On Wed, Jun 13, 2007 at 11:17:34PM +0100, Phil Endecott wrote: > I agree with your analysis, but note that the control components are > all marked "NC", which I imagine means "not connected", while the 0-ohm > link is not marked "NC". Ahh, very good... err bad. > > C3) Bypass cap for the power supply node of the fan? > > This is odd; the Fintek datasheet shows a capacitor in this position, > but doesn't give a value. Putting a largish capacitor here (this is 10 > uF I think) will rather screw up the PWM control in exactly the way > that Jean and I have been discussing, i.e. even if the "on" period is > quite short it will be enough to charge the capacitor for the whole > cycle. Having experimented I have not added a capacitor in that > position on my hack. > > They may be imagining that it has the effect of "smoothing" the PWM > signal (e.g. 50% 12v smoothed to 6v). To make this work you would need > an inductor and a diode and you'd have built yourself a simple > switch-mode supply. I was confused on this too, since you would now have an exponential discharge at every pulse... so you get that weird ramp / triangle-wave behavior you were seeing. Is there something about the fans that makes it better to always have some sort of DC available instead of switching to ground? The only other thing I could think of would be for noise / EM reasons -- so that you are not switching all the way to ground? > Do you have any firm indication that the EPIA C7 board does have fan > control? Only annectodal evidence of people getting lm-sensors/fancontrol working on some EPIA boards. I have a request open with the local Via FAE to get an official answer. I'm hoping that the local electronics store might have some in stock and I can go eyeball em. -m
