Abstract: An uninterruptible power supply provides internal or external connection of an auxiliary power source to a main power supply so as to provide continuous device power without requiring any modification to the main power supply itself and without duplicating its power circuit. The uninterruptible power supply (UPS) comprises a switch power supply (SPS) including a rectifier, DC to DC converter and at least one holdover capacitor connected across the input terminals of DC to DC power converter. A backup system supplies DC power from a battery to the holdover capacitors, and a back up control circuit supplies high voltage DC energy to the SPS at fixed time intervals and ensures smooth transition to AC utility power upon resumption of such power following a failure. An auxiliary AC power output signal is maintained for a video monitor or other device during operation of the uninterruptible power supply.