4 wire speed control computer fans

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Anyone know how these things work, ie how is the speed control
implemented with the 4th wire.
Most modern motherboards now seem to have these for the CPU and
sometimes one case fan.


Re: 4 wire speed control computer fans

"Mauried"
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**  Google  " PWM fan ".

Or read this or both.

http://www.formfactors.org/developer/specs/4_Wire_PWM_Spec.pdf




.....   Phil




Re: 4 wire speed control computer fans
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Excellent
thanks for that

Re: 4 wire speed control computer fans

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It's a 5V 25khz PWM signal.
You need to let it spin up first. I have one motherboard that always cycles
them at 50% duty right from power on, only problem is it does this straight
from bootup, so some fans don't even spin up and the bios just cuts out with
CPU fan fail error for under 800rpm... the only way I found around it was to
hold reset until the fans spin up. Because of all the dodgy brands of fans
you will find that some need way longer than 30% PWM to keep spinning and
some won't even start spinning unless they have 100%.



Re: 4 wire speed control computer fans

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Thanks guys.
Looks like I have a dodgy fan.
Buzzes a lot when powered then  turns slowly then stops completely.
If the 4th wire is left unconnected it spins at full speed.
This one is CPU fan, so better to have it run flat out that not at
all.


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