--- title: "Zener voltage regulator working" subject: "Basic Electronics Engineering" chapter: 2 key: "semester_2/basic_electronics_engineering:zener-voltage-regulator" source: https://www.bctengineer.com/q/semester_2/basic_electronics_engineering/zener-voltage-regulator site: BCT Engineering --- # Zener voltage regulator working Basic Electronics Engineering · Chapter 2: Diodes - Asked in: 2067 Mangsir, 7 marks - Video: [Using Zener Diodes (Part 1) - Voltage Regulator and Zener Theory](https://www.youtube.com/watch?v=E4IIiIhIYAg) — Simply Electronics - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_2/basic_electronics_engineering/zener-voltage-regulator - Whole chapter: https://www.bctengineer.com/md/semester_2/basic_electronics_engineering/chapter-2 ## Question Draw the Zener voltage regulator circuit and explain clearly the working principle of this circuit to produce a regulated dc output. ## Short answer A Zener regulator uses a reverse-biased Zener diode in parallel with the load and a series resistor. In breakdown, VZ stays constant; changes in input voltage or load current are absorbed by the Zener current, keeping output voltage fixed. Design ensures IZ stays between IZmin and IZmax. ## Answer A **Zener voltage regulator** keeps the dc output voltage almost constant when the input voltage or the load current changes. It uses a Zener diode operated in reverse breakdown. #### Circuit ```text Rs Vin o--/\/\/--+----------+--o Vo = VZ (unregulated) | | Z RL (reverse biased) | | | GND o----------+----------+--o ``` The unregulated dc Vin (from a rectifier and filter) is applied through a series resistor Rs. The Zener is connected in reverse across the load RL. #### Working principle In breakdown, the Zener voltage stays almost constant at VZ while its current can vary from IZmin to IZmax. So Vo = VZ. - Series current: IR = (Vin − VZ)/Rs - Load current: IL = VZ/RL - Zener current: IZ = IR − IL **Input voltage change (line regulation):** if Vin rises, IR rises. The extra current flows through the Zener (IZ rises), the voltage drop across Rs increases by the same amount as the rise in Vin, and Vo stays at VZ. If Vin falls, IZ falls in the same way. **Load change (load regulation):** if RL decreases, IL increases; the Zener current decreases by the same amount, so IR (and therefore Vo) stays constant. If RL increases, IL falls and IZ rises. #### Design conditions - Vin(min) must exceed VZ + (IZmin + ILmax)·Rs so that the Zener stays in breakdown. - Rs = (Vin − VZ)/(IZmin + ILmax) - IZ must not exceed IZmax = PZ/VZ (power rating). - IZ must not fall below IZmin (the knee current); otherwise regulation is lost. Example: Vin = 12 V, VZ = 6.8 V, Rs = 100 Ω → IR = 52 mA. With a 340 Ω load, IL = 20 mA and IZ = 32 mA, and the output stays at 6.8 V. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In which region is the Zener operated for regulation? A. Forward bias B. Cut-off C. Reverse breakdown D. Saturation Answer: C. In reverse breakdown the voltage stays near VZ over a wide current range. (Key point: Zener region) 2. When Vin rises, what happens? A. IZ rises and the extra drop appears across Rs, so Vo stays at VZ B. Vo rises by the same amount C. IZ falls and IL rises D. RL changes to compensate Answer: A. The extra current flows through the Zener and the extra voltage drops across Rs. (Key point: Line regulation) 3. If RL decreases, what happens to the currents? A. IR increases B. IZ increases and IR falls C. IL increases and IZ decreases by the same amount so IR is constant D. Vo drops to zero Answer: C. The load takes more current from the Zener branch, leaving IR and Vo unchanged. (Key point: Load regulation) 4. Which expression gives the Zener current? A. IZ = IR + IL B. IZ = IR - IL C. IZ = IL - IR D. IZ = Vin/RL Answer: B. KCL at the output node gives IZ = IR - IL. (Key point: Zener current relation) 5. What limits must IZ obey? A. IZ must be zero at no load B. IZ must exceed IZmax C. IZ must stay between IZmin (knee) and IZmax = PZ/VZ D. IZ must equal IL Answer: C. Below IZmin regulation is lost; above IZmax the power rating is exceeded. 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