We found this protection circuit for power supplies with the 12 V power supply itself in a 1982 documentation. This power supply can supply a 12 V voltage under a current of up to 2 A, depending on the transformer, and has a simple system, however efficient, protective. A relay that provides power to the output circuit is powered by its own voltage in a feedback system. If an accidental short circuit occurs at the output, lowering the voltage beyond the operating point of the relay, it trips by opening the contacts and therefore disconnecting the source from the local network. To "start" the source or reset it, after an accidental short circuit, just press the S1 switch for a few moments. Originally the project is indicated to provide a voltage of 12 V, in which case the diode is 12 V or 13 V5 x 400 mW (zener), and the relay with a 12 V coil such as RU 101 012, but it can be perfectly use zener diodes for lower voltages such as 6 or 9 V and relays with corresponding voltages.
