In addition to the power, ground, and tach signal, 4-wire fans have a PWM input, which is used to control the speed of the fan. Instead of switching the power to the entire fan on and off, only the power to the drive coils is switched, making the tach information available continuously. Switching the coils on and off generates some commutation noise. Driving the coils at rates greater than 20 kHz moves the noise outside of the audible range, so typical PWM fan-drive signals use a rather high frequency (>20 kHz). Another advantage of 4-wire fans is that the fan speed can be controlled at speeds as low as 10% of the fan’s full speed.
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