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LM317 Resistor Calculator

Calculate R2 resistor value for LM317/LM338 adjustable voltage regulator output voltage, with actual Vout and resistor power dissipation.

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Formula

R2=Vout−VrefVref/R1+IadjR_2 = \frac{V_{out}-V_{ref}}{V_{ref}/R_1 + I_{adj}}

Reference: Texas Instruments LM317 Datasheet (SNVS774)

Vref — LM317 reference voltage (1.25V) (V)
R1 — Output to ADJ resistor (Ω)
R2 — ADJ to GND resistor (Ω)
Iadj — ADJ pin current (A)

How It Works

The LM317 is a versatile voltage regulator that allows precise output voltage adjustment using two resistors. It operates on the principle of maintaining a constant 1.25V reference voltage between its adjustment and output terminals. By carefully selecting the values of R1 and R2, engineers can create a configurable voltage source for various electronic applications.

Worked Example

Problem: Design an LM317 voltage regulator circuit to output 5V with a typical 240Ω R1 resistor Solution: 1. Use Vref = 1.25V 2. Rearrange formula: R2 = (5V - 1.25V) / (1.25V/240Ω + 0.00005A) 3. Calculate R2 = 750Ω 4. Verify: Vout = 1.25V * (1 + 750/240) + 0.00005A * 750 ≈ 5.00V

Practical Tips

  • ✓Use 1% tolerance resistors for more precise voltage regulation
  • ✓Add heat sinking for higher current applications
  • ✓Include input and output capacitors for stability

Common Mistakes

  • ✗Neglecting the small adjustment current (Iadj)
  • ✗Choosing resistor values with poor tolerance
  • ✗Ignoring thermal considerations and power dissipation

Frequently Asked Questions

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