---
title: "Design 6 V Zener regulator, regulation factor"
subject: "Electronics Devices and Circuit"
chapter: 6
key: "semester_3/electronics_devices_and_circuit:zener-regulator-design-6v-12v"
source: https://www.bctengineer.com/q/semester_3/electronics_devices_and_circuit/zener-regulator-design-6v-12v
site: BCT Engineering
---

# Design 6 V Zener regulator, regulation factor

Electronics Devices and Circuit · Chapter 6: Power Supplies, Breakdown Diodes, and Voltage Regulators

- Asked in: 2071 Shrawan, 5 marks
- Video: [Zener Diode as Voltage Regulator | Parameters & Example of Zener Diode as Voltage Regulator](https://www.youtube.com/watch?v=F6eGVaNcavg) — Engineering Funda
- 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-regulator-design-6v-12v
- Whole chapter: https://www.bctengineer.com/md/semester_3/electronics_devices_and_circuit/chapter-6

## Question

Design DC voltage regulator for 6V output. Given data are Vz = 6V at Iz = 20 mA, Izk = 2 mA, PZMAX = 500 mW and rz = 20Ω. The nominal input voltage is 12V±20% DC. Find voltage regulation factor and maximum current it can deliver to the load.

## Short answer

Use series resistor R=110Ω with a6V Zener (Vz0=5.6V, rz=20Ω). Maximum load current is34mA. Line regulation154mV/V, load regulation -16.9mV/mA.

## Answer

**Given:** Vz = 6 V at Iz = 20 mA, Izk = 2 mA, Pzmax = 500 mW, rz = 20 Ω. Vin = 12 V ± 20 % → Vin(min) = 9.6 V, Vin(max) = 14.4 V.

#### Circuit
```text
 Vin o----[ R ]-----+------+----o Vo ≈ 6 V
                    |      |
                  (Dz)    [RL]
                    |      |
                   GND    GND
```

#### Zener model
Vz0 = Vz − rz Iz = 6 − 20 × 0.02 = 5.6 V. So Vz(Iz) = 5.6 + 20 Iz.

#### Maximum Zener current and R
Pz = Vz0 Iz + rz Iz² = 0.5 W → 20 Iz² + 5.6 Iz − 0.5 = 0 → Iz(max) = 71.2 mA (Vz = 7.02 V).

Worst case for the Zener power: Vin(max) with no load:

R ≥ (14.4 − 7.02)/71.2 mA = 103.6 Ω → choose the standard value **R = 110 Ω**

#### Maximum load current
At Vin(min), the Zener must stay at the knee current Izk = 2 mA (Vz = 5.6 + 0.04 = 5.64 V):

IR = (9.6 − 5.64)/110 = 36 mA

**IL(max) = IR − Izk = 36 − 2 = 34 mA** (RL(min) ≈ 5.64/34 mA = 166 Ω)

#### Voltage regulation factors
**Line regulation:** ΔVo/ΔVin = rz/(R + rz) = 20/(110 + 20) = 0.154 V/V = **154 mV/V**. For the full input swing of 4.8 V (9.6 V to 14.4 V), Vo changes by about 0.74 V. At no load: Vo = 6.2 V at 9.6 V; 6.95 V at 14.4 V; and 6.58 V at 12 V nominal. In percent: line regulation ≈ 0.154 × 100/6.6 = 2.3 % per volt of input change.

**Load regulation:** ΔVo/ΔIL = −(rz ∥ R) = −(20 × 110/130) = **−16.9 Ω** (−16.9 mV per mA). For the full load swing of 34 mA, Vo falls by about 0.57 V.

Note: because the Zener current at the design point differs from 20 mA, Vo is about 6.6 V and not exactly 6 V; to obtain exactly 6 V the Zener current should be set at 20 mA (R about 300 Ω at no load), but then IL(max) falls.

**Answer: R = 110 Ω; IL(max) = 34 mA; line regulation 154 mV/V; load regulation −16.9 mV/mA.**

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How is the maximum load current found?
   A. At Vin(min) keep Iz = Izk: IL(max) = (9.6 - 5.64)/110 - 2 mA = 34 mA
   B. At Vin(max) with Iz = 71.2 mA: IL = 36 mA
   C. At 12 V nominal: IL = 20 mA
   D. IL = 500 mW/6 V = 83 mA
   Answer: A. The Zener must stay at its knee current at the lowest input, so IL(max) = IR - Izk. (Key point: Maximum load current)

2. What is the line regulation?
   A. R/(R + rz) = 846 mV/V
   B. rz x R/(R + rz) = 16.9 Ω
   C. ΔVo/ΔVin = rz/(R + rz) = 20/130 = 154 mV/V
   D. rz/R = 182 mV/V
   Answer: C. Input changes divide between R and rz, and the Zener resistance rz gives the output change. (Key point: Line regulation)

3. What is the load regulation?
   A. -rz = -20 mV/mA
   B. -(rz ∥ R) = -16.9 mV per mA of load current
   C. -(rz + R) = -130 mV/mA
   D. +154 mV/V
   Answer: B. Output resistance looking into the regulator is rz in parallel with R. (Key point: Load regulation)

## Close variants set in other exams

- [Design DC voltage regulator for 6 V](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3Azener-regulator-design-6v) — Electronics Devices and Circuit, chapter 6, asked 4×

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