![]() The +12VDC supply line in this system is always at that full voltage (does not get reduced by the controller). In this system, the Ground and +12 VDC power supply lines and the speed pulse return line are all there still, but there's a 4th line carrying the PWM (Pulse Width Modulation) signal. For the Case Fans, the mobo has its own temp sensor built in and uses that to measure the actual inside-the-case temp.Ī 4-pin fan plugged into a 4-pin fan mobo port works very similarly, but the details of how the fan speed is controlled are different, and the wire color codes are different, also. ![]() For the CPU, the actual temperature measurement is done with a sensor built into the CPU chip itself and fed to the mobo on one of the CPU's pins. Note that the actual measured fan speed is NOT used to control the fan speed - that control is based only on measured temperature. As the actual temp changes from the target, the control loop changes its output (the voltage supply to the fan) to bring it back to target. In that system, the result of the loop's control action (a measured actual temperature) is fed back into the mobo controller and compared to the target. The basis for speed control is what's called a feedback loop. This is how the mobo can warn you if the fan stops. The Yellow wire carries a pulse train (2 pulses per revolution) signal generated in the fan motor back to the mobo for display and monitoring. The fan's power is on the Black (Ground) and Red (+12 VDC) wires, and the mobo lowers the voltage on the Red wire to reduce fan speed as needed. If you plug a 3-pin fan into a 3-pin fan port of the mobo and allow the mobo's automatic systems to control the fan speed, here's what happens. Fan connectors and the mating mobo ports have been designed, physically and electrically, so that they always will fit together with reasonable electrical connections, but the details of fan speed control are not always the same. If you don't plug the CPU fan in there, leave that fan port unused.Ī 3-pin fan plugs into EITHER a 3-pin or a 4-pin fan port on the mobo. But because of that checking feature, if you opt to run your CPU fan straight from a Molex PSU connector at full speed, you must tell your mobo's BIOS NOT to watch that fan, or it will get very upset that there is no fan speed signal to watch.ĭo NOT plug into the mobo CPU_FAN connector ANY other fan. It also allows the mobo to check that the fan is working - it will shut the system down fast if that fan suddenly stops. This ensures that the CPU cooling fan (assuming it's working properly) will run fast enough to keep the CPU at the right temperature, but not too fast so it makes more noise than necessary. One IMPORTANT note: it is advisable to plug your CPU cooler fan into the CPU_FAN port of your mobo, and let the mobo's automatic systems control the CPU fan speed. Such a system has no way for the fan's speed to be sent to the mobo. Details below.Ī 4-pin Molex connector from the PSU will supply 12 VDC to the fan at all times over 2 wires, so it always runs at full speed. Start with: all case fans are designed to run at 12 VDC for full speed, but will run slower under the right conditions.
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