Re: How does a Computer Fan work?
White or yellow is the tachometer lead. That tells the controller how fast the fan is turns by sending out pulses to it. The pulses are not true rpms and may have to wash through a multiplier or divider to get true rpms.
The red lead lead is positive and is connected to the fan (if speed controlled) through a device that uses PWM. Pulse Width Modulation. Simply means how often and how long power is applied to the fan. This determines speed.
The tach lead is the feedback to the PWM controller.
Black is ground. Common to both power and the tach lead.