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PWM vs voltage control
My understanding is that the mobo can either use PWM to pulse the 4th wire to ground (turning it all the way off & on rapidly), or in the absence of the 4th pin, regulate voltage to the 2nd pin (12v +) down to 5-7 volts (on certain boards) while reading RPM from the 3rd pin. I know my Asus P8Z77-V controls 3 pin fans, other brands or cheaper Asus boards may not.
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