This is 7.5V Uninterpretable Power Supply simple circuit. This circuit was used in many applications as a battery backup for low power requirements. This circuit contains a bridge rectifier, an electrolytic capacitor,  and a transformer followed by a Zener controlled series pass transistor. This circuit can generate +7.5V stable output. Here is the circuit:

7.5V Uninteruptible Power Supply circuit schematic

The stand-by battery, 7.5V, is connected in series with D7, ready o take over in case of main failure. The power supply is reduced to about 7V by The voltage drop across D7. when working off the mains, R3 will trickle charge the storage battery or dry cells. By dividing the voltage potential difference between the Zener diode D6 and the battery, the correct resistance will be found. [Source: Sam Electronic Circuit]