[mythtv-users] Fancontrol with replaced CPU fan

[Brian Wood]

On Monday 12 October 2009 16:24:41 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.

Yes, simply varying the voltage isn't the best way to control a DC motor's 
speed, but you can get away with it at the low currents most fans use. PWM is 
in fact a more efficient way to control the speed, less power thrown away as 
heat. The mobo may in fact be using a PWM variable voltage source, I'm not 
sure.

I have a few little modules that can control a fan's speed using PWM, they can 
be had cheaply, but use a manual potentiometer to set the speed. I use them 
to set the maximum speed the mobo can run the fan(s) at.

When you're talking about comsumer electronics, Cheap is the watchword, though 
even on high-end server boards I think they use the same control method.

-- 
Brian Wood
beww at beww.org