Fan Tachometer Block Diagram

Majority of personal computer (PC) cooling fans are two-phase DC motors, consisting of a driver integrated circuit (IC). This IC usually consists of a built-in Hall Effect sensor, which detects the change in magnetic field as the shaft rotates.

As you can see, the output from the pre-driver feeds an internal transistor in an open collector configuration. This is a fast switching transistor, which conducts when it receives a pulse at the base.

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