Patent ID: 8344778

Claim:
A control circuit for controlling a rotation speed of a fan, the control circuit comprising: a triangular wave generating circuit to output a first triangular wave signal; a temperature sensing circuit to sense a temperature surrounding the fan and output a temperature signal; a first comparator, wherein a non-inverting terminal of the first comparator is connected to the triangular wave generating circuit to receive the first triangular wave signal, an inverting terminal of the first comparator is connected to the temperature sensing circuit to receive the temperature signal, the first comparator compares the first triangular wave signal with the temperature signal to output a control signal; and a switching circuit connected between a power supply and the fan, wherein the switching circuit turns on or off according to the control signal; wherein the temperature sensing circuit comprises a first resistor and a positive temperature coefficient thermistor, a first terminal of the first resistor is connected to the power supply, a second terminal of the first resistor is connected to the inverting terminal of the first comparator, a first terminal of the positive temperature coefficient thermistor is grounded, and a second terminal of the positive temperature coefficient thermistor is connected to a node between the inverting terminal of the first comparator and the first resistor.