LM317 Voltage Regulator Calculator

LM317 Adjustable Regulator

The LM317 is a common adjustable linear voltage regulator. Two external resistors set the output voltage above the internal 1.25 V reference. This calculator solves for output voltage or the required R2 value.

LM317 Output Formula

$$V_{out} = 1.25 \times \left(1 + \frac{R_2}{R_1}\right)$$

Example: with R1 = 240 Ω and R2 = 720 Ω, output is $1.25 \times (1 + 3) = 5.00\,\text{V}$.

Solving for R2

$$R_2 = R_1 \times \left(\frac{V_{out}}{1.25} - 1\right)$$

Choose R1 from standard values (often 120 Ω to 1 kΩ), then compute R2 for your target voltage. Use the nearest standard resistor and verify output with a multimeter.

Frequently Asked Questions

What is the minimum output voltage?

The LM317 reference is about 1.25 V, so that is the practical minimum regulated output in this simple two-resistor configuration.

Which resistor should I pick first?

Designers usually choose R1 (often 240 Ω) and calculate R2. Keep current through the divider modest to limit wasted power.

Does load current affect the formula?

The resistor formula sets the nominal output. Actual voltage may vary slightly with load, temperature, and dropout conditions.

Can I use this for the LM317T?

Yes. The same resistor equation applies to common LM317 variants in TO-220 and similar packages.

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LM317 Calculator

LM317 Adjustable Regulator

LM317 Output Formula

Solving for R2

Frequently Asked Questions

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