The purpose of this circuit is to regulate the output voltage of a source at 11.4 V or other value, given by zener minus 0.6 V. The transistor can be the BD135 for currents up to 500 mA or the TIP31 for currents up to 2 A. The transistor must be equipped with a heatsink. The circuit was obtained from 1980 documentation. The input voltage must be at least 2 V higher than the desired output voltage. The voltage can be changed by changing the zener diode.
