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LM317 voltage regulator calculator

Find adjustment resistors, output voltage and heat.

CalculatorHow to useFrequently asked questions

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R1 connects OUT to ADJ; R2 connects ADJ to ground. Their ratio sets the output voltage.

Choose a target voltage or enter both resistors. The calculation includes a nominal 50 µA adjustment current.

How to use

  1. Enter the input voltage and load current.
  2. Enter R1 and the target voltage, or switch to calculating the output from R2.
  3. Check the commercial resistor, resulting voltage and operating limits.

Formula and operating limits

Vout = 1.25 × (1 + R2/R1) + Iadj × R2. Leave at least 3 V input headroom. Check the package and heat sink in the datasheet. Texas Instruments.

Frequently asked questions

Can I always use R1 = 240 Ω?

Its divider current is below the legacy chip's worst-case 10 mA minimum load. Additional load may be necessary; R1 = 120 Ω supplies over 10 mA.

Does the LM317 always deliver 1.5 A?

No. Available current also depends on input-output voltage, temperature, package and cooling.

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