Fan Tachometer Schematic / Circuit Diagram

Here is a typical CPU fan tachometer circuit. The main IC drives a pair of coils, and produces the tach signal. The diode D provides protection against incorrect insertion of the fan plug. The tach output is typically a yellow coloured wire and is the centre pin.

The value of the pull-up resistor R is calculated from the transistor saturation voltage Von and its sink current Ic. Typically, Ic is at 5 mA with a saturation voltage of 0.5 V, hence R is 1 kΩ.

Working in IT, I came across a number of fans where the tach signal had failed, and when opening the fan I always found that the main IC was faulty. I hope this article helped someone figure out which fan to buy.

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