LM317 Adjustable Voltage Regulator Calculator

LM317 Adjustable Voltage Regulator Calculator

Voltage Out (Vout):

Resistor R1:

Resistor R2:

Dropout Voltage (Vdo):

Minimum Input Voltage (Vin):

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Using the LM317 Voltage Regulator Calculator

This calculator was designed around the equation for the popular LM317 Adjustable Voltage Regulator. There are two methods of calculation included in this tool:

  1. Calculate the output voltage, given the resistor values for R1 and R2.
  2. Calculate R2 for a desired output voltage, given resistor R1.

Additionally, this LM317 includes a calculation for the minimum input voltage (Vin) given the dropout voltage (Vdo).

To calculate the output voltage of an LM317 voltage regulator circuit, you must first define the resistor values for R1 and R2. The default values for R1 and R2 are 124 ohms and 205 ohms, respectively. (These values were inspired by the popular Art of Electronics by Horowitz & Hill). Next, choose Vout in the Method to Calculate field. Finally, click Calculate for the result.

Alternatively, you can calculate R2 for a desired output voltage Vout. First, choose a resistance R1 and your desired output voltage Vout. Then, select R2 under Method to Calculate. Finally, click Calculate for the result.

Calculating the LM317 Output Voltage (Vout)

To calculating the LM317: take the ratio of R2 over R1, add 1 to it, and multiply the result by 1.25V. This is described in the following equation, which is what the tool above is based on.

Equation for calculating the output voltage of a LM317 adjustable voltage regulator circuit.
Equation for calculating the output voltage of a LM317 adjustable voltage regulator circuit.

Calculating Resistor R2

Sometimes you already have a desired output voltage in mind when designing a voltage regulator. Most times this is the case, so I’ve included the following equation. It takes in the desired output voltage and a resistor value for R1. The calculation is simply a rearrangement of the previous equation for calculating the output voltage Vout.

Equation for calculating R2, given a desired output voltage and R1.
Equation for calculating R2, given a desired output voltage and R1.

Rules for Resistor Values

The tool only allows an integer or decimal value into the resistance fields. Use the drop-down box on the side of each input box to convert the value between kilo ohms and ohms.

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