Patent ID: 7394215

Claim:
A thermal control variable-speed circuit without switching noise, connected with a driving circuit, for controlling the rotation speed of a fan motor, comprising: a reference voltage output circuit, which is connected to a power supply and outputs a corresponding reference voltage signal, wherein said reference voltage signal is changeable according to ambient temperature and said reference voltage output circuit comprises a first resistor, a thermistor and a second resistor, wherein one end of said thermistor is coupled to one end of the first resistor and the power supply and the other end of said thermistor is grounded, the other end of the first resistor outputs the reference voltage signal and is coupled to one end of the second resistor, and the other end of the second resistor is grounded; and a driving voltage control circuit, which receives said reference voltage signal and outputs a corresponding driving voltage according to the amplitude of said reference voltage signal, wherein said driving voltage is proportional to said reference voltage signal and used to control the rotation speed of said fan motor.