DIY Small Uninterruptible Power Supply

This small uninterruptible power supply (UPS) can provide 5V, 9v and 12v DC power supply. The maximum supply current can reach 1A. It can supply power to the load without delay when the main power is interrupted. For devices using 12v power supply, when the battery voltage drops to 10.5V, the circuit will immediately disconnect the load to avoid deep discharge of the battery. After the battery voltage is fully charged, LED1 will be used to indicate it. During power failure at night, two white LED2 and LED3 small light-emitting diodes can be used as emergency lighting.

When the main power is turned on, diode D3 is forward biased to charge the battery, and R1 is used to limit the charging current. 10k potentiometer VR1 and transistor T1 act as a voltage comparator to indicate the voltage level. VR1 should be adjusted so that LED1 remains off during the charging phase and only lights up when the battery is fully charged, at which time the battery voltage is 12V.

If the main power fails, D3 becomes reverse biased and D4 becomes forward biased. At this time, the battery automatically continues to supply power to the load without any delay. When the battery voltage or input voltage drops to 10.5V, the "cut-off" circuit composed of R3, ZD1, VR2 and T2 immediately prevents the deep discharge of the battery. If the voltage is above 12V, T2 is turned on. However, if the battery voltage decreases to less than 10.5V, the voltage regulator diode ZD1 is cut off, and the base potential of T2 becomes positive. It enters the cut-off mode and blocks the current of the output stage. VR2 should be adjusted so that when the battery voltage is higher than 10.5V, its emitter-base voltage is maintained at 0.6V to make T2 work.

After the main power supply is restored, all output voltages can supply power to their loads normally. But remember that when the main power fails, the load can only be powered if the battery voltage is sufficient (LED1 is on). For partially charged batteries, only 9V and 5V are available, and 12V is no longer usable. Also, if the battery voltage is lower than 10.5V, the three voltages of 12V, 9V and 5V are no longer usable. If the battery voltage is between 10.5V and 13V, the voltage range at the output terminal "A" is only between 10.5V and 12V. The reason is that a 12V Zener diode ZD2 is connected to the "A" terminal.

The "B" and "C" terminals provide 9V and 5V respectively through the voltage regulator chips IC1 and IC2. LED2 and LED3 are two super bright white LEDs. They are used for emergency lighting. R5 and R6 are current limiting resistors.

Before connecting the circuit to the battery and transformer, first connect the circuit to the variable DC power supply, add 12V, and adjust VR1 until LED1 lights up. Then reduce the voltage to 10.5V. Adjust VR2 until the output is cut off. Then remove the variable power supply and connect a fully charged 12V battery to the battery end to see if LED1 lights up. After everything is correct, connect the circuit to the battery and transformer. The battery should be a 12V 4.5Ah UPS-specific rechargeable battery.