Patent ID: 8344674

Claim:
A fan speed control circuit to control a fan speed of a fan, the fan speed control circuit comprising: a voltage input terminal to receive a voltage signal; a thyristor; a negative temperature coefficient thermistor; first to fourth resistors; and first and second electronic switches each comprising a control terminal, a first terminal, and a second terminal; wherein the voltage input terminal is grounded through the first resistor and the thermistor in that order, connected to an anode of the thyristor, and connected to the second terminal of the second electronic switch through the second resistor, a node between the first resistor and the thermistor is connected to the control terminal of the first electronic switch, the second terminal of the first electronic switch is grounded, the first terminal of the first electronic switch is connected to a control terminal of the thyristor through the fourth resistor, the third resistor is connected between the anode and the control terminal of the thyristor, the control terminal of the second electronic switch is connected to the control terminal of the thyristor, the first terminal of the second electronic switch is connected to a cathode of the thyristor and a voltage pin of the fan, the first electronic switch is turned on in response to the control terminal of the first electronic switch being in a high voltage state, and the second electronic switch is turned on in response to the control terminal of the second electronic switch being in a low voltage state.