---
title: "Design LM317 regulator 5 V to 18 V"
subject: "Electronics Devices and Circuit"
chapter: 6
key: "semester_3/electronics_devices_and_circuit:lm317-design-5-18"
source: https://www.bctengineer.com/q/semester_3/electronics_devices_and_circuit/lm317-design-5-18
site: BCT Engineering
---

# Design LM317 regulator 5 V to 18 V

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

- Asked in: 2082 Shrawan, 4 marks
- Video: [LM317 Adjustable Voltage Regulator - English - Design adjustable voltage regulator using LM317](https://www.youtube.com/watch?v=enhQhQmW-a0) — Electronics Subjectified
- 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/lm317-design-5-18
- Whole chapter: https://www.bctengineer.com/md/semester_3/electronics_devices_and_circuit/chapter-6

## Question

Design a DC voltage regulator to give output voltage from 5V to 18V using LM 317.

## Short answer

Use R1=240Ω, a fixed resistor of 720Ω (e.g., 680Ω+39Ω) in series with a 2.5kΩ potentiometer for R2. Provide an unregulated input of about 22V dc. Output adjusts from 5V to 18V.

## Answer

**LM317** is a three-terminal adjustable positive regulator (1.25 V to 37 V, up to 1.5 A). Between OUT and ADJ it holds a constant reference Vref = 1.25 V, so with R1 from OUT to ADJ and R2 from ADJ to ground:

Vout = 1.25 (1 + R2/R1) + Iadj R2  (Iadj ≈ 50 µA, neglected)

#### Circuit
```text
            +--------------+
 Vin o--+---| IN       OUT |---+------+--o Vout
        |   |    LM317     |   |      |
      [C1]  |     ADJ      | [R1]   [C2]
        |   +------+-------+   |      |
       GND         +-----------+     GND
                   |
                 [R2 = fixed + pot]
                   |
                  GND
```
C1 = 0.1 µF (input bypass), C2 = 1 µF (output).

#### Design for 5 V to 18 V
1. Choose R1 = 240 Ω. The current in R1 is 1.25/240 = 5.2 mA, which is above the minimum load current of the LM317 (about 3.5 to 5 mA).
2. R2 = R1 (Vout/1.25 − 1)
3. **Minimum output** 5 V: R2(min) = 240 (5/1.25 − 1) = **720 Ω**
4. **Maximum output** 18 V: R2(max) = 240 (18/1.25 − 1) = **3216 Ω**
5. A potentiometer alone cannot give a minimum above 1.25 V, so use a fixed resistor in series with the pot:
   - Fixed resistor = 720 Ω (not a standard value; use 680 Ω + 39 Ω in series)
   - Potentiometer = 3216 − 720 = **2496 Ω** → use the next standard value, a **2.5 kΩ** pot, and set it so that the maximum output is 18 V
6. Input voltage: the LM317 needs a dropout of about 3 V, so Vin ≥ 18 + 3 = 21 V. Use an unregulated supply of about **22 V** dc (the maximum Vin − Vout is 40 V).

The pass transistor inside the IC dissipates (Vin − Vout) IL, so a heat sink is needed at high current.

**Answer: R1 = 240 Ω, R2 = 720 Ω (680 Ω + 39 Ω) fixed + 2.5 kΩ pot, Vin ≈ 22 V.**

## Check yourself

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

1. With R1 = 240 Ω, what are R2(min) and R2(max) for 5 V to 18 V?
   A. 720 Ω and 3600 Ω
   B. 720 Ω and 3216 Ω
   C. 960 Ω and 3216 Ω
   D. 300 Ω and 2160 Ω
   Answer: B. 240(5/1.25 - 1) = 720 Ω and 240(18/1.25 - 1) = 3216 Ω. (Key point: R2 limits)

2. What pot value follows after the fixed 720 Ω?
   A. 3216 - 720 = 2496 Ω
   B. 3216 Ω
   C. 720 Ω
   D. 1.25 kΩ
   Answer: A. The pot supplies the range between minimum and maximum R2. (Key point: Pot value)

3. Why must current through R1 matter?
   A. It sets the dropout to 3 V
   B. It limits Vin to 22 V
   C. It cancels the pass transistor heat
   D. 1.25/240 = 5.2 mA gives the regulator its minimum load current
   Answer: D. The LM317 needs about 3.5 to 5 mA minimum load for regulation. (Key point: Why R1 = 240)

4. Why choose Vin about 22 V?
   A. Vin must be 1.25 times 18
   B. Vin must be below 5 V
   C. Vin ≥ 18 + 3 V dropout = 21 V, plus a margin
   D. Vin must equal Vout(min) + 3 V
   Answer: C. Headroom of about 3 V above the maximum output is required. (Key point: Input voltage)

## Close variants set in other exams

- [Design LM317 regulator 7 V to 21 V](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3Alm317-design-7-21) — Electronics Devices and Circuit, chapter 6, asked 8×
- [Design LM317 regulator 6 V to 15 V](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3Alm317-design-6-15) — Electronics Devices and Circuit, chapter 6, asked 8×
- [Design LM317 regulator 3 V to 12 V](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3Alm317-design-3-12) — Electronics Devices and Circuit, chapter 6, asked 8×
- [Design LM317 regulator 3.7 V to 12 V](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3Alm317-design-3p7-12) — Electronics Devices and Circuit, chapter 6, asked 8×

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