no matching lead on the fan or motherboard. The labeling (CPU, SYS, PWR) is just so you can match any connected fans with the monitored speeds. Scott.


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 Post subject: Re: Difference between PWR_FAN and SYS_FAN
 Post Posted: Sat Aug 30, 2014 8:31 pm 
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Why the "PWR_FAN"?, or "Power Fan"?, instead of just "Fan" or "Sys Fan"?. I mean I understand I'd match in the cmos set up so I can see the rpm of each fan. I'm just confused as to why the use of the term "Pwr" on one instead of just "Sys_Fan 3"?.


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 Post subject: Re: Difference between PWR_FAN and SYS_FAN
 Post Posted: Sat Aug 30, 2014 9:11 pm 
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Some power supplies include a fan speed monitor connector that allows the motherboard to monitor the speed of the PSU fan. You could connect that monitor plug to any 3 or 4-pin fan connector on the motherboard, but some motherboards include a connector specifically labeled for that purpose. Scott.


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 Post subject: Re: Difference between PWR_FAN and SYS_FAN
 Post Posted: Sat Aug 30, 2014 9:34 pm 
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Ok then, so that is why it is called PWR. Thanks!. Otherwise it just doesn't matter, it works the same as any other fan connector?. Is monitoring the speed of the power supply fan common?. Or is it something left over from years ago?. I mean is it even useful to monitor the power supply fan speed?.


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 Post subject: Re: Difference between PWR_FAN and SYS_FAN
 Post Posted: Sat Aug 30, 2014 10:56 pm 
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Quote:
...it works the same as any other fan connector?

Yes indeed.

Quote:
Is monitoring the speed of the power supply fan common?

It is commonly provided on motherboards, but not all power supplies have the connector.

Quote:
...is it even useful to monitor the power supply fan speed?

It is useful to know if it is slowing down or stopping due to failure. Scott.


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 Post subject: Re: Difference between PWR_FAN and SYS_FAN
 Post Posted: Sun Aug 31, 2014 11:10 am 
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Useful to know, but I don't know about practical. How often have you personally used the PWR_FAN(specifically) connection to diagnose a failing power supply?. Wouldn't you just throw in a known good spare if the power supply was suspected?. As for the fan, you could just look and see if it's still spinning and if it looked abnormally slow, then something is obviously wrong. Monitoring the speed of the power supply fan electronically just seems silly and unnecessary. It seems like an extra step in the diagnosis process that's not needed. Maybe comparable to having to plug in each single little connection for the FNT_PANEL instead of having one keyed block connection!


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 Post subject: Re: Difference between PWR_FAN and SYS_FAN
 Post Posted: Sun Aug 31, 2014 12:20 pm 
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It wouldn't be used for diagnosis, more as a warning instead. Kind of like a temperature guage (or light) on a car. More useful on systems in environments where the fans can get clogged with dust and debris. Scott.


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 Post subject: Re: Difference between PWR_FAN and SYS_FAN
 Post Posted: Thu Sep 04, 2014 11:20 am 
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Ahh, Ok. So something that may be useful in say a factory type setting. Thanks for all your help Scott!. I applaud your patience with me!


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