Abstract: In a digital data communications device for use in an ISDN environment, a power supply apparatus and method are provided for energizing telephone danger circuitry or other alerting mechanisms. Embodiments of the present invention employ signal generators which control switching circuitry that converts a low voltage DC power source into a low voltage alternating current. A step-up transformer is used to convert or transform this low voltage into a high voltage alternating current, which is rectified and filtered to generate a high voltage direct current. The signal generators also control the switching circuitry which applies this high voltage direct current to an alerting mechanism and alternately provides a path to discharge energy which may be stored in the alerting mechanism, causing the alerting mechanism or other ringer circuitry to resound.