4 Simple 12V UPS circuit

This project is one of the simplest 12V Uninterruptible Power Supply (UPS) using few parts. When AC source is available, the battery is charged and there is dc supply coming from regulator IC. On the other hand, when AC supply is not available, the battery will supply power to its output.

The R3 limits the charging current while R1 and R2 combination limits the output voltage of the IC to 13.9V or 13.2V for the battery. D1, D3, D4 prevents the battery from draining or supplying power to the IC. You can modify the circuit by changing the values of R1 and R2 and Bat only (you can use LM317 calculator to change this values).

Part List:
R1 - 120 ohms 1/4w
R2 - 1.2 kilo ohms 1/4W
R3 - 56 ohms 1/2W
C - 470uF (higher the better) electrolytic capacitor rated 25V
D1, D2, D3, D4 - 1N4004, 1N4007, or similar (ge diode is much better)
Bat - 12V lead acid battery
317 IC - LM317 regulator IC
DC input - see AC to DC converter below

The DC input is made out from a simple circuit shown below.

AC-DC converter

Part List:
Transformer - 220V to 12V center-tap min 12VA
D - 1N4004, 1N4007, or similar diode
C - 470uF (higher the better) electrolytic capacitor rated 25V