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Abstract
The project adopted the research and development design approach. The system was made up of 4 units, these are, the rectifier/charger, the battery, the inverter and the switch units. The rectifier/charger unit is responsible for the stepping down of the 220V AC from the main source to 12V DC and for charging the 12V lead acid battery, provision was made for the external battery so that the system can use high capacity batteries to provide longer backup period during the power outage. The inverter unit of this UPS was made up of both the oscillator and power circuit, the oscillator which performs the major function of converting DC to AC was built with NPN transistors of various values and power transistors. The output unit was made of a self-wounded step-up transformer that steps the voltage up to 220V AC and capacitor that removes the ripples which may occur at the output unit. The switch inform of a relay was used to build change-over such that immediately there is power failure it changes over to the UPS. The constructed 1.5KVA (500V AC) was given out from the 12V DC lead acid battery during power failure. Keywords: UPS, DC, Oscillator, Capacitor, Battery