Patent ID: 7983539

Claim:
A control circuit for a fan mounted on a chip, the control circuit comprising: a power interface connected to a power supply, to receive a high or low level signal from the power supply; a latch circuit connected to the power interface, to receive the high or low level signal from the power interface, and connected to a first and a second power sources, to receive voltages from the first and the second power source; a control chip connected to the latch circuit, to turn on or turn off the latch circuit, and also connected to a fan header mountable for the fan, to output pulse width modulation (PWM) signals with different duty factors to a control pin of the fan header; an adjusting circuit connected to the latch circuit, to rectify a voltage output from the first power source as an analog voltage signal; and a control circuit connected to the adjusting circuit, and a third power source, to control the third power source to output a constant driving voltage according to the analog voltage signal, and provide the constant driving voltage to a voltage pin of the fan header, to provide power for the fan; wherein when the fan inserted in the fan header is a 4-pin fan, the control chip output the PWM signals with different duty factors to the control pin of the fan header, to automatically change the rotary speed of the 4-pin fan, wherein when the fan inserted in the fan header is a 3-pin fan, the control chip outputs the PWM signals whose duty factor changes with a temperature of the chip under the fan to control the first power source to provide a voltage to the adjusting circuit, the adjusting circuit rectifies the voltage output from the first power source as an analog voltage signal to the control circuit, and the control circuit controls the third power source to output a changeable driving voltage to the power pin of the fan header to control the rotary speed of the 3-pin fan.