On Wed, 14 Jan 2009 11:22:59 -0700, David Hubbard wrote: > > What does this have to do with 4-pin fans? This is indeed what I have, > > but I can't see the relation. I thought that the 4th pin was simply > > there to get proper speeds reported even when low PWM duty cycles were > > used. If there more to it than that? > > > > This is the only 4-pin fan I have and, as far as I know, the only > > mainboard I have that support a 4-pin fan, so I can't do that many > > tests. > > I've seen it on a motherboard with an it87 driving an intel 4-pin fan > that came with the CPU. It seems to be circuitry inside the fan. (So, > replacing the fan might work, no guarantees.) Ah, I get the idea now. On 4-pin fans, PWM is only a signal, not a power source. This explains why the PWM response curve is very different from what you get with a 3-pin fan: it's almost linear and has a non-zero minimum, while usually you get a non-linear response (upper half of PWM values has little effect) and PWM=0 stops the fan. This indeed suggests some embedded electronics in the fan itself. And it also means that each 4-pin fan can have its own max _and min_ speed limit. I'll make sure to pay attention to this next time I must buy a CPU fan. -- Jean Delvare
