Abstract: A speed-control drive circuit for a D.C. brushless fan motor includes a pulse width modulation input end for receiving an external pulse width modulation signal for controlling (and changing) the motor speed, and a motor winding drive circuit. The motor winding drive circuit and the pulse width modulation input end are connected to a set of motor windings. The pulse width modulation signal controls the motor winding drive circuit to cause excitation/unexcitation of the set of motor windings and to alternately changes a direction of current such that the set of motor windings adjusts a speed of the fan motor in response to excitation of the set of motor windings responsive to the pulse width modulation signal. The speed of the fan motor increases when an operational period of the pulse width modulation signal increases, and the speed of the fan motor decreases when the operational period of the pulse width modulation signal decreases.

Abstract: A speed-control drive circuit for a D.C. brushless fan motor comprises a pulse width modulation input end for receiving a pulse width modulation signal and a motor winding drive circuit. The motor winding drive circuit and the pulse width modulation input end are connected to a set of motor winding. The pulse width modulation signal controls the motor winding drive circuit to cause excitation/unexcitation of the set of motor winding and to alternately change a direction of current such that the set of motor winding adjusts a speed of the fan motor in response to excitation of the set of motor winding responsive to the pulse width modulation signal. The speed of said fan motor increases when an operational period of the pulse width modulation signal increases, and the speed of the fan motor decreases when the operational period of the pulse width modulation signal decreases.