--- title: "Series regulator with current limiting" subject: "Electronics Devices and Circuit" chapter: 6 key: "semester_3/electronics_devices_and_circuit:series-regulator-current-limit-working" source: https://www.bctengineer.com/q/semester_3/electronics_devices_and_circuit/series-regulator-current-limit-working site: BCT Engineering --- # Series regulator with current limiting Electronics Devices and Circuit · Chapter 6: Power Supplies, Breakdown Diodes, and Voltage Regulators - Asked in: 2083 Baisakh, 6 marks; 2081 Baisakh, 7 marks; 2079 Baisakh, 6 marks; 2076 Ashwin, 6 marks; 2074 Chaitra, 1+5 marks; 2072 Chaitra, 5 marks; 2073 Chaitra, 7 marks; 2071 Shrawan, 6 marks (asked 8 times) - Video: [How Current Limiting Circuit Works in Voltage Regulator](https://www.youtube.com/watch?v=yoLBv2gPrl0) — root tech - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/electronics_devices_and_circuit/series-regulator-current-limit-working - Whole chapter: https://www.bctengineer.com/md/semester_3/electronics_devices_and_circuit/chapter-6 ## Question Draw series voltage regulator with current limiting circuit and explain how this protection circuit works. ## Short answer A sense resistor Rsc in series with the load develops IL·Rsc. When it reaches ~0.6 V, Q2 turns on and shunts base current from the pass transistor, limiting output current to IL(max) ≈ 0.6 V / Rsc. ## Answer A series regulator with current limiting protects the pass transistor and the load from damage during an overload or short circuit. A sense resistor Rsc and a transistor Q2 limit the maximum output current. #### Circuit diagram ```text Vi o----+--------------------+ | | C [R] B1 +-----| Q1 (pass) | | | E +--------------+ +------+---[Rsc]---+------+----o Vo | | | | | | | | | [RL] | +---+ C | | | | |--B-----+ | GND | | E Q2 | | +--------------------+ [Zener / error amp] ``` Connections: Q1 is the pass transistor (collector to Vi). Its emitter goes through the small sense resistor Rsc to the output Vo. The base of Q2 is connected to the Q1-emitter side of Rsc and the emitter of Q2 to the output side, so VBE2 = IL Rsc. The collector of Q2 is connected to the base of Q1 (node B1). Q1 is driven by the regulator (error amplifier or Zener) in the usual way. #### Working 1. **Normal load:** IL Rsc < VBE2(on) ≈ 0.6 V, so Q2 is OFF and has no effect. Q1 is controlled by the regulator and keeps Vo constant. 2. **Overload or short circuit:** IL rises. When IL Rsc reaches about 0.6 V, Q2 turns ON. 3. Q2 now diverts (shunts) base current away from Q1, because Q2's collector is connected to the base of Q1. Q1 conducts less, so IL cannot rise further. 4. Thus the output current is clamped to **IL(max) = VBE2 / Rsc ≈ 0.6 V / Rsc** 5. In a short circuit the output voltage falls while the current stays at IL(max), so the power dissipation of Q1 is limited. This is called constant-current limiting. **Example:** For IL(max) = 0.5 A: Rsc = 0.6/0.5 = 1.2 Ω. A disadvantage is the drop IL·Rsc in the output path (about 0.6 V at full current), which reduces the efficiency. Foldback limiting reduces the current further as the output voltage falls, giving lower dissipation in a short circuit. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What gives the voltage that turns on Q2? A. IL x Rsc across the sense resistor, equal to VBE2 B. The Zener voltage C. Vi D. The load capacitor Answer: A. VBE2 = IL Rsc. (Key point: Sense element) 2. When Q2 turns on, what does it do? A. Shunts base current away from Q1 so IL stops rising B. Increases Q1 base current C. Opens the load D. Grounds the output Answer: A. Q2 collector is on Q1 base. (Key point: Limiting action) 3. What is IL(max)? A. VBE2/Rsc B. Vz/Rsc C. Vi/RL D. VBE2 Rsc Answer: A. Q2 turns on at about 0.6 V across Rsc. (Key point: Limit current) 4. For IL(max) = 0.5 A, what is Rsc? A. 1.2 Ω B. 0.3 Ω C. 3 Ω D. 12 Ω Answer: A. 0.6/0.5 = 1.2 Ω. (Key point: Rsc design) ## Close variants set in other exams - [Series regulator current limiting; LM317 design 7-21 V](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3Aseries-regulator-current-limit-lm317) — Electronics Devices and Circuit, chapter 6, asked 9× ## Related in Electronics Devices and Circuit - [Current limiting circuit in dc regulator](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3Acurrent-limiting-circuit-draw) — Electronics Devices and Circuit, chapter 6 - [Series regulator stability factor and overload protection](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3Aseries-regulator-stability-overload) — Electronics Devices and Circuit, chapter 6 - [Series regulator circuit and stability factor](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3Aseries-regulator-stability-factor) — Electronics Devices and Circuit, chapter 6, asked 12× - [LM317 voltage regulator circuit](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3Alm317-regulator-circuit) — Electronics Devices and Circuit, chapter 6 - [Reverse breakdown; Zener as voltage regulator](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3Areverse-breakdown-zener-regulator) — Electronics Devices and Circuit, chapter 6 - [Series regulator efficiency; bandgap-reference regulator](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3Aseries-regulator-low-efficiency-bandgap) — Electronics Devices and Circuit, chapter 6, asked 2× ## Related in other subjects - [Headworks sketch, regulator section, protection works](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Aheadworks-sketch-regulator-section-dissipators) — Irrigation and Drainage Engineering, chapter 7 - [Zener circuit output voltage and current](https://www.bctengineer.com/question.md?key=semester_1%2Fbasic_electrical_and_electronics_engineering%3Azener-circuit-120v-5k) — Basic Electrical and Electronics Engineering, chapter 3 - [Current for flux in cast steel series circuit with air gap](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Acast-steel-series-circuit-airgap-200turns) — Electrical Machines I, chapter 1 - [RL series circuit current, pf, power](https://www.bctengineer.com/question.md?key=semester_1%2Fbasic_electrical_and_electronics_engineering%3Arl-series-230v-numerical) — Basic Electrical and Electronics Engineering, chapter 1