All it knows is that it's getting 12V, so it runs at full speed. If the header can't switch modes, the result is that if you plug a 3pin fan into it, the mobo/header can provide it 12V and monitor it's RPM, but it's also sending a PWM control signal that the fan isn't capable of receiving. Some mobos have PWM fan headers that you can switch the mode on from PWM control to voltage control. The speed of the fan is controlled by pulse width modulation (PWM), which basically means the fan gets a signal through the 4th pin to turn on and off rapidly at a rate calculated to result in a specific RPM. 12V gets 100% of the fan's max rated RPMs, 9V gets 50%, etc.Ī 4pin fan only gets one voltage level - 12V. Many, if not most, newer mobos can control these fans by adjusting the voltage level fed to the fans based on the RPM reading it gets from the fan. It's designed to provide fan control for PWM fans that have 4 pins.Ī 3pin fan has connectors for power, common (ground), and RPM reporting (tachometer).
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