Deriving High Current from 7805, 7812 Voltage Regulator Power Supply Citcuits

If, for instance, a power transistor is connected in parallel with the IC, the supply will no longer be protected against short-circuits. 

The circuit given here shows that a simpler solution is possible: the power transistor, T1, is provided with an emitter resistor! This effectively solves the problem, because the current through Tl is then proportional to the current supplied by the voltage regulator.

But this solution A suffers from a heavy power loss during short-circuit conditions, which is not really acceptable either. 

There are various ways and means of drawing more current from a voltage regulator IC than it was originally intended to supply, but most methods have their disadvantages.
lf the 7805 or 7812 regulator and T1 are mounted onto the same heatsink, the transistor is also thermally protected! The output voltage is dependent only on the type of voltage regulator used and, as drawn here, the circuit is suitable for currents up to 2 A. 

lf higher values are required, some components need to be changed according to the table. For currents above 7 A, transistor T1 must be replaced by two parallel-connected transistors each of which has an emitter resistor, R1 and R1' respectively.

That can, of course, be remedied by adding a current sensor in the shape of an extra transistor which, during overload conditions, cuts off the base current to the power transistor.