Detecting fan rpm? How's it done?

TimTim Southwest PA Icrontian
edited February 2004 in Hardware
I've noticed on case and CPU fans that there's a 3rd wire in the harness. One for +, one for -, and the third, which must be the rpm wire.

But does anyone know how the rpm wire works? Does it pulse a signal to the MB every time the fan turns 1 turn?

Comments

  • edcentricedcentric near Milwaukee, Wisconsin Icrontian
    edited February 2004
    Basicaly, yes. It is a actually a timer that times between the pulses. Some fans send one/rev and some more. 2 or 4 counts/rev aren't uncommon either. If you plug a 2400rpm fan in and mbm tells you that it is running 9600rpm then you know to change the divider.
    You will notice if you use fan controlers that when you slow the fan down at some point mbm will loose track of the correct speed. This is because the time between pulses is either too long or it is eratic.
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