Power supply having selectable operation based on communications with load

A power supply includes signaling circuitry coupled to output terminals for engaging in bidirectional communications with a load. A controller is capable of conducting the bidirectional communications and selecting among different operating modes for the power supply based on the communications. The modes may include a constant-current mode suitable for applications such as battery charging and power LED lamps, and a constant-voltage mode suitable for a variety of conventional uses such as powering electronic circuitry. The signaling circuitry may include a power switching transistor in series with the load, which is pulsed in a binary fashion to transmit communications to the load. A signal-forming resistor in parallel with the power switching transistor develops a signaling voltage monitored by the controller to receive communications from the load.