Adjustable Voltage Regulator LM338 Circuit
Learn how the LM338 adjustable voltage regulator works, wiring diagrams, components, calculations, and troubleshooting for safe DC power supply circuits.
What is an LM338 Adjustable Voltage Regulator?
LM338 is a voltage regulator IC with 3 terminals that are adjustable terminals. LM338 is also used in place of fixed voltage regulators (7805, 7812) with the output voltage being adjustable between 1.2V and 32V. It is safe to use on different DC circuits since it offers overload and thermal protection.
High current LM338 regulator
Adjustable voltage regulator LM338 Circuit is a flexible DC voltage regulator circuit designed to find application in electronics, power supply, and battery chargers. LM338 can provide up to 5A load current and can be varied to 1.2V to 32V with two resistors. This renders it appropriate for both leisure and business.
This is a guide on the working of LM338, pin arrangement, components needed, Wiring diagram, calculation of the output voltage, and safety measures. You will also be taught how to troubleshoot the common problems, which include overheating, ripple, or even unregulated voltage. Using this guide, you will be able to create a stable, adjustable DC power supply with LM338 when using it in microcontroller projects, LED drivers, or any other electronic circuits that need a stable, adjustable power supply.
LM338 Pin Configuration and Features
- Pin 1 (Adjust): Connects to the resistor divider to set the output voltage.
- Pin 2 (Output): Regulated voltage output.
- Pin 3 (Input): Connect to unregulated DC supply.
Features:
- Adjustable output voltage 1.2–32V
- Maximum load current 5A
- Built-in current limiting and thermal shutdown
- Low dropout voltage
Components Required for LM338 Circuit
- LM338 IC
- Two resistors (R1 fixed, R2 variable)
- Capacitors: 0.1µF (input), 1µF (output)
- Heatsink for LM338
- DC input supply (unregulated)
- Wires and breadboard or PCB
LM338 Adjustable Voltage Regulator Wiring Diagram
- Connect the input DC supply to Pin 3 (Input).
- Connect Pin 2 (Output) to the load.
- Connect R1 (fixed resistor, typically 240Ω) between Output and Adjust.
- Connect R2 (variable resistor) between the Adjust pin and ground.
- Connect input/output capacitors for stability.
- Add heatsink to prevent overheating.
Step-by-Step Construction Guide
- Prepare components: LM338 IC, resistors, capacitors, heatsink.
- Mount IC on breadboard or PCB.
- Wire resistors between the output and adjust pins.
- Add capacitors: 0.1µF on input, 1µF on output.
- Connect the DC input and test with a multimeter.
- Adjust R2 to set the desired output voltage.
- Check load response to ensure stable output.
Calculating Output Voltage (Vout) with LM338
Output voltage is calculated using Ohm's Law and the LM338 formula:
$$ V_{out} = 1.25 \times \left(1 + \frac{R_2}{R_1}\right) + I_{adj} \times R_2 $$
Where:
R1 = 240Ω (fixed), R2 = variable resistor, Iadj ≈ 50µA (negligible)
Example: R1 = 240Ω, R2 = 1kΩ → Vout ≈ 1.25 × (1 + 1000/240) ≈ 6.5V
Applications of LM338 Adjustable Regulator
- Variable DC power supply
- Battery charger circuits
- LED driver circuits
- Microcontroller projects requiring stable voltage
- DIY lab power supplies
Troubleshooting Common Problems
Output Voltage Not Adjustable
- Check resistor connections
- Ensure R2 is not shorted or open
Overheating Issues
- Add proper heatsink
- Avoid exceeding 5A current
Ripple in Output Voltage
- Add smoothing capacitors
- Ensure input voltage is higher than desired output by ≥3V
