Abstract: Uninterruptible power supply apparatus which provides for supplying ac power to equipment even if and when the primary ac power is interrupted is disclosed and comprises an ac to dc converter connected to receive the primary ac power. The output of the ac to dc converter is applied to a dc power source. The resultant output of the ac to dc converter and the dc power source is provided as a dc input to a dc to ac inverter. The dc to ac inverter comprises an output transformer having a center-tapped primary. A first switching device is connected between a first end of the center-tapped primary and the negative terminal of the dc input to provide power pulses to one-half of the center-tapped primary. A second switching device is connected between a second end of the center-tapped primary and the negative terminal of the dc input to provide power pulses to the remaining half of the center-tapped primary.