What is Current Limiting Protection?
The circuit arrangement for IC 723 as low voltage or high voltage regulator has no protection. If short circuiting takes place (when load demands more current), regulator IC makes efforts to provide it at a constant output voltage which results in heating that may finally burn the IC.
To prevent the load current from increasing a set value, say ILIMIT is used so that current gets limited. As current increases due to want of load the output voltage falls, and current limiting protection is provided by connecting an internal resistance Rsc between pin No. 10 (Vout) and pin No. 3 (emitter of Q2). The emitter of Q2 is also connected to the load.
The load current generates a small voltage drop, VSENSE across Rsc which is directly applied to base-emitter junction of Q2. When the transistor becomes ON (when VBE ≡ 0.6 V), the current from error amplifier reaches collector of Q2, and IB of Q1 decreases, and hence IE of Q1 too. Thus, any increase in load current gets cancelled. Similarly, if the load current decreases, VBE of Q2 drops, the cycle gets repeated such that load current is held constant to produce a voltage drop VSENSE across Rsc to turn ON Q2.
This method of current limiting protection is also called as current sensing technique.