Subject: Temp Controlled fan circuit trouble
By: Jpoulsen (IP: 208.254.149.*)
Written on: 16-11-2005 15:19

First i would like to thank the contributor of the temp controlled fan circuit article.

Next i have a problem with the circuit. The fan appears to not really 'come on' or 'shut off' when the temp thresholds to turn it on are reached. What occurs is that the fan buzzes and then starts to turn very slowly as the temp rises.

When i put a voltmeter on the fan leads and turn the pot screw, the voltage at the fan leads varies linearly with the adjustment of the pot screw.

Shouldnt the transistor keep this from occuring. I am using the radio shack transistor? Anyone have any advice?

By: Phil (IP: 212.159.31.*)
Written on: 17-11-2005 18:38

This is how it's supposed to work. They come on at a low speed which will depend on your specific fans and how you've set it via the trimming pot, then turn faster as the temperature rises. The idea being to only turn the fan fast enough to keep the temperature at about the same level, rather than on or off like the electric engine cooling fans in most cars.

I have built quite a few of these, and find that they seem to have a temperature range of about 4 or 5 degrees C. By this I mean the difference between being off (or just starting to spin the fans) and the fans being at maximum RPM.

In one PC I have 6 auxilliary fans controlled by one of these great gadgets. Most of the time, the PSU fan takes care of the cooling and the extra fans either just idle, barely turning over, or if the room's cool enough they don't turn at all. If I load it up, then the fans will start up spin up when the internal case air temperature gets above 29 degrees C, and by 34 degrees C they're all going hell for leather. (I'm so sad I bought a cheap digital thermometer with a remote probe to measure this!). Most of the time it just sits quietly in the corner.

I position the thermistor just above the CPU heatsink fan, so it's measuring the air temperature that the CPU fan is drawing in, rather than the room temperature air the intake fans suck in. Then, with the PC running I open the case and adjust the potentiometer on the controller so the fan is running full-speed, then slowly turn it backwards until it stops altogether. Then, when you close the case, the fan should start to spin slowly, speeding up as necessary to keep the case air temp. close to this level. I also use Vcool on my Athlon PCs where possible.

Hope this helps!
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