A 7805 type regulator has a maximum current of 1A. With the circuit presented here, this current is increased by adding a power transistor (T3 in the circuit diagram below) on a heat sink. Thus, a 3A voltage regulator may be build based on the standard 7805.
The T3 transistor is switched on at a current above 15mA, when the voltage drop across R2 is large enough to switch it on. The T3 transistor is also protected against short circuits by T2. When the current through the MJ2955 rises above 3A, the voltage drop across R1 is large enough to switch on the T2. This limits the base-emitter voltage of T3, so that the output current cannot increase much more.
T1, switches on a LED as soon as current limiting occurs. Resistor R3 has been added to limit the current through the regulator as soon as the current limiting circuit operates, since R2 is then short circuited by T2: in the absence of R3, the full current would flow through the 7805.
The input voltage must be 10V for an output current of 3A, instead for the standard 8.5V required by the 7805 for currents up to 1 A. An input voltage of about 12V is actually the recommended value for this circuit. T2 and T3 must be insulated from the heat sink.
The 7805 does not really need a heat sink.
The pcb Artwork for the 5V/3A linear regulator