[mythtv-users] Fancontrol with replaced CPU fan

[Johnny]

>> 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.