[mythtv-users] Fancontrol with replaced CPU fan

[Paul Gardiner]

Johnny wrote:
>>> I'd assumed this was the way all fans worked in PCs - they do indeed vary
>>> the voltage to vary fan speed.
>> Oops, should have added "on moderately modern hardware". Old hardware does
>> in fact just supply a fixed 12V and the fan runs at constant speed.
> 
> Actually newer hardware can often use pulse-width modulation (PWM).
> Basically, they always apply the full voltage but they cycle it on and
> off at different duty cycles to vary the speed. It is more efficient
> when done this way. What I don't completely understand is they can do
> this with the 3-pin fans. But the 4-pin was introduced with a pin
> designated for PWM. I know what PWM is but I don't really understand
> the details of how it is implemented in these two cases. I used
> pwmconfig to set things up and it can control my fans on both the
> 3-pin and 4-pin headers.

I've found a good explanation on the web. The main points are:

* Just 2 pins are required to supply and control power.
Both the PWM mode and varying voltage modes can be used through 2 pins.
Varying the voltage is inefficient because there will be power loss
through whatever drops the voltage, whereas PWM is almost always running
full on or full off.

* One problem with PWM: the PWM switching frequency can sometimes
become audible, with the fan playing the part of a loud speaker.

* The 3rd pin is for feedback of the speed (the tachometer signal).

* There's a problem with using PWM through 2 pins in that during
the off phase of the PWM signal, there is nothing to power the
tachometer circuitry. Some motherboards get around this by setting
small periods of full power during which to sense (although why
they don't just store power in a big capacitor I don't know).

* The idea behind the 4th pin is to send the power and the PWM
switching signal separately. This has several advantages. The
tachometer circuitry is permanently powered. The motherboard
doesn't have to switch the power - it just need to send a low
power switching signal. Higher switching frequencies can be used
outside the audible range.

The source is: 
http://www.analog.com/library/analogdialogue/archives/38-02/fan_speed.html

My guess would be that most motherboards with 4pin connectors would
be able to run in a 3pin mode and still control speed. Probably
a setting in the BIOS. I've seen a 3pin/4pin setting in at least
one BIOS.

Paul.