An identifying circuit is connected between a Universal Serial Bus (USB) interface and a controller. The identifying circuit includes first to fourth electronic switches. When a power adapter connects to the USB interface, the first and fourth electronic switches are not turned on, and the second and third electronic switches are turned on. An identification pin of the controller receives a low level signal and determines that the power adapter connects to the USB interface. When a computer connects to the USB interface, the first and fourth electronic switches are turned on, and the second and third electronic switches are not turned on. The identification pin receives a high level signal and determines that the computer is connected to the USB interface.