Mini uPS SySteM
D. Mohan KuMar
his circuit provides an uninterrupted power supply (UPS) to operate 12V, 9V and 5V DC-powered instruments at up to1A current. The backup battery takes up the load without spikes or delay when the mains power gets interrupted. It can also be used as a workbench power supply that provides 12V, 9V and 5V operatingvoltages. The circuit im-
When the mains power fails, diode D3 gets reverse biased and D4 gets forward biased so that the battery can automatically take up the load without any delay. When thebattery voltage or input voltage falls below 10.5V, a cut-off circuit is used to prevent deep discharging of the battery. Resistor R3, zener diode ZD1 (10.5V) and transistor T2 form the cut-off circuit.When the voltage level is above 10.5V, transistor
vary between 10.5V and 12V, when the UPS system is in battery mode. Outputs at points B and C provide 9V and 5V, respectively, through regulator ICs(IC1 and IC2), while output A provides 12V through the zener diode. The emergency lamp uses two ultra-bright white LEDs (LED2 and LED3) with current limiting resistors R5 and R6. The lamp can bemanually
mediately disconnects the load when the battery voltage reduces to 10.5V to prevent deep discharge of the battery. LED1 indication is provided to show the full charge voltage level of thebattery. miniature white LEDs (LED2 and LED3) are used as emergency lamps during power failure at night. A standard step-down transformer provides 12V of AC, which is rectified by diodes D1 and D2.Capacitor C1 provides ripple-free DC to charge the battery and to the remaining circuit. When the mains power is on, diode D3 gets forward biased to charge the battery. Resistor R1 limits the chargingcurrent. Potentiometer VR1 (10k) with transistor T1 acts as the voltage comparator to indicate the voltage level. VR1 is so adjusted that LED1 is in the ‘off’ mode. When the battery is fully charged,...