LM317 Calculator

Introduction to the LM317 Voltage Regulator

The LM317 is a popular adjustable voltage regulator used in various electronic applications. Unlike fixed regulators that provide a constant output voltage, the LM317 allows for a customizable output, making it versatile for different projects. It operates with an input voltage that is at least 2V higher than the desired output voltage, ensuring stable and precise regulation.

How the LM317 Works

The LM317 works by adjusting the output voltage through a pair of resistors (R1 and R2) connected to its adjustment pin. By selecting appropriate values for these resistors, you can set the desired output voltage. The basic formula for calculating the output voltage is:

Vout = 1.25 * (1 + R2 / R1)


  • Vout is the output voltage.
  • R1 is the resistor connected between the output and the adjustment pin.
  • R2 is the resistor connected between the adjustment pin and ground.

How to use the LM317 Calculator

Our LM317 Calculator simplifies the process of finding the right resistor values for your desired output voltage. The calculator features three fields: Output Voltage (Vout), Resistor R1, and Resistor R2. By entering any two of these values, the third one is automatically calculated.

  1. Determine the desired output voltage: Decide on the output voltage you need for your project.
  2. Select known resistor value: Enter one known resistor value (either R1 or R2).
  3. Calculate the missing value: The calculator will automatically compute the required value for the other resistor.

Example Calculation

Suppose you need an output voltage of 5V, and you have a 240-ohm resistor for R1. Enter 5V in the Output Voltage field and 240 ohms in the R1 field. The calculator will provide the required R2 value of 720 ohm.

R2 = 240 * ( 5 / 1.25 - 1) = 720 Ω

Important Note on Input Voltage

Due to the internal voltage drop within the LM317 regulator, your input voltage should be at least 2V higher than the desired output voltage. For example, if you need a 5V output, ensure your input voltage is at least 7V to maintain proper regulation and avoid instability.

Compatibility with LM350 and LM338

It's worth noting that the same calculator can be used for the LM350 and LM338 voltage regulators, as they operate on similar principles and have almost identical characteristics. The LM350 is a 3A adjustable regulator, while the LM338 is a 5A adjustable regulator. Both follow the same formula for calculating the output voltage and can be used in a similar manner to the LM317.

Applications of the LM317, LM350, and LM338

These voltage regulators are widely used in power supply circuits for:

  • Adjustable power supplies: Provides a customizable output for different electronic projects.
  • Battery chargers: Ensures a stable and precise charging voltage.
  • LED drivers: Maintains consistent current and brightness.
  • DIY electronics: Ideal for hobbyists and professionals working on custom voltage requirements.


The LM317, along with the LM350 and LM338 voltage regulators, are versatile components in electronic design, offering adjustable voltage regulation with simple resistor calculations. Our calculator makes it easy to find the right resistor values for your desired output voltage, ensuring a stable and efficient power supply for your projects. Always remember to maintain the necessary voltage difference between input and output to achieve optimal performance.

Feel free to experiment with different values and explore the wide range of applications these regulators can support. Happy regulating!