--- title: "Zener current for two load values" subject: "Electronics Devices and Circuit" chapter: 6 key: "semester_3/electronics_devices_and_circuit:zener-current-rl-numeric" source: https://www.bctengineer.com/q/semester_3/electronics_devices_and_circuit/zener-current-rl-numeric site: BCT Engineering --- # Zener current for two load values Electronics Devices and Circuit · Chapter 6: Power Supplies, Breakdown Diodes, and Voltage Regulators - Asked in: 2073 Shrawan, 4 marks - Video: [Solved Problems on the Zener Diode](https://www.youtube.com/watch?v=kYtebfx-yHc) — ALL ABOUT ELECTRONICS - 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/zener-current-rl-numeric - Whole chapter: https://www.bctengineer.com/md/semester_3/electronics_devices_and_circuit/chapter-6 ## Question Find the zener current from the given circuit if (i) RL = 1.2 KΩ (ii) RL = 3 KΩ. [Figure: 16 V source in series with R = 1 KΩ; Zener diode (PzM = 80 mW, Vz = 10 V) in parallel with load RL] ## Short answer For RL=1.2 kΩ, open-circuit voltage 16×1.2/(1+1.2)=8.73 V < 10 V, so Zener off, Iz=0. For RL=3 kΩ, voltage 12 V > 10 V, Zener on: IR=(16−10)/1k=6 mA, IL=10/3k=3.33 mA, Iz=6−3.33=2.67 mA. ## Answer **Given:** Source = 16 V, R = 1 kΩ, Zener Vz = 10 V with PZM = 80 mW, load RL in parallel with the Zener. #### Step 1: Maximum Zener current IZM = PZM/Vz = 80 mW / 10 V = 8 mA #### Step 2: Check whether the Zener is ON (breakdown) Remove the Zener and find the voltage across RL: VL = 16 × RL/(R + RL). The Zener conducts only if VL ≥ Vz = 10 V. #### (i) RL = 1.2 kΩ VL = 16 × 1.2/(1 + 1.2) = 16 × 0.5455 = 8.73 V Since 8.73 V < 10 V, the Zener is **OFF** (not in breakdown). **Iz = 0** (The load voltage is then 8.73 V, and the load current IL = 8.73/1.2 kΩ = 7.27 mA.) #### (ii) RL = 3 kΩ VL = 16 × 3/(1 + 3) = 12 V Since 12 V > 10 V, the Zener is **ON**, with Vo = 10 V. IR = (16 − 10)/1 kΩ = 6 mA IL = 10 V / 3 kΩ = 3.33 mA **Iz = IR − IL = 6 − 3.33 = 2.67 mA** This is less than IZM = 8 mA, so the Zener is safe. **Answer: (i) Iz = 0 (Zener off); (ii) Iz = 2.67 mA.** ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How is the maximum Zener current found from the data given? A. IZM = PZM x Vz = 800 mA B. IZM = PZM/Vz = 80 mW/10 V = 8 mA C. IZM = Vz/R = 10 V/1 kΩ = 10 mA D. IZM = (16 - 10)/1 kΩ = 6 mA Answer: B. Power rating equals Vz x IZM, so IZM = 8 mA. (Key point: Maximum Zener current) 2. How do you check whether the Zener is in breakdown? A. Assume it conducts and compute IR = (16 - 10)/1k; it is always ON B. Compare 16 V with the power rating C. Remove the Zener and compute VL = 16 x RL/(R + RL); it conducts only if VL >= Vz D. Check whether RL is larger than 1 kΩ Answer: C. If the open-circuit voltage across RL is below Vz the Zener cannot break down. (Key point: Is the Zener ON?) 3. What is the Zener current for RL = 1.2 kΩ? A. Zero, since VL = 8.73 V is less than 10 V so the Zener is OFF B. 6 mA, because (16 - 10)/1 kΩ flows in the Zener C. 2.67 mA D. 8 mA, the maximum Zener current Answer: A. With the Zener off, only the 1 kΩ and 1.2 kΩ divider acts, giving 8.73 V. (Key point: Case RL = 1.2 kΩ) 4. What is the Zener current for RL = 3 kΩ? A. Iz = 6 mA B. Iz = IR + IL = 9.33 mA, which exceeds the 8 mA limit C. Iz = 0 since VL = 12 V is above 10 V D. Iz = IR - IL = 6 mA - 3.33 mA = 2.67 mA Answer: D. VL = 12 V > 10 V, so the Zener is ON and holds 10 V; IR = 6 mA and IL = 10/3k = 3.33 mA. 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