LM117 / LM317 Output Voltage Calculator

Precision Vout from resistor divider — design linear power supplies, battery chargers, and lab benches. Includes step‑by‑step derivation, thermal design tips, and reference circuit.

? Calculate Vout (R1,R2)
? Find R2 (given Vout, R1)
Typical 220Ω–240Ω ensures minimum load current (10mA).
Connect between ADJ and OUT.
? 5V Output (R1=240, R2=720)
⚡ 12V Output (R1=240, R2=2064)
? 3.3V Output (R1=240, R2=393.6)
? 1.25V (R2=0, R1=240)
?️ 9V (R1=220, R2=1364)
No server upload: All calculations run locally in your browser. Datasheet-grade accuracy.

LM317 / LM117 Precision Voltage Regulation

The LM117 and LM317 are iconic three-terminal adjustable linear regulators capable of supplying over 1.5A of load current with an output voltage range from 1.25V to 37V (input-output differential up to 40V). Developed by Robert "Bob" Dobkin at National Semiconductor in the 1970s, the LM117 set the standard for adjustable regulators with built-in current limiting, thermal overload protection, and safe-area compensation. The LM317 is the commercial version while the LM117 meets military temperature range. This calculator implements the fundamental equation derived from the internal bandgap reference: VOUT = VREF × (1 + R2/R1) + IADJ × R2 where VREF = 1.25V (typical, 1.2V–1.3V guaranteed). IADJ ≈ 50µA flows from the ADJUST terminal, negligible for most designs but important when R2 is high.

✍️ Classic design formula (ignoring Iadj):

VOUT = 1.25V × (1 + R2/R1)

For precision designs: VOUT = 1.25V × (1 + R2/R1) + (IADJ × R2). With IADJ = 50μA.

Engineering Best Practices & Design Guide

  • Minimum load current: LM317 requires 3.5mA typical (10mA max) to maintain regulation. Choose R1 ≤ 240Ω to guarantee Imin = 1.25V / R1 ≥ 5mA.
  • Capacitor selection: Place a 0.1µF ceramic at input (if far from filter caps), a 1µF tantalum or 10µF aluminum at output, and a 10µF from ADJ to GND for 80dB ripple rejection.
  • Protection diodes: Add 1N400x across input-output and output-ADJ to prevent reverse biasing when output capacitance discharges.
  • Thermal considerations: Power dissipation PD = (VIN – VOUT) × ILOAD. Use adequate heatsink (TO-220: θJA ~ 50°C/W without heatsink).
  • Stability: Output capacitor ESR between 0.1Ω and 10Ω ensures loop stability.

Real‑World Applications

Bench power supply – variable output with potentiometer.
Lead‑acid charger – constant voltage float charge.
Low‑noise audio rails – extra filtering with bypass cap.
Lab instrumentation – precise reference voltage.
Typical output configurations (VREF=1.25V, R1=240Ω)
Desired Vout (V) R2 theoretical (Ω) Nearest E96 1% resistor (Ω) Actual Vout (V)
3.3 393.6 392 3.292
5.0 720.0 715 / 732 4.98 / 5.06
9.0 1488 1500 9.06
12.0 2064 2050 / 2100 11.93 / 12.19
15.0 2640 2610 14.84
Case Study: Variable Power Supply for Prototyping

A hobbyist builds a 1.25V–15V variable supply using LM317, R1 = 240Ω, and a 5kΩ potentiometer as R2. Our calculator shows that with R2=0 → 1.25V, R2=5kΩ yields Vout ≈ 1.25×(1+5000/240) ≈ 27.3V (exceeds input limit). Use 2.5kΩ pot for 1.25V–14.2V range. Also, include a fixed resistor in series to limit max voltage. Tool instantly verifies settings and highlights proper potentiometer selection.

Frequently Asked Questions

LM117 is rated for -55°C to +150°C (military/aerospace), while LM317 is 0°C to 125°C (commercial). Both share identical pinout and electrical characteristics at 25°C.

For R2 < 5kΩ, error < 0.25%. For high R2 (e.g., 12kΩ), error adds up to ~0.6V. Enable "Include Iadj" for precision.

Thermal drift of resistors and Vref (0.02%/°C). Use low-TCR metal film resistors and proper heatsinking.

Minimum load current may not be met, causing output to rise. Our calculator warns if R1 > 470Ω.
Authoritative references: Texas Instruments LM317 Datasheet (SNVS774X), Analog Devices LM117 Datasheet, “Linear Regulator Design” by ON Semiconductor. Verified with industry-standard equations.