LM317 Current Limit Calculator

Design precise current sources using the classic LM317 adjustable voltage regulator. Enter desired current or sense resistor value — get accurate resistor values, power dissipation, dropout margin, and thermal estimates.

Find Current from Resistor (R1)
RSET (Ω)
Find Resistor for Desired Current
IOUT (mA)
? LED 350mA → R = 3.57Ω
? 100mA LED → R = 12.5Ω
? Battery 500mA → R = 2.5Ω
⚡ Laser 200mA → R = 6.25Ω
? R=10Ω → I = 125mA
? R=5Ω → I = 250mA
On‑device computation: All calculations run locally. Your circuit values remain private.
LM317 Constant Current Parameters
Current IOUT = 125.00 mA
Sense Resistor RSET = 10.00 Ω
RSET Power Dissipation = 0.156 W
Reference Voltage VREF = 1.25 V (typical)
Min. Input Voltage (VIN,min) = — V (dropout + load)
Recommended RSET tolerance = ±1% metal film
Max current (LM317 typical) = 1.5A (with heatsink)
Thermal & Dropout Verification (Optional)
Load Voltage VLOAD = — V
LM317 Dropout VDROP = — V
LM317 Power Dissipation = — W
Junction temperature estimate (RθJA=50°C/W) = — °C
LM317
RSET (current sense)
Load
Current flow

LM317 as a Precision Current Limiter

The LM317 adjustable positive voltage regulator is widely used as a constant current source due to its inherent 1.25V reference between the output and adjustment pins. By connecting a single resistor (RSET) across these pins, the regulator enforces a constant current IOUT = 1.25V / RSET into the load. This simple yet robust topology delivers exceptional line/load regulation and short‑circuit protection.

IOUT = VREF / RSET where VREF = 1.25 V (typical)

For high precision, include IADJ (~50 µA) error: IOUT = (1.25V / RSET) + IADJ. The calculator above uses the standard formula (error < 1% for IOUT >> 50µA).

Design Guidelines & Practical Considerations

  • Resistor Power Rating: PR = 1.25² / RSET. Always derate by 2× (e.g., for 0.25W use ≥0.5W resistor).
  • Dropout Voltage (VDO): LM317 requires VIN - VLOAD ≥ 3V typical (up to 3.5V at high current). Lower dropout may cause current regulation loss.
  • Thermal Management: PD = (VIN - VLOAD) × IOUT. For >1W, attach a heatsink. Junction temperature TJ = TA + PD × RθJA (TO‑220 ~50 °C/W).
  • Minimum Load Current: LM317 needs ~10mA minimum to maintain regulation; constant current mode naturally satisfies this if IOUT > 10mA.
  • Input/Output Capacitors: Use 0.1µF at input and 1µF tantalum at output to prevent oscillations.
Real‑World Application: High‑Power LED Array

A designer needs to drive three 1W LEDs in series (forward voltage 3.4V each, total 10.2V) at 350 mA constant current. Using our calculator: Target 350 mA → RSET = 3.57 Ω (nearest 3.6 Ω, 1% metal film). Power dissipation in resistor = 0.437 W → recommend 1W resistor. With VIN = 15 V, load voltage = 10.2 V, dropout = 4.8 V (>3V sufficient). LM317 power = 4.8 V × 0.35 A = 1.68 W → small heatsink (TO‑220 with 15 °C/W heatsink keeps TJ under 110°C). This calculator instantly verifies each parameter, saving prototyping time.

Step-by-Step Calculation Methodology

  1. Determine required current – LED forward current, battery charge rate, or load requirement.
  2. Calculate RSET using R = 1.25 V / IOUT. Choose nearest standard E96 value for precision.
  3. Compute resistor power PR = (1.25 V)² / R, select wattage 2× that value.
  4. Verify dropout margin: VIN ≥ VLOAD + 3 V (minimum). Higher VIN increases dissipation.
  5. Evaluate thermal conditions – PD = (VIN - VLOAD) * IOUT. Use heatsink if TJ > 125 °C.

Reference Table: Common Current Values & Standard Resistors

Desired IOUT (mA) Calculated RSET (Ω) Nearest E96 Resistor Power in R (W) Recommended Rating
100 12.50 12.4 Ω 0.125 0.25 W
200 6.25 6.19 Ω 0.25 0.5 W
350 3.571 3.57 Ω / 3.6 Ω 0.438 1 W
500 2.50 2.49 Ω 0.625 1 W
700 1.786 1.78 Ω 0.875 2 W
1000 1.25 1.24 Ω 1.25 2 W+ heatsink req.

Advanced: IADJ Correction & Temperature Stability

For ultra‑precise current sources (< 0.5% error), the adjustment pin current (IADJ ≈ 50 µA) introduces a systematic offset: IOUT = VREF/RSET + IADJ. At 100 mA this adds 0.05% error; at 5 mA it becomes ≈1%. Moreover, VREF has a temperature coefficient of ~0.02%/°C. For designs demanding high stability (e.g., sensor excitation), use low‑TC resistors and consider the LM317L (100mA) or LM338 (5A) variants. The calculator on this page assumes the standard formula ideal for 95% of applications; advanced users can factor IADJ manually.

Frequently Asked Questions

The LM317 can deliver up to 1.5 A (with adequate heatsinking). For higher currents, use LM338 (5 A) or LM350 (3 A). Our calculator supports values up to 1.5 A, but warnings appear near limits.

Within the dropout range (VIN - VLOAD ≥ dropout voltage), the LM317 maintains constant current regardless of load variations. If dropout margin is insufficient, current drops. Use the “Thermal & Dropout Verification” section to check.

Power in RSET can be significant for low resistance values. Always choose a resistor with a power rating at least twice the calculated dissipation. The calculator shows recommended rating.

Yes, LM317 current limit is perfect for charging NiMH, lead‑acid, or Li‑FePO₄ cells (with proper voltage limiting). Ensure input voltage covers maximum battery voltage plus dropout.

Based on LM317 datasheet values from Texas Instruments, ON Semi, and STMicroelectronics. Typical VREF varies 1.2V–1.3V, so tolerance ±5% is expected. Use trimmer for critical applications.
References: Texas Instruments LM317 Datasheet (SNVS774X), “Linear Voltage Regulator Handbook” (ON Semi), Bob Pease’s “Troubleshooting Analog Circuits”. Verified with industry design notes.
Last update: May 2026.