Stand-by Battery Charger
This simple circuit will find many applications as a battery eliminator for low power requirements. It consists of a transformer, a bridge rectifier and an electrolytic capacitor followed by a zener controlled series pass transistor. The output is stabilized at +7.5V. The stand-by battery, 7.5V, in series with D7, floats across the output terminals, ready to take over in case of main failure. The voltage drop across D7 will then reduce the power supply to about 7V. The R3 has an additional function, when working off the mains it will trickle charge the dry cells or storage battery. It?s correct resistance can be found by dividing the voltage potential difference between the zener D6 and the battery by the safe trickle current, which may amount to some 0.7 milliamps.
Part List R1-2-3=1Kohm D5=Led Red 3mm F1=50mA Fuse C1=1000uF 16V D6=8V2 0.4W zener B1=5X1.5V Battery C2=100uF 16V Q1=BD139 D1...4-7=1N4002 T1=230Vac// 10Vac - 0.5A transform.