This means that the CPU_FAN header IS using the PWM Control system, but the SYS_FAN headers are NOT - they are using Voltage Control Mode. It says that the signal on Pin #4 is "VCC", whereas the label for the CPU_FAN header has the label "Speed Control" on its Pin #4. 15 where the diagrams are shown for the three SYS_FAN headers that have 4 pins. You have to play detective with the mobo's manual. You can NOT tell from the count of pins on the mobo header which type of control it is using. The motor receives a fixed +12 VDC on Pin #2, and has no way to accept or use the PWM signal on Pin #4, so the fan motor always runs full speed. But the other way - plug a 3-pin fan into a header using PWM Mode - does not give control. BUT the supply on Pin #2 is a VARYING voltage which then gets fed to the motor windings unchanged, and the motor's speed IS under control by the mobo. If you plug a 4-pin fan into a mobo header that uses Voltage Control Mode the fan receives no PWM signal and cannot modify the power supplied. Obviously, of you don't mix, each type works as intended. The signals are so similar that mixing them sort of works. The connectors of the two fan types are almost the same so that either type can plug into either type of mobo male header.
Inside the fan a small chip uses that PWM signal to control the flow of current through the motor from the +12 VDC supply, thus manipulating speed. Pin #2 always has +12 VDC on it, never reduced. They still use almost the same signals on those first 3 pins, but with one important difference.
Interestingly, this signal is NOT used for actual control of the speed.Ĥ-pin fans work differently. The fan speed signal is a series of pulses (2 per revolution) generated inside the motor and sent back on Pin #3 to the mobo for counting and display. (At voltage less than 5 VDC, the fan may stall and not re-start until the voltage is increased.) This method is called "Voltage Control Mode" or sometimes "DC Mode". To control fan speed, the mobo header must alter the voltage on Pin #2, ranging form +12 VDC (max) to about +5 VDC. The speed of BOTH 3-pin and 4-pin fans can be controlled, but the method is different for the two types.įor 3-pin fans, the connections to it are Ground to Pin #1, +VDC (varying) to Pin #2, and Speed signal on Pin #3. Actually, you MISunderstand that 4-pin fans only can be controlled.
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