Hi Phil, On Mon, 11 Jun 2007 00:56:45 +0100, Phil Endecott wrote: > Jean Delvare wrote: > > I published my script here: > > http://khali.linux-fr.org/devel/lm-sensors/draw_pwm_response.pl > > It writes the results to a CSV file, it's up to you to make a graph out > > of it (I think I used OOo calc). > > Mine is here; I modified your script a bit: duty cycle increases on the > X axis, it uses a different range of PWM frequencies, and I plotted it > using gnuplot: > > http://chezphil.org/tmp/fanplot.png > > Based on this I think I'll reduce mine below 40 kHz, maybe 25 kHz or > something like that. You graph looks good, you fan seems to be better than mine. Admittedly, this is a rather old 80 mm fan which I nailed on top of my CPU heatsink just because it was relatively silent, compared to everything I bought since then. It didn't originally meant to control its speed. I agree that my X axis is in the wrong direction and that's somewhat confusing. I did it that way because it's easier (and safer) to sample the speeds while the PWM duty cycle decreases rather than increases - but I should have reversed the array of values before drawing the graph. > > I confirm that I didn't modify my board in any way, maybe it includes > > some additional circuitry. The problem isn't totally fixed though, as > > you can see on the graph, below 1000 RPMs I lose the fan speed readings > > (there were a couple bogus values which removed from the graph to make > > it clearer.) Is it even worst on your side? > > Are you sure that it was a reading problem, not that the fan had > actually stopped? I could believe that fans have a minimum speed below > which they don't have enough rotational momentum to get past the "dead > points" in the magnetic field. > > As you can see, my fan runs down to a few hundred RPM. I haven't > experimented further with that. This was one part of the effect I observed back then, and my graph shows it for low frequencies: the fan stops way before the 0% duty cycle. But on top of that, before the full stop, some values were obviously bogus: I could see the fan spin relatively slowly as expected, but the driver was telling me speeds like 3000 RPM - more than this fan can actually do. So I think that artifacts were confusing the tachometer somehow. This is something to remember, because when this happens, this means that the given frequency isn't suitable for automatic fan speed control. -- Jean Delvare
