Patent ID: 7702223

Claim:
A circuit for controlling rotation speed of a computer fan, the circuit comprising: a fan header configured for connecting to a 4-pin fan or a 3-pin fan; a jumper device having a first pin configured for receiving a controlling signal which is controlled by a super input/output (SIO) controller, and connected to a first power source, a second pin connected to the fan header, and a third pin connected to the first power source; an amplifier comprising a negative input terminal connected to the third pin of the jumper device via an integrator; and a controller comprising a first terminal connected to an output terminal of the amplifier, a second terminal connected to a second power source, and a third terminal connected to the fan header and connected to a positive input terminal of the amplifier via a first resistor; when a 3-pin fan is connected to the fan header, the first pin of the fan header is connected to the third pin, the controlling signal is converted to an analog signal by the integrator, the analog signal is amplified by the amplifier, an amplified signal output from the amplifier adjusts the rotation speed of the 3-pin fan by controlling a quiescent operating point of the controller, when a 4-pin fan is connected to the fan header, the first pin of the fan header is connected to the second pin, the duty cycle of the controlling signal is adjusted by the SIO controller, to adjust the rotation speed of the 4-pin fan.