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Current limiting circuit in dc regulator

Short answer

A current limiting circuit uses a series-pass transistor Q1 with a current-sense resistor Rsc in its emitter path. A limiting transistor Q2 has its base-emitter junction across Rsc and collector tied to Q1’s base. When load current causes Rsc voltage drop to reach ~0.6 V, Q2 turns on, stealing Q1’s base drive and limiting output current to IL(max) ≈ 0.6 V / Rsc.

Draw the circuit of current limiting circuit in dc voltage regulator.

 Vi o----+-----------------------+
         |                       | C
        [R]                      |
         |                 B     |
         +---------+-------------Q1  (series pass)
         |         |             |
 (error amplifier  |             | E
  / Zener drive    |             +-----+---[Rsc]---+-----+----o Vo
  to Q1 base)      |                   |           |     |
                   | C                 |           |    [RL]
                   Q2  B---------------+           |     |
                   E                               |    GND
                   |                               |
                   +-------------------------------+
  • Q1 is the series pass transistor of the regulator, and Rsc is the small current-sense resistor in series with the load.
  • Q2 is the limiting transistor. Its base-emitter junction is connected across Rsc (base on the Q1 side, emitter on the output side), and its collector is connected to the base of Q1.
  • Normal operation: IL Rsc < 0.6 V, so Q2 is OFF.
  • Overload: when IL Rsc ≈ 0.6 V, Q2 turns ON and takes away the base drive of Q1. The output current is limited to

IL(max) ≈ 0.6 V / Rsc

✓ Answer checked by Claude Opus 5.5 (high effort)

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