"JAY EMRIE" wrote to "ROY J. TELLASON" (11 Feb 03  10:05:00)
 --- on the topic of "Fan Monitoring?"

 RJT>JB>extra components would be needed to get a rate signal from a
2 wire fan.

 RJT>JB>it's easist to tap the monitioring terminsl on the fan PCB. (typically
 RJT>JB>right next to the power terminals)

 RJT>JE> HUH? None of the fans in my computers have EVER had a fan PCB. Am I
 RJT>JE> not understanding?

 RJT>Ever take one apart?  :-)

 RJT>There's likely much more than just a simple dc motor in there.

 JE> That "likely" indicates that you "THINK" but do
not "KNOW"
 JE> for sure that there MAY be more than a simple DC motor in
 JE> there.

 JE> IIRC, the subject was PC case fans - and simply looking at
 JE> one will show there is no circuit board in/on one of these.

Brushless DC fans have a circuit board in them. It is needed because of
the problem of commutation (pole rotation). These brushless fans use an
elegant electronic solution which employs a Hall Effect switch(es) to
detect the position of the armature magnet. The rotor is fixed!
The speed of brushless motors can be varied quite a bit by simply
varying their supply voltage. I suppose that's all an external circuit
might do in response to the monitored temperature.

 Mike
 ****

... Modulation in all things...
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