battery stand-by circuit

This simple circuit will find many applications as a battery eliminator for low power Iequirements. It consists of a transformer, a bridge rectifier and an electrolytic capacitor followed by a zener controlled series pass transistor. The output is stabilised at 7.5 volts. The stand-by battery, 7. 5 volts, in series with D2, floats across the output ter- minals, ready to take over in case of mains failure.