How to use LM317 to construct a voltage source circuit?

The LM317 is one of the most versatile and popular linear voltage regulators. Constructing a variable voltage source with it is straightforward.

Here is a comprehensive guide on how to use the LM317 to construct a voltage source circuit, from the basic formula to practical considerations.

The Fundamental Circuit

The most common configuration for a variable voltage source is as follows. You only need two resistors!

Components Required:

1. LM317 IC

2. Two Resistors: One fixed (R1), one variable potentiometer (R2). Typical values are 240Ω for R1 and a 5kΩ pot for R2.

3. Input and Output Capacitors: C_in and C_out (typically 0.1µF to 10µF).

The Golden Formula

The output voltage (Vout) is set by the ratio of the two resistors using this formula:

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

Since the adjustment pin current (Iadj) is very small (typically 50µA) and constant, the Iadj * R2 term is often negligible, simplifying the formula to:

Vout ≈ 1.25V * (1 + R2 / R1)

How it works:
The LM317 is designed to maintain a constant 1.25V reference (Vref) between its Output (Vout) and Adjustment (ADJ) pins. This 1.25V is dropped across resistor R1. The current flowing through R1 also flows through R2 (ignoring the tiny Iadj). The voltage drop across R2 is therefore 1.25V * (R2 / R1). Adding this to the 1.25V reference gives the total output voltage.

Example Calculation:
Let's use R1 = 240Ω and R2 = 5000Ω (a 5kΩ pot).

• Vout ≈ 1.25 * (1 + 5000 / 240)

• Vout ≈ 1.25 * (1 + 20.83)

• Vout ≈ 1.25 * 21.83

• Vout ≈ 27.29V

By adjusting the potentiometer (R2), you can vary the output voltage from about 1.25V (when R2 = 0Ω) up to the maximum your input voltage allows (minus dropout voltage).

Step-by-Step Construction Guide

1. Identify the Pins:

   • Input (Vin): Connects to the positive supply.

   • Output (Vout): Provides the regulated output voltage.

   • Adjustment (ADJ): Used to set the output voltage.

2. Connect the Resistors:

   • Connect R1 (240Ω) between the Output and Adjustment pins.

   • Connect one end of the potentiometer R2 to the Adjustment pin and the other end to Ground.

3. Add Capacitors (Highly Recommended):

   • Place a 0.1µF to 10µF capacitor (C_in) between the Input pin and ground. This improves transient response and stability.

   • Place a 1µF to 10µF tantalum or 25µF aluminum electrolytic capacitor (C_out) between the Output pin and ground. This is crucial for stability and improves ripple rejection. Without it, the circuit may oscillate.

4. Provide Input Power:

   • Connect your unregulated DC input voltage (e.g., from a wall adapter or battery) to the Input pin. The ground of the input supply connects to the circuit's common ground.

5. Get Your Output:

   • Your regulated DC voltage is available between the Output pin and ground.

Important Considerations & Limitations

1. Input Voltage (Vin): Must be at least 2.5V to 3V higher than the desired output voltage. This is called the dropout voltage. For a 12V output, you need at least ~14.5V input.

2. Maximum Voltage:

   • The LM317 can handle a maximum input-to-output differential voltage of 40V.

   • The absolute maximum voltage between any two pins is 40V.

3. Maximum Current: The standard LM317 can provide up to 1.5A of continuous output current.

4. Heat and Power Dissipation (CRITICAL!):

   • A linear regulator like the LM317 dissipates excess power as heat.

   • Power Dissipated (Pd) = (Vin - Vout) * Iload

   • Example: If Vin=12V, Vout=5V, and Iload=1A, the power dissipated is (12V - 5V) * 1A = 7W.

   • You MUST use a heatsink for currents above 100mA or large voltage differences. Without a proper heatsink, the IC will overheat and shut down.

Practical Circuit with Protection Diodes

For a more robust design, especially when using large output capacitors, add protection diodes.

• D1 protects the regulator if the input is shorted while the output capacitor (C_out) is still charged.

• D2 protects the regulator from a discharged capacitor on the ADJ pin if the output is shorted.

Fixed Output Voltage Circuit

If you need a fixed voltage, you can calculate and use fixed resistors for R1 and R2.

Formula for Fixed Output:
R2 = R1 * ((Vout / 1.25V) - 1)

Example for a 5V Supply:
Using R1 = 240Ω:

• R2 = 240 * ((5 / 1.25) - 1)

• R2 = 240 * (4 - 1)

• R2 = 240 * 3

• R2 = 720Ω (You can use a standard 680Ω and a 39Ω in series, or a 1kΩ trimpot to set it precisely).

Summary

Using an LM317 to build a voltage source is simple and reliable. Remember the key points:

• Use the formula Vout ≈ 1.25V * (1 + R2 / R1).

• Always use the output capacitor (C_out) for stability.

• Calculate power dissipation and use a heatsink when necessary.

• Ensure your input voltage is high enough (Vout + ~3V).

This circuit is perfect for benchtop power supplies, battery chargers, and providing a stable voltage for various analog and digital circuits.