LM317 Calculator

The LM317 adjustable regulator sets its output with two resistors.

How the Math Works

The LM317 Calculator uses a fundamental voltage divider principle to determine the output voltage of the adjustable regulator. The formula Vout = 1.25 * (1 + R2 / R1) works because the LM317 maintains a constant 1.25V reference voltage between its output and adjustment pins. This reference voltage forces a current through R1 that remains nearly constant, and this same current flows through R2. The ratio of these two resistors determines how much additional voltage is added to the base 1.25V, creating the final regulated output voltage.

Practical Applications

To use this calculator practically, first identify the resistor values in your LM317 voltage regulator circuit. R1 is typically a fixed resistor between 240Ω and 1kΩ connected between the output and adjustment pins. R2 is your variable resistor or potentiometer that adjusts the output voltage. Enter these values into the calculator to determine your exact output voltage, which is essential for designing power supplies for electronics projects, LED drivers, or any circuit requiring a specific voltage from a higher input source.

Day-to-Day Use

This calculation helps in everyday applications like building custom power adapters for electronic devices, setting the correct brightness for LED lighting strips, or powering sensitive electronics that require precise voltage. For example, if you're creating a USB phone charger from an old computer power supply, you'd use this formula to ensure you get exactly 5V output. It also helps when replacing damaged voltage regulators or upgrading existing circuits to meet new voltage requirements without expensive trial and error.

Worked example

R1 240, R2 720 gives 5 V.

FAQ