An electric adapter system includes an adapter including a circuit connected between input and output connection devices respectively connectable with an external power source and a power consuming device. The output connection device is mateable with a connector forming an output member for engaging the power consuming device. The circuit includes a voltage level control device and a feedback device. The voltage level control device supplies an output voltage to the output connection device via positive and negative power terminals. The feedback device includes a plurality of voltage selection lines each having a different electric resistance whereby by selectively connecting one of the voltage selection lines to the negative power terminal, a feedback signal is generated by the feedback device and applied to the voltage level control device for changing the voltage level of the output voltage of the voltage level control device.