Modifying the Output Voltage of an LM317 Voltage Regulator

The LM317 is a widely used linear voltage regulator that can be adjusted to provide a wide range of output voltages. This article discusses the methodology for fine-tuning the output voltage of an LM317 by using external resistors. We'll explore the typical configuration and the mathematical relationships that allow for precise control over voltage settings.

Principles and Formulas

The output voltage (V_{out}) of an LM317 can be adjusted using an external resistor network, comprising resistors (R_1) and (R_2). The formula provided for calculating the output voltage is:

(V_{out} V_{ref} left(1 frac{R_2}{R_1}right) I_{adj} cdot R_2)

Where: (V_{ref}) is typically 1.25V for the LM317 (I_{adj}), the adjustment pin current, is usually minimal (around 50 μA) and can often be neglected for practical purposes

Steps to Change the Output Voltage

Typically, (R_1) is chosen to be a standard value, often 240 ohms.

Rearranging the formula, we can find the value of (R_2) as follows:
(R_2 R_1 left(frac{V_{out}}{V_{ref}} - 1right))

Example Calculation

For instance, if you aim for an output voltage of 5V with (R_1) set at 240 ohms, the calculation would be:

(R_2 240 left(frac{5}{1.25} - 1right) 240 left(4 - 1right) 240 times 3 720 , Omega)

Connecting the Circuit

To modify the output voltage, follow these steps:

Example Configuration

R1: 240 ohms
R2: 720 ohms for a 5V output

Additional Considerations

Input Voltage and Regulation

For proper operation, the input voltage should be at least 3V higher than the desired output voltage. This ensures stable regulation.

Power Dissipation and Heat Management

Monitor the power dissipation of the LM317. If the heat sinks, use a heat sink or decrease the load to avoid overheating.

Stability and Capacitors

To enhance stability, add capacitors of 0.1 μF and 1 μF near the input and output pins respectively.

By adjusting (R_1) and (R_2), you can set the output voltage to your desired levels, providing a flexible and reliable voltage regulation solution.