Abstract: A dual mode external DC power supply system for supplying DC power to an electronic device and which operates under the control of the electronic device is provided. The disclosed embodiment of the power supply system includes a first input adapted to receive an input signal of unregulated DC power, a second input adapted to receive a control signal from the electronic device, and an output adapted to provide either a constant voltage DC output signal or a constant current DC output signal depending on the magnitude of the control signal. The control signal also determines the level of constant current delivered to the electronic device when the system is operating in the constant current mode.