Tune output voltage using practical resistor relationships today. Estimate dropout, divider current, and heating quickly. Check LM317 behavior for reliable adjustable power supply builds.
LM317 output formula: Vout = Vref × (1 + R2 / R1) + Iadj × R2
Minimum input estimate: Vin(min) = Vout + Dropout
Regulator power loss: Preg = (Vin − Vout) × Iload
Temperature rise: ΔT = Preg × θJA
Estimated junction temperature: Tj = Tambient + ΔT
Efficiency estimate: Efficiency = Output Power ÷ Input Power × 100
The calculator also estimates divider current with Vref ÷ R1. It includes the adjustment current term for better accuracy.
| Case | Vin | R1 | R2 | Estimated Vout | Load Current | Estimated Heat |
|---|---|---|---|---|---|---|
| 5V Supply | 12 V | 240 Ω | 720 Ω | 5.04 V | 0.50 A | 3.48 W |
| 9V Supply | 15 V | 240 Ω | 1480 Ω | 9.03 V | 0.35 A | 2.09 W |
| 12V Supply | 18 V | 240 Ω | 2050 Ω | 12.04 V | 0.25 A | 1.49 W |
The LM317 is a classic adjustable linear regulator. It is simple. It is reliable. It is used in power supplies, lab projects, chargers, and test fixtures. This calculator helps you size the feedback resistors, check output voltage, estimate dropout needs, and review heat loss before you build.
Many LM317 circuits fail for easy reasons. The input voltage is too low. The resistor values are poor. The load current is too high. The regulator overheats. Small design mistakes can cause drift, shutdown, or unstable output. This page lets you test those points quickly with one form.
You can solve for output voltage from R1 and R2. You can solve for R2 from a target output. You can estimate the minimum input voltage from your chosen dropout margin. You can also review divider current, regulator power loss, output power, input power, efficiency, temperature rise, and estimated junction temperature.
The usual starting value for R1 is 240 ohms. That keeps divider current high enough for stable regulation in many common circuits. The exact output still depends on the adjustment pin current. This tool includes that term, so the result is more realistic than using the short formula alone.
Linear regulators waste extra voltage as heat. That heat can rise fast when load current increases. A modest voltage drop at one amp can already create several watts. That may demand a heatsink. The thermal section helps you compare ambient temperature, package resistance, and junction limit before parts are stressed.
Real boards still need correct capacitors, safe margins, and clean layout. Always compare the result with the device datasheet, load profile, and worst case input conditions. For high current or large input to output differences, a switching regulator may be the better choice. Use this calculator for planning, then verify on the bench.
This calculator also helps when you compare several resistor options during prototyping. You can enter a desired output, see the required resistor value, then review thermal impact without opening a second sheet. That saves time and reduces wiring errors during repeated bench adjustments and quick rebuilds and checks.
R1 is commonly set to 240 ohms. That value keeps enough programming current flowing through the divider in many standard LM317 circuits and usually gives predictable regulation behavior.
Use the regulator dropout margin. Add the chosen dropout voltage to the desired output voltage. The input should stay above that minimum during normal load and line variation.
The LM317 wastes the extra voltage as heat. Multiply input-to-output voltage drop by load current. Higher current or higher drop means more heat and stronger cooling needs.
Yes. The adjustment pin current slightly changes the result. It is often small, but including it improves accuracy, especially when resistor values are larger than the usual design range.
No. The LM317 is a linear regulator. It is better for simple, clean, adjustable supplies. For large current or large voltage drops, switching designs are usually more efficient.
The calculator gives planning values. Real results still depend on the datasheet, tolerance, layout, capacitors, heatsinking, and operating conditions. Always test the final circuit on hardware.
You can, but standard values are easier to source and test. After calculation, choose the nearest preferred resistor and check the resulting output again before assembly.
If the estimated junction temperature is above your safe limit, reduce voltage drop, reduce load current, improve airflow, add a heatsink, or choose a more efficient regulator topology.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.