Abstract: In order to extend battery life when batteries are used and to conserve energy when commercial power is used, it is highly desirable that power not be applied to a data processor when it is not operating. A power supply in accordance with the present invention includes a power-up and supply subcircuit (100) that applies power to an associated data processor (500) when a data input circuit (700) is operated to select a particular operation of the processor. When the processor completes its operation, it provides an output signal to a power-down subcircuit (200) of the power supply which then removes power from the processor.