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Posted on 2003-12-09
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ok, I'm debating what the volages are supposed to be for a motherboard fan header. Usually there are 3 pins on a fan header. Since a power supply supplies 5v, 3.3v, and 12v, you would think that the fan header would output a combo of these voltages. I know that one pin will be the ground. The other 2 on older systems I think would be 5v and 12v. My main question is this. I know that the fan speed on many systems is voltage regulated by the motherboard. I saw a site that said the 12v rail can range from 1v to 12v. When the voltage dropped below 1v, the fan would turn off, when voltage rises above 10.5 volts, the fan would spin at full speed. I'm curious what the other pin should be...a constant 5v? Or does that voltage vary too? I cant find any more solid info on this. Any additonal info would be greatly appreciated. In my job, we check voltages of the fan header and I need to know what voltages constitute a good fan header and what voltages are considered bad. PLEASE POST THE CORRECT VOLTAGES OR RANGE OF VOLTAGES FOR MOTHERBOARD FAN HEADERS THAT VARY FAN SPEED BY GIVING DIFFERENT VOLTAGES.

Thank you!!

TAZ :-)
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Question by:tazlastic
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LVL 18

Accepted Solution

chicagoan earned 672 total points
ID: 9908079
the pins are 12v, ground and a 'sense' connection that reads the speed of the fan.
DC fans can run from 0-12 volts

see http://www.heatsink-guide.com/tempcontrol.htm
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LVL 69

Expert Comment

ID: 9908096
You are right in that one pin is ground and another is 12v.  The third is not supplied power, it is used by the motherboard to read fan speed - that is how you are able to see it reported in the BIOS screen and in monitoring programs.  If you have a fan with just two wires, no speed monitoring can be done.
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Expert Comment

ID: 9908097
I have to type faster.
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LVL 17

Assisted Solution

rayt333 earned 664 total points
ID: 9908098
Most MB's supply a contast 12 volts to the fan header, of course this could be from 11 1/2 to 12 1/2 actual voltage depending on the power supply and the load againgt it. with a heavy load the voltage may drom into the 10 volt range (which is not good)
What MB are you using?  this would help us give you more detailed specs maybe.
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LVL 18

Expert Comment

ID: 9908127
2 minutes to type THAT! use the other index finger!
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Author Comment

ID: 9908239
wow, quick response, thanks all. One part of the questions isnt answered, if one pin is ground, the other is between 0v and 12v, and the third is used to sense the speed...when testing a mb, I do get voltage from both pins other than ground...for example, the MB I just tested readed 1.3v on one pin and 3.9v on the other...would the 3.9v be what would be considered the 12v rail and the 1.3v be the sensing pin? if so, what is the correct coltage that should be coming from this 'sense' pin? thanks again for help...

TAZ ;-)
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LVL 18

Expert Comment

ID: 9908365
This is an anaolg circuit which uses voltage pulses, which are generated by the fan current
flowing through a sense resistor, are detected at the SENSE pin and used to calculate the fan speed.
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Author Comment

ID: 9908413
so there should be no voltage from the 'sense' pin?
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LVL 9

Expert Comment

ID: 9908544
you are correct.
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LVL 69

Expert Comment

ID: 9908576
>2 minutes to type THAT! use the other index finger!
Heh, you got my number... :-)

I was actually thinking and composing as I went along - I said "Self, no one's gonna answer this one before I hit enter"... wrong!
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Author Comment

ID: 9908587
ok, so if I'm getting over 1v on the 'sense' pin, is that indicate a faulty mb fan header or is it prolly just negligable voltage leaking through...are there any fan headers that are supposed to give vltg out of both pins? thnx again.

TAZ :-)
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LVL 69

Assisted Solution

Callandor earned 664 total points
ID: 9908631
I just popped the cpu fan connector off on my Asus P4B533 motherboard and measured the voltage - I got 11.78v and 5v, so the presence of voltage on that line is not an indication of trouble.
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LVL 31

Expert Comment

ID: 9908695
Fan operation can be sensed in various ways... Volatge on the "sense" pin is not necessarily a problem, it may be used by the sense function, by a circuit  detecting a speed-related demand from the fan. The putput voltage should probably be 12V or less, depending on how fast the fan is supposed to run, according to temp etc.
/RID
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Author Comment

ID: 9908793
so basically on the system I just tested, I got around 1v from 1 pin and about 4v from the other pin. The cpu fan wasnt spinning. Should I assume that the 1v is from the 'sense' pin and the 4v is from the power pin that could go up to 12v but wasnt because the temp was low? In that case, I would assume that 4v would be enough to make a fan spin and that the cpu fan is bad? Is 4v enough to spin a fan? thnx again, this will be my last question.

TAZ :-)
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LVL 18

Expert Comment

ID: 9908892
> Is 4v enough to spin a fan
It's plenty for a 4 volt fan

every case and CPU fan i've seen is 12v

Whether a 12v fan would spin - prolly - although a diry old bushing type might not spin with 50v

If the CPU fan header has low voltage, and there isn't some sort of onboard rpm control, I'd say there's a problem somewhere
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LVL 69

Expert Comment

ID: 9909143
I just tried running a 12v case fan with 6v and it spins, but barely pushes any air.  It is also completely silent in operation.
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LVL 69

Expert Comment

ID: 9909159
The fact that you're not getting anything near 12v and there's no attenuation going on, as chicagoan says, says it's broken to me.
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Author Comment

ID: 9913294
Ok, but....if the mb gives the fan more or less voltage depending on temperature...and if the system was cold, then the 4v would be enough to spin the fan slightly and the 1v could be just leakage through the sense line. I read that a fan that was voltage regulated by the mb would spin if it received 1 volt or higher....under 1v and the fan would shut off...over 10.5v and the fan would run at full speed. So, basically if its a fan speed voltage regulating mb, what range of voltage is acceptable. It seems to me that 1v ~ 12v would be acceptable. Also, it seems that you all agree that any low voltage is acceptable coming from the sense line...even 0v? So when testing the 3 pins on a fan header, one pin is ground, the other pin is a sense line, but what is the acceptable range for the 3rd pin. Can anyone find any solid info on this? Can anyone find anything to back up the info I found about the voltage ranging from 1 - 10.5? thanks again.

TAZ :-)
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LVL 18

Expert Comment

ID: 9913607
This analogue circuit uses amplitude modulation to do signalling. Current is applied to the circuit and a mechanism changes it.
Here we're talking about a tachyometer. As the fan spins, each revolution attenuates the voltage and the sense circuit counts the pulses. The voltage is not important, it is determined by the designer of the sense circuit.
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LVL 69

Expert Comment

ID: 9913632
Which motherboard is it?  If you're driving an AMD, I would think that the fan is always running at full speed, because if it didn't, you would end up with a fried cpu.  Case fans, however, are another story - I would expect them to kick in when the case interior started to heat up.
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Author Comment

ID: 9914316
The actual system I'm talking bout is a sony sys with a p4 cpu, however, this is more of a general question about the correct voltage that a 3 pin fan header supplies that is voltage regulated. There should be some solid documentation onj this but I have yet to find any....

TAZ
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LVL 18

Expert Comment

ID: 9914336
99.9% of motherboard fan headers run supply full voltage 100% of the time.
There is usually a thermistor that will alarm and shutdown at temperatures stated in the BIOS setup.
Some systems will shut down if CPU_FAN RPMs fall below a certain level.
It's very unusual to see a motherboard with a temerature controlled fan circuit, case or cpu.
Many such aftermarket devices exist, though.
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LVL 3

Expert Comment

ID: 9916637
Even a speed controlled header should deliver 12V albeit over a reduced duty cycle.  In my experiments, anything less than about 7V won't overcome the static friction to start the fan.  As others have noted, the sense voltage is unimportant (a nominal 5V on the open pin sounds pretty reasonable).  If you are trying to figure out which is which, look at the fan wires:  black is ground, red is +12 and yellow or blue is sense.

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LVL 18

Expert Comment

ID: 9916872
> In my job, we check voltages of the fan header and I need to know what voltages constitute a good
A fan is pretty forgiving - 11 - 13 volts and it will perform within spec.
That is not to say anything else on the motherboard will work properly, but that's not your job.
Somebody else must test the power supplies I gather.

>PLEASE POST THE CORRECT VOLTAGES OR RANGE OF VOLTAGES FOR MOTHERBOARD
>FAN HEADERS THAT VARY FAN SPEED BY GIVING DIFFERENT VOLTAGES
If there are motherboards that incorporate variable speed fan headers, this information would be proprietary to the design of each motherboard.
AFAIK, there are no IEEE, CIGRE, UL, CEC or Good Housekeeping Magazine guidlelines.
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LVL 69

Expert Comment

ID: 9917087
>Good Housekeeping Magazine guidlelines
LOL! My computer interior would fail this, with all the dust blowing around!
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LVL 18

Expert Comment

ID: 9917123
those were the main regulatory bodies I was able to research, if this is inadequate, I'll look into GQ's suggestions
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LVL 69

Expert Comment

ID: 9917915
>if this is inadequate, I'll look into GQ's suggestions
You're cracking me up...
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