Op-amp Regulator with Current Limiting and Overload Protection
If IL ≅ 100 mA, voltage drop across Rsc ≈ 0.12 volts and therefore, is not adequate to turn Q2
ON. When IL ≥ 494 mA or so VBE Q2 ≅ 0.6 and Q2 is turned on and therefore, diverts part of
the base current of Q1 towards collector Q2 thus limiting the current. Therefore, Q2 - Rsc combination comes into picture only while load current tries to become more than the intended value (500 mA, in this case).
Figure: Op-amp Regulator with Current Limiting and Overload Protection