Patent ID: 7756402

Claim:
A method for controlling rotation speed of a computer fan, comprising: providing a circuit comprising: an NMOS transistor having a gate to receive a first control signal generated by a BIOS, a source to receive a PWM signal, and a drain grounded via a first resistor and a capacitor in turn; an amplifier comprising a positive input terminal grounded via a second resistor, a negative input terminal connected to a node between the first resistor and the capacitor and connected to a first power source via the first resistor, and an output terminal; a PNP transistor, the base of the PNP transistor connected to the output terminal of the amplifier via a third resistor, the emitter of the PNP transistor connected to a second power source, and the collector of the PNP transistor connected to the positive input terminal of the amplifier via a fourth resistor, the PNP transistor outputting a fan control signal via the collector thereof; and an output port configured for the computer fan connecting thereto, the output port comprising two pins respectively receiving the PWM signal and the fan control signal; detecting a rotation speed of the fan; changing duty cycle of the PWM signal; detecting a rotation speed of the fan after the duty cycle of the PWM signal is changed; comparing the two rotation speeds, if the rotation speeds are different, the fan is a first type computer fan whose rotation speed is controlled by PWM signals; if the rotation speeds are equal, the fan is a second type fan whose rotation speed is controlled by voltage signals; transmitting PWM signals to the fan if the fan is the first type computer fan via the BIOS controlling the control signal GPIO at a low level to turn off the NMOS transistor, and converting the PWM signals to voltage signals and transmitting the voltage signals to the fan if the fan is the second type fan via the BIOS controlling the control signal GPIO at a high level to turn on the NMOS transistor.