LM317 Calculator
Example Data Table
| Target Output | R1 | Approx R2 | Common Use | Design Note |
|---|---|---|---|---|
| 3.3 V | 240 Ω | 390 Ω | Logic supply | Check dropout at low input voltage. |
| 5 V | 240 Ω | 720 Ω | Control board | Power loss rises with high input voltage. |
| 9 V | 240 Ω | 1.5 kΩ | Analog circuit | Use measured load current for heat checks. |
| 12 V | 240 Ω | 2.0 kΩ | Bench adapter | Use suitable capacitors near regulator pins. |
Formula Used
The main LM317 output formula is:
Vout = Vref × (1 + R2 / R1) + Iadj × R2
R1 and R2 are measured in ohms. Iadj is converted to amperes. Vref is often near 1.25 V. The ideal R2 value is found by rearranging the same equation:
R2 = (Vtarget - Vref) / ((Vref / R1) + Iadj)
The regulator power loss is estimated as:
Power loss = (Vin - Vout) × Load current
The junction estimate is:
Junction temperature = Ambient + Power loss × Thermal resistance
How To Use This Calculator
- Enter the desired target output voltage.
- Enter input voltage and expected load current.
- Set R1 and R2 if you want a direct output result.
- Select the preferred standard resistor series.
- Enter tolerance, dropout, and thermal data.
- Enable pair search to compare standard R1 and R2 values.
- Press calculate to view results above the form.
- Download CSV or PDF for records.
LM317 Design Guide
Why This Calculator Helps
The LM317 is a popular adjustable linear regulator. It is simple, stable, and easy to tune. Designers set the output with two resistors. R1 sits between output and adjust. R2 sits between adjust and ground. The calculator helps choose useful values.
How The Resistor Network Works
A fixed R1 creates a small program current. The regulator holds about 1.25 volts across R1. That current flows through R2 and sets the output voltage. The adjust pin current is small, yet it can matter with large resistor values. This tool includes that current, so high value designs are checked more carefully.
Choosing Standard Values
Standard resistor values rarely match the exact ideal value. A target of 5 volts may need a calculated R2 that is not sold. The calculator compares the ideal value with common E series choices. It then shows the nearest outputs, errors, and resistor currents. You can select E12, E24, E48, or E96 values.
Power And Heat Review
Power is just as important as accuracy. A linear regulator burns the unused voltage as heat. The loss rises when input voltage or load current increases. This page estimates dissipation, efficiency, dropout margin, and junction temperature. These checks help you avoid designs that look correct on paper but run too hot.
Tolerance And Practical Safety
Tolerance also changes the final voltage. A one percent pair is normally tighter than a five percent pair. The tool estimates worst case high and low output. Use these limits when powering sensors, microcontrollers, amplifiers, or battery chargers.
Design Workflow
The calculator is useful for quick design reviews. Start with the expected input voltage and load current. Enter the target output, then choose a resistor series. Review the recommended standard pairs. Check the thermal result before selecting a package or heatsink. If the temperature margin is small, lower the input voltage, reduce load current, add cooling, or choose a switching regulator.
Build Notes
Real circuits still need good layout. Place input and output capacitors near the regulator pins. Follow the device datasheet for stability values. Keep power traces wide. Measure the output under load after assembly. The calculator gives a strong estimate, but the final circuit should always be tested with actual components. It also supports saved reports through CSV and PDF exports. That makes documentation easier for students, technicians, practical lab projects, and maintenance teams.
FAQs
What does this LM317 calculator do?
It calculates output voltage, ideal R2, standard resistor matches, power loss, dropout margin, and thermal estimates for adjustable LM317 regulator circuits.
Why is R1 often 240 ohms?
A 240 ohm R1 creates enough program current for many LM317 designs. It helps maintain regulation and keeps the resistor network predictable.
Can I use 1 kΩ for R1?
Yes, but check minimum load current. A larger R1 reduces program current, which may affect regulation in lightly loaded circuits.
Why is my exact R2 not available?
The calculated ideal resistor may not exist in a standard series. The calculator finds nearby E series values and shows the voltage error.
What is dropout margin?
Dropout margin is the input voltage left after output voltage and dropout allowance. A negative value means regulation may fail.
Why include adjust pin current?
Adjust pin current is small, but it can shift output voltage when R2 is large. Including it gives a better advanced estimate.
Does this replace circuit testing?
No. It gives design estimates. Always measure the real output under load and confirm heat performance with the final parts.
When should I avoid LM317?
Avoid it when power loss is high, efficiency matters, or input voltage is close to output voltage. A switching regulator may fit better.