JAY EMRIE wrote in a message to JASEN BETTS:

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

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

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

Ever take one apart?  :-)

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

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