[mythtv-users] Fancontrol with replaced CPU fan

[John Drescher]

On Mon, Oct 12, 2009 at 6:24 PM, Johnny <jarpublic at gmail.com> 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 always thought if you plugged a 3 pin fan in these 4 pin pwm headers
you would get no voltage control and the fan would run at full power
always.

John