A power supply includes an alternating current (AC)/direct current (DC) converter, a control unit, and a selection switch. The AC/DC converter is connected to an AC power source to convert an AC voltage into a DC voltage. The control unit includes a controller and a switch. The selection switch is connected to the controller, and connected between the AC converter and the power module of a motherboard to connect the AC converter to either the power module or the controller. When the AC/DC converter is connected to the controller, the controller receives the DC voltage to be started, the controller turns on the switch for a predetermined connection time to make the DC voltage be input to the power module, and then turns off the switch for a predetermined disconnection time to make the DC voltage fail to be input to the power module.