How to Build a Variable Linear Voltage Regulator with LM317

Introduction

A variable linear voltage regulator like the LM317 is a useful circuit for generating a stable and adjustable DC output voltage from a higher voltage DC source. With just a few external components, the LM317 can be configured as a variable voltage regulator with adjustable output between 1.25V to 37V.

Here's what I'll cover in this guide on how to build a variable linear voltage regulator with the LM317:

How the LM317 Works

The LM317 is a 3-terminal adjustable linear voltage regulator. It works by comparing a fraction of the output voltage against a stable reference voltage of 1.25V. Based on the difference, it adjusts the pass element to maintain a constant output voltage.

The key features of the LM317 are:

• Output voltage adjustable from 1.25V to 37V
• Up to 1.5A load current
• Line regulation better than 0.01%
• Load regulation better than 0.1%

Internally, the LM317 contains the following key components:

• Reference voltage - Provides stable 1.25V reference
• Error amplifier - Compares fraction of output to reference and amplifies difference
• Pass transistor - Adjusts current flow to maintain regulated output voltage
• Current limit - Prevents overcurrent damage

By adding just two external resistors, we can configure the LM317 as an adjustable voltage regulator.

Parts Needed

To build a variable voltage regulator with the LM317, you'll need the following parts:

• 1 x LM317 regulator IC
• 1 x 240Ω resistor
• 1 x 2.2kΩ potentiometer
• 2 x 10μF capacitors
• 1 x Heat sink for LM317
• Wires, breadboard/PCB, etc.

Optionally, you can add:

• 1 x LED and resistor to indicate power
• Capacitors after the output for ripple reduction

LM317 Variable Voltage Regulator Circuit

Here is the basic circuit diagram for a variable voltage regulator using the LM317:

Let's go through each component:

• The 240Ω resistor sets the maximum output current. A higher value allows less current.
• The 2.2kΩ potentiometer adjusts the output voltage.
• The 10μF capacitors - Input and output capacitors help stabilize the circuit.
• A heatsink is vital for dissipating heat from the LM317.

The key formula for the LM317 is:

Vout = 1.25V * (1 + R2/R1) + Iadj * R2

Where:

• Vout is the ouput voltage
• R1 is the 240Ω resistor
• R2 is the potentiometer

This allows the output voltage to be adjustable from 1.25V to about 37V.

Construction Tips

Here are some tips for constructing the circuit:

• Use a suitable heatsink and thermal compound for the LM317. The regulator can get very hot.
• Ensure the input voltage does not exceed 40V for the LM317.
• Start with low resistance values for R1 and R2. Gradually increase to fine tune the adjustment range.
• Use capacitors before and after the regulator to improve stability.
• Twist wires connecting the potentiometer to reduce noise.
• Double check polarity when connecting electrolytic capacitors!

Testing and Adjustment

After constructing the circuit, test it as follows:

• Apply input power without any load first.
• Measure the output voltage as you adjust the potentiometer.
• Verify you can adjust from 1.25V up to 37V.
• Connect a load and measure regulation and ripple.
• If needed, adjust R1 to limit max current for your load.

That's it! You now have a working variable linear voltage regulator.

Example Applications

Some examples of how this LM317 regulator can be used:

• Variable bench power supply
• Adjustable bias voltage source
• LED driver with selectable brightness
• Battery charger with adjustable float voltage
• Step-down converter from 24V to 12V system voltage

With some additional components, the LM317 can provide a wide range of useful and stable DC voltages.

Summary

• The LM317 adjustable regulator provides 1.25-37V from a higher input voltage.
• Only 2 resistors are needed to configure the LM317.
• Proper heatsinking is vital to prevent overheating.
• Output capacitors improve stability and reduce ripple.
• Useful for variable voltage power supplies, battery chargers, LED drivers etc.

So that's how to build a simple variable linear voltage regulator using the LM317 IC! Let me know if you have any other questions.