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Building my first PC tomorrow, and one thing is confusing me. How do the fans know how fast to spin? The CPU fan is connected to the motherboard's 4pin cpu_fan socket, so I can understand how it can interface with thermometers there, but my case fans only have Molex cables, directly from the PSU. Are the car fans only meant to spin at a standard speed?

4 pin power supply + and - cables

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up vote 4 down vote accepted

A slightly longer explanation - Molex adapters will supply just one voltage unless they're regulated by a controller.

Three-pin motherboard sockets have the same kind of setup - the third pin is for sending speed signals back to the controller, and the motherboard is intelligent enough to know how fast the fan is spinning and can vary the voltage applied.

Four-pin fans use PWM (pulse-width modulation) to achieve greater control than just varying the voltage supplied to the fan.

More details are on Wikipedia here:

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Motherboards may also have case fan connectors, and if you connect your fan to them, then you'll have speed control.

If the case fans do not have the appropriate 3-pin connectors (or an adapter), then when connected directly to a Molex, they'll only spin at a fixed speed.

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in short, yeah. case fans not connected to the motherboard will just go at one standard speed.

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