It also may check this signal for failure - that is, are there speed pulses coming or not? - and issue a failed fan warning if it gets no signal.

It will count pulses and generate a fan speed measurement for display by software. The fan motor generates a pulse signal (2 pulses per revolution) which is sent on the Yellow wire back to the mobo for monitoring. The third pin (Pin #3 on the port) is connected usually to a Yellow wire to the fan. 2-pin fans have no way to send out a speed signal, so you cannot measure the speed of such a fan.Ī 3-pin fan is generally connected to a mobo fan port and its speed is changed by changing the supply voltage on Pin #2 of the mobo port.

It also might be possible to get an adapter to allow you to connect such a fan to a mobo 3-pin fan port, thus allowing the mobo to control the fan's speed (but see comment later on speed monitoring). Instead, if it is plugged into a third-party fan controller, that unit will supply a voltage that may be reduced (for slower fan speed) by changing the manual setting on the controller - usually, a knob that turns. Done that way, it always runs at full speed. That mates with a power supply connector directly from the PSU. All fans in a computer are designed for max 12 VDC supply.Ī 2-pin fan generally has on the end of its wires a male 4-pin Molex connector, but only 2 pins are used. On both, the Black wire is Ground, and the Red wire is the + DC voltage supply. 2- and 3-pin fan speeds can only be changed by altering the voltage supplied to them.

Cpu speed fan control