NOTE that the actual fan speed as measured by the pulse signal is NOT used for control - it is only measured so you can see it if you wish. You also have options to turn off automatic fan control and use fixed manually-set fan speed. On many mobos, you can alter the target and the fan speed control strategy if you so choose, either in BIOS Setup or using a software monitoring utility that comes with the mobo. The automatic control system constantly compares the measured temperature to the target and alters the fan speed in an attempt to keep actual measured temp close to the target. The mobo has a temperature sensor built into itself somewhere the maker thinks is vital, (like close to the Northbridge chip) and its software setup has some control parameters like a target temperature and a controller gain, etc. For this reason, if you are using this fan port for some other fan type (like an adapted 2-pin fan) many mobos allow you to tell the fan port control system to Ignore this port's speed signal, thus eliminating false failure warnings.Ī mobo case fan port usually has an automatic fan speed system available and activated by default. 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.
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