Free LM317 Calculator

LM317 Voltage Regulator Calculator: An Overview

The LM317 adjustable voltage regulator is a staple in electronics for creating stable DC power supplies. Determining the correct external resistors to achieve a desired output can be done quickly with an LM317 calculator (also called a resistor divider calculator or LM317 circuit calculator). This free online voltage regulator output tool helps both hobbyists and professionals compute the necessary resistor values without manual trial‑and‑error.

Understanding the LM317 Adjustable Regulator

An LM317 is a three‑terminal adjustable positive voltage regulator. It can supply output currents from 0.01 A to 1.5 A and generate a steady voltage anywhere between 1.25 V and 37 V. For proper operation, the difference between the input voltage and the output voltage must be kept in the range of 3 V to 40 V.

The three terminals are:

• Input – receives the unregulated supply voltage.
• Output – delivers the regulated voltage.
• Adjust – connects to an external resistor divider that sets the output level.

The adjust pin works with two resistors, commonly labeled R1 and R2, that form a voltage divider. By altering the values of these resistors, you can select any output voltage from 1.25 V up to 37 V.

How the Output Voltage Is Determined

The exact output voltage of an LM317 circuit is given by:

$$V_{\text{out}} = V_{\text{ref}} \left(1 + \frac{R_2}{R_1}\right) + I_{\text{adj}} \times R_2$$

Here:

• $V_{\text{ref}}$ is the constant reference between the output and adjust pins, fixed at 1.25 V.
• $I_{\text{adj}}$ is the current flowing out of the adjust pin (typically 50 µA).

Because the $I_{\text{adj}} \times R_2$ term is very small, it is usually omitted, leading to the simple expression most designers rely on:

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

Thus, selecting appropriate R1 and R2 lets you set the regulator’s output to any value within its operating range.

Using the LM317 Calculator: A Practical Example

Assume you choose R1 = 240 Ω and R2 = 1200 Ω. Using the simplified formula:

$$V_{\text{out}} = 1.25 \times \left(1 + \frac{1200}{240}\right) = 1.25 \times (1 + 5) = 7.5\, \text{V}$$

The LM317 output voltage calculator confirms this result of 7.5 V. The tool also works in reverse: if you know the desired output voltage and either R1 or R2, it will compute the missing resistance. This makes the resistor divider calculator useful for both verifying existing designs and planning new ones.

For those who also need to identify unknown resistor values, a resistor color code calculator can assist in reading color bands.

Common Applications and Use Cases

LM317 regulators are employed in many electronic devices that require a regulated DC supply. Typical examples include:

• Desktop PCs
• Ethernet switches
• Power banks
• Hydraulic valves
• Refrigerators

These applications take advantage of the LM317’s ability to maintain a stable output voltage despite variations in the input supply. For more detailed specifications and application notes, the datasheet from Texas Instruments provides extensive guidance.

The LM317 voltage regulator calculator presented here simplifies the selection of R1 and R2, saving time and reducing errors in circuit design. By entering any two parameters (Vout, R1, or R2), you immediately receive the third value.