LM317 Voltage Regulator Calculator

Design reliable linear power supplies using the classic LM317. Calculate output voltage from resistor divider (R1/R2), find standard resistor values for a target voltage, and verify dropout, power dissipation, and load regulation

Find VOUT from R1 & R2
Find R2 for Desired VOUT
? 3.3V (R1=240, R2=394Ω) ? 5.0V (R1=240, R2=720Ω) ⚙️ 9.0V (R1=240, R2=1488Ω) ? 12.0V (R1=240, R2=2064Ω) ?️ 15.0V (R1=240, R2=2640Ω) ? R1=240, R2=1500 → Vout=9.06V
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LM317 Voltage Regulator Parameters
Output Voltage VOUT = — V
Resistor R1 = 240 Ω  |  R2 = — Ω
Reference Voltage VREF = 1.25 V (typical)
IADJ contribution = — mV
Minimum load current (R1 selection) = — mA
R2 power dissipation = — W
R1 power dissipation = — W
Dropout & Thermal Verification
VOUT (computed) = — V
Dropout voltage VIN - VOUT = — V
Power dissipation PD = — W
Junction temp (RθJA=50°C/W) = — °C
LM317 R1 (programming) R2 (adjust) Load

LM317 Voltage Regulator Theory

The LM317 is an adjustable three-terminal positive voltage regulator capable of supplying over 1.5A over an output voltage range of 1.25V to 37V. It uses a bandgap reference of 1.25V between the output and adjustment terminals. The output voltage is set by two external resistors R1 and R2:

VOUT = 1.25V × (1 + R2/R1) + IADJ × R2

where IADJ ≈ 50µA typical, often negligible for R2 < 10kΩ.

The formula shows that the output voltage is directly proportional to the resistor ratio. R1 is usually chosen between 120Ω and 240Ω to guarantee a minimum load current of 5–10mA, which improves regulation. The adjustment pin current (IADJ) introduces a small offset; for precision designs, include it in calculations.

Design Guidelines & Best Practices

  • R1 selection: Use 240Ω for most designs (Imin = 1.25V/240Ω ≈ 5.2mA). Lower R1 increases quiescent current but reduces IADJ error.
  • R2 tolerance: For precise VOUT, use 1% metal film resistors. The calculator provides exact theoretical values; pick the nearest E96 series.
  • Dropout voltage: VIN must exceed VOUT by at least 3V (or up to 3.5V at 1.5A). Insufficient dropout causes output to sag.
  • Thermal management: Power dissipation PD = (VIN - VOUT) × ILOAD. For PD > 1W, attach a heatsink. Max junction temperature 125°C.
  • Protection diodes: Add 1N4002 across input-output and output-adjust to prevent capacitor discharge damage.
  • Capacitors: 0.1µF at input, 1µF tantalum at output, and a 10µF electrolytic at output for transient response.
Real‑World Application: Variable Lab Power Supply

A hobbyist builds a 1.25V–15V adjustable bench supply using an LM317, a 24V transformer, and a 5kΩ potentiometer as R2. Using our calculator: with R1=240Ω, R2 at maximum 5000Ω gives VOUT = 1.25×(1+5000/240) ≈ 27.3V, but limited by VIN (24V rectified ≈ 32V DC, so 27V is feasible). At 12V output and 1A load, dropout = VIN (≈20V) - 12V = 8V → PD = 8W, requiring a substantial heatsink (RθSA ≈ 5°C/W). The calculator instantly validates these numbers, preventing overheating failures.

Step‑by‑Step Calculation Workflow

  1. Choose R1 (120Ω to 240Ω) to satisfy minimum load current ≥ 5mA.
  2. Determine desired VOUT and compute R2 = R1 × (VOUT/1.25 - 1).
  3. Select the nearest standard resistor value (E96 series for 1% precision).
  4. Calculate actual output voltage with the chosen resistor.
  5. Verify dropout: VIN ≥ VOUT + 3V.
  6. Estimate PD and required heatsink.

Standard Resistor Values for Common Voltages (R1=240Ω)

Target VOUT (V) Calculated R2 (Ω) Nearest E96 R2 (Ω) Actual VOUT (V) Error
3.3 394 392 3.29 -0.3%
5.0 720 715 4.98 -0.4%
9.0 1488 1500 9.06 +0.7%
12.0 2064 2050 11.93 -0.6%
15.0 2640 2670 15.16 +1.1%

Precision Enhancement: IADJ Correction

For outputs above 25V or when R2 exceeds 10kΩ, the adjustment pin current (50µA) produces a voltage drop across R2 of up to 0.5V. The exact formula VOUT = VREF(1+R2/R1) + IADJ·R2 should be used. Our calculator includes this term (displayed as IADJ contribution) for full accuracy. For most designs below 15V, ignoring IADJ yields <0.5% error.

Frequently Asked Questions

LM317 requires a minimum load current of ~5mA to maintain regulation. 240Ω draws 1.25V/240Ω = 5.2mA, satisfying the requirement while minimizing wasted power.

The LM317 can regulate up to 37V (input-output differential ≤40V). Actual max depends on input voltage and dropout.

Based on Texas Instruments LM317 data sheet. Typical VREF tolerance is ±4%; using 1% resistors yields overall accuracy ±5%, which matches industry practice.

Yes, a 5kΩ or 10kΩ linear pot works well. Add a fixed resistor in series to set a minimum output voltage.

Depends on dropout. At VIN-VOUT=5V, PD=5W → heatsink mandatory. Use our thermal verification section.
LM317 Design Checklist
  • ✅ R1 between 120Ω and 240Ω? (Imin load ≥ 5mA)
  • ✅ R2 tolerance 1% for precision?
  • ✅ Input capacitor ≥0.1µF ceramic near IC?
  • ✅ Output capacitor ≥1µF tantalum?
  • ✅ VIN ≥ VOUT + 3V?
  • ✅ Power dissipation < (TJmax - TA) / RθJA with heatsink?
  • ✅ Protection diodes across IN‑OUT and OUT‑ADJ?
References: Texas Instruments LM317 Datasheet (SNVS774X), ON Semiconductor LM317 Application Note, “Linear & Switching Voltage Regulator Handbook” (Fairchild). Validated against real‑world lab measurements.