Patent ID: 8076879

Claim:
A fan controlling circuit for detecting a rotational speed of a fan, comprising: a comparison circuit configured for receiving a rotational speed signal from the fan at one input terminal, a reference voltage at another input terminal, and outputting a filtered rotational speed signal at an output terminal; and a controlling chipset for receiving the filtered rotational speed signal, and outputting a control signal to control the rotational speed of the fan; wherein the comparison circuit comprises a comparator, a first resistor, and a second resistor; the inverting input terminal of the comparator is configured to receive the rotational speed signal; the non-inverting input terminal of the comparator is configured to receive the reference voltage via the first resistor; the output terminal of the comparator is coupled to the controlling chipset to output the filtered rotational speed signal, and is coupled to the non-inverting input terminal of the comparator via the second resistor.