Op-amp Regulator with Current Limiting and Overload Protection

If I_L ≅ 100 mA, voltage drop across R_sc ≈ 0.12 volts and therefore, is not adequate to turn Q₂

ON. When I_L ≥ 494 mA or so V_BE 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 - R_sc 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