Quote:
Originally posted by kfowler
The fan has three wires. Red, yellow and black. I currently have the red and yellow wired together to the hot wire from the transformer. I've read that the yellow wire is typically your speed control wire.
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The yellow wire is not a speed control wire. It is an RPM sensor wire that allows your computer motherboard to determine the speed of your fan. Some motherboards can vary the speed of the fan by varying the voltage.
There are 4-wire fans known as Pulse Width Modulation (PWM) fans. These fans have a additional blue wire. Instead of varying the voltage to control the fan speed, a PWM controller will feed the fan an ON/OFF square wave. I don't recommend these fans for a hobbyist because at certain speeds, you will hear noise from the fan clicking on and off. Another disadvantage is that the RPM sensor will not work on a PWM fan unless the fan is running 100%.