A universal serial bus (USB) power supply is disclosed as including a main body (12) and a cable (24) releasably connectable with each other, the main body (12) being releasably connectable to a direct current (DC) power source and having a USB receptacle (20) electrically connectable with the DC power source. The cable (24) includes a USB plug (26) releasably connectable with the USB receptacle (20) to establish electrical connection and data communication there-between and a connector (30) electrically connected with the USB plug (26). The power supply is operable in a detached mode in which the main body (12) is out of connection with the cable (24) and in an attached mode in which the main body (12) is releasably connected with the cable (24). The power supply is adapted, when in the attached mode, to automatically change the voltage of a DC power output from the connector (30).