Fan Tachometer Open Collector Circuit

An open collector design, typically used for CPU fan tach signals, is a very simple digital way to transfer rpm information. As you can see in the circuit diagram, the internal transistor sinks current to ground when it is conducting. The transistor conducts at the same rate as the pulses at the base junction. In this type of configuration, the motherboard provides the +5 V (TTL) to the open collector, which becomes close to zero volts each time the transistor conducts.

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