Fan motor speed control circuit

[0019] Referring to FIG. 2, a fan motor speed control circuit 30 in accordance with the invention comprises a digital / analog converting unit 301 and a driving unit 302. The digital / analog converting unit 301 is used for converting an inputted PWM digital signal 3 into an analog signal such as a voltage signal. The driving unit 302 takes the analog signal and compares it with two predetermined voltages Vref1 and Vref2 set by the driving unit. After that, a digital signal representing the comparison result is used to control the rotation speed of the fan motor 40.

[0020] The fan motor speed control circuit according to one embodiment of the invention is shown in FIG. 3A. In this embodiment, a digital / analog converting circuit 22 is employed as a digital / analog converting unit, and a drive IC 23 is employed as a driving unit. In this embodiment, the digital / analog converting circuit 22 is mainly composed of a transistor 221, a diode 222, a capacitor 223, and a plurality of resis