Uninterruptible Power Supply (UPS) Systans are extensively used by modern society for electrical support of computers, communications and other critical systems. Over the last few years, power transistorized UPS's have become much more practical and their numbers have increased accordingly. The performance of these transistorized UPS's has been further improved since they were described two years ago [1]. The inverter has been refined and simplified to the extent that the system now contains only one output transformer. The control circuitry now consists of a single circuit board. The improved rectifier has reduced input current distortion significantly.
