Well I'll be hornswoggled! I just took one that has three
wires apart and it DOES have a circuit board

Mea Culpa - I was all wrong!

The one I took apart was only 13/32" deep and 2 3/8th in
square. The only has 2 components - 1 diode and one IC.
Who would have dreamed a ckt board could/would have been put
is such a small place? Actually I shouldn't have been
surprised with all the miniature stuff that has come out in
the past dozen years.

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

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

The only way one could possibly tap the monitoring terminal
on the PC board would be to destroy the fan getting to it.
However, the monitoring wire seem to be (on this fan) the
yellow wire (on one side of the plug) which goes to the
opposite side of the 3 wire plug where the black wire
is. - easy to tap there.

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

MR> RJT>Ever take one apart?  :-)

Not until tonight.  :-(

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

It is a simple DC motor and there is more in there. You are
correct.

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

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

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

See all the above - I stand or rather humbly bow and am
corrected.

Jay

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