A power supply circuit includes a connector for a fan, a speed adjusting circuit, a speed feedback circuit, a control module, first and second power sources, and a switching module. The speed adjusting circuit is connected to a modulation pin of the connector. The speed feedback circuit is connected to a speed pin of the connector. The control module receives a speed signal from the fan through the speed feedback circuit and outputs signals correspondingly. The switching module selects the first or the second power source to supply power for the fan according to the signals from the control module.