Using a 7805 voltage regulator and some classic electronic components can be made a very simple voltage regulator capable of providing a maximum current of 3 amperes.
T1 acts as a current limiter. Once the voltage on R2 + R3 is greater than 0.6 0.7 V, the transistor opens, which will reduce to zero the current in the base of T2.
Voltage from which enters in operation the protective circuit is gived by the sum of drops voltage on the R2 and R3. Resistors R3 and R4 form a voltage divider on T2.