Abstract: An uninterruptible power supply which is advantageous for use with a computer receives power alternatively from a utility main power source or a rechargeable battery. The uninterruptible power supply includes an input for receiving AC utility power, an AC to DC converter coupled to the input, receiving AC power from the input, having at least one regulated D.C. output, and generating DC power at an output of the converter, a transformer having a primary winding and a secondary winding, an inverter coupled between the AC to DC converter and the primary winding of the transformer and arranged to supply a chopped DC voltage to the primary winding to cause a corresponding chopped DC voltage having an amplitude V.sub.s to be generated across a secondary winding of the transformer coupled to the output, an auxiliary power supply having an energy store having a supply terminal that is operable at a voltage V.sub.

Abstract: An uninterruptible power supply includes first and second inputs for receiving AC and DC power respectively, first and second switching circuits, an auxiliary power supply, a transformer, a selection circuit, an A to DC converter and a hold-up capacitor. The hold-up capacitor can provide energy storage for both AC and DC operation.