This circuit consists of a voltage regulator for 6 V batteries with a very low voltage drop. The transistor must be mounted on a heatsink and for the BD135 the maximum current is 500 mA. The BC548 can be used for currents up to 50 mA. The input voltage must be at least 3 V higher than the desired output voltage. The output voltage will be given by Vz (Rb / Rb + Ra) and Rz depends on the input voltage and the voltage of the zener so that we have a current in the zener of the order of 1 mA.
