LM317 Voltage Calculator: Set the Output Voltage

An LM317 voltage calculator determines the resistor value needed to set an adjustable LM317 regulator’s output, or predicts the output from resistor values you already have. Enter the resistor between the output and adjust pins and the resistor from adjust to ground; the calculator returns the ideal output voltage and the standard resistor value for a chosen target.

Formula

The standard LM317 output-voltage equation is:

V_out = V_ref × (1 + R2/R1) + I_adj × R2

• V_out is the predicted output voltage, in volts.
• V_ref is the regulator’s reference voltage; the usual nominal value used for a first estimate is 1.25 V.
• R1 is the resistor from the output pin to the adjust pin, in ohms.
• R2 is the resistor from the adjust pin to ground, in ohms.
• I_adj is current flowing from the adjust pin, in amperes. This calculator uses 0.00005 A, or 50 µA, as a nominal estimate.

For a target voltage, rearrange the same equation: R2 = (V_out − V_ref) / (V_ref/R1 + I_adj). The calculator uses this rearranged form to estimate R2. Actual reference voltage and adjust-pin current vary with the specific device and operating conditions, so these are design estimates rather than precision guarantees.

Worked Example

With the calculator’s defaults, R1 is 240 Ω, R2 is 720 Ω, and the target is 5 V. First calculate the predicted output:

V_out = 1.25 × (1 + 720/240) + 0.00005 × 720
= 1.25 × 4 + 0.036
= 5.04 V

For a 5 V target, the estimated lower resistor is:

R2 = (5 − 1.25) / (1.25/240 + 0.00005)
= 3.75 / 0.0052583…
= 713.15… Ω, or 713.1 Ω to one decimal place.

A real circuit would normally use an available standard resistor value near this estimate, then measure the output or account for tolerances in the design. The two outputs answer different questions: the first predicts voltage for the entered resistor pair; the second estimates the resistor for the entered target.

How the LM317 Adjustment Network Works

The LM317 is an adjustable linear voltage regulator. Its internal control circuit maintains roughly 1.25 V between its output and adjust pins. R1 carries current from the output pin toward the adjust pin, while R2 connects that point to ground. The regulator’s control action therefore produces an output above ground set mainly by the ratio R2/R1.

In a common circuit, R1 is chosen near 240 Ω. This provides a load current of about 1.25 V / 240 Ω = 5.2 mA through the adjustment network, helping maintain regulation for many LM317 versions. The exact minimum-load requirement is device- and condition-dependent, so consult the data sheet for the specific part. Some circuits use a different R1 value, particularly when standby current matters.

The formula’s adjust-current term is often omitted in quick calculations because it is small. It can still affect the result: with 720 Ω, a nominal 50 µA contributes 0.036 V. For a basic supply that may be negligible; for tighter accuracy, the regulator’s specifications, resistor tolerances, temperature effects, and measurement conditions matter.

Choosing Resistors for a Target

Use the calculated R2 as a starting point, not as a requirement that a resistor exist at exactly that value. Standard resistor series offer preferred values, and their actual resistance has a tolerance. Choose a nearby value, recalculate using that value, and check whether the resulting output meets the circuit’s needs. A trimmer can permit adjustment, but it should be selected and wired with suitable fixed-resistor limits rather than treated as an automatic accuracy fix.

• Keep the resistor values in ohms. Entering 0.72 to mean 720 Ω would produce a very different result.
• Check resistor tolerance: a 1% resistor can vary from its nominal value, changing the output.
• Use the exact pin arrangement shown in the data sheet for the package. Pin order is not interchangeable between all regulator types or packages.
• Confirm the target is within the regulator’s allowed operating range, with enough input voltage above the output to maintain regulation.

Limits, Dropout, and Heat

The equation predicts the adjustment-network set point; it does not prove that the regulator can deliver that voltage under a particular load. The input must exceed the output by enough headroom for the LM317 to regulate. The required margin depends on load current and device conditions, so use the data sheet rather than assuming a fixed universal dropout voltage. The maximum input-to-output differential and operating limits also depend on the specific part and package.

As a linear regulator, an LM317 dissipates power largely according to the voltage it drops: approximately (V_in − V_out) × I_load, with adjustment-network current also contributing. For example, dropping 12 V to 5 V at 0.2 A implies roughly 1.4 W from the load term alone. That heat may require attention to the manufacturer’s thermal data and the actual mounting and cooling arrangement. A voltage calculator alone cannot determine safe current, temperature, or heatsink requirements.

Use the manufacturer’s recommended protection components where the circuit conditions call for them, such as when large capacitors or unusual input/output sequencing are involved. Follow the specific data sheet’s application guidance. Do not assume that a calculated voltage remains stable if the input source sags, the load exceeds capability, or the device enters thermal or current limiting.

Common Calculation Mistakes

• Swapping R1 and R2: R1 goes between output and adjust; R2 goes between adjust and ground. Reversing them changes the divider relationship.
• Ignoring the adjust current: This is usually a small correction, but it can matter when seeking a closer estimate or using a large R2.
• Expecting exact voltage: The reference and resistors have tolerances, so a calculated 5.00 V is not a guarantee of a measured 5.00 V.
• Confusing set voltage with usable regulation: Input headroom, load current, thermal conditions, and minimum-load requirements can prevent the expected output.
• Using an unsuitable resistor power rating: Estimate resistor dissipation from the actual circuit current and voltage, then verify component ratings and margins.

Frequently Asked Questions

What resistor values do I need for 5 V from an LM317?
With R1 = 240 Ω, the idealized calculation gives R2 close to 713 Ω when using the calculator’s nominal adjust-current estimate. A nearby standard value may be used, but recalculate its predicted output and check the circuit under real operating conditions.

Why does an LM317 calculator use 1.25 V?
The regulator is designed to maintain a nominal reference voltage of about 1.25 V between its output and adjust pins. The actual value has specified variation, which is why calculator results are estimates.

Can I use the LM317 to make 1.25 V?
The basic adjustment relationship approaches 1.25 V when R2 is zero, but a practical circuit must still satisfy the regulator’s load and operating requirements. Check the particular device’s data sheet and circuit recommendations.

Why is my measured output different from the calculated value?
Possible causes include resistor tolerances, reference-voltage variation, adjust-pin current, insufficient input headroom, excessive load, wiring or pinout errors, and thermal or current limiting. Measure the input and output under load and verify the part’s pin connections.

Does the LM317 voltage formula tell me the maximum current?
No. The formula sets an approximate voltage from the resistor network. Deliverable current depends on the specific regulator, input-output voltage difference, temperature, package, and thermal design; use the data sheet to evaluate those limits.

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in r1 | R1 from output to adjust | ohm | 240 | 1 | 1000000
in r2 | R2 from adjust to ground | ohm | 720 | 0 | 1000000
in vo | Target output voltage | V | 5 | 1.25 | 37
out voltage | Calculated output voltage | V | 1.25 * (1 + r2 / r1) + 0.00005 * r2 | 2
out r2needed | R2 for target output voltage | ohm | (vo - 1.25) / (1.25 / r1 + 0.00005) | 1