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fan rpm question

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Mar 15, 2001
just wondering on how if a fan is a 2 wire set up...how do programs like mbm5.0 and others know the rpm's of the fans?....there are only 2 wires....i can see how a 3 wire hook up could send a reading through the third wire...but when you only have 2????? just wondering.
If the fan is supplied with a constant voltage through 2 wires, the rotation speed is related to how much current the fan draws, which would be measured through those same 2 wires.
The third usually yellow wire measures the RPM's of the fan.If u remove it or u have a two wire fan than say bye bye to monitoring the RPM's.