The sinewave UPS circuit using PIC16F 84A microcontroller IC is cheap, versatile and can be a perfectly small UPS which can be used in homes, offices, shops to get a ready 24x7 power whenever the AC power fails.
The circuit design is not complex and comprises all the necessary stage which a standard UPS ought to possess. To be precise the circuit includes a center tap transformer based inverter circuit, a fully automatic battery charger circuit with over charge and low charge cut off, an output overload protection and over voltage protection stage, a buzzer beeper stage when the UPS is running in the inverter mode, and most importantly it includes a rapid relay changeover stage which makes sure that the transition from mains to inverter or vice versa is triggered with least delay and as quickly as possible, this is to ensure that any critical load associated with the UPS is not affected by a short transition lapse, rather switched with a uninterrupted accuracy.
The main stages of the circuit can be understood from the following points:
A PIC16F84A microcontroller stage primarily monitors the frequency generation for the inverter stage. It not only generates the basic 50Hz with utmost accuracy, the microcontroller also becomes responsible for driving power transistors with a pure sinewave frequency (SPWM) ensuring a perfectly optimized sine waveform at the output of the transformer. It also handles the buzzer triggering in the form of intermittent beeps whenever an AC mains failure is detected.
The other features are monitored the external opamps which execute the required battery charing and other related protection features of the UPS circuit.
The inverter section is built using a basic push pull center tap transformer circuit which is switched alternately using power BJTs to keep things simple and manageable. Mosfets are avoided since mosfets require much stringent PCB designing parameters.
The HEX codes and the other crucial information for this sinewave UPS circuit using PIC16F84A MCU can be downloaded from the above link: