Calculator mode
Common output voltages:
V
Max 40V, min Vout + 3V
V
Min 1.25V, max 37V
For power dissipation calc
Ω
Typically 120–240 Ω
Circuit diagram
Common output voltages — click to load
| Vout | R1 (240 Ω) | R2 exact | R2 E24 | Actual Vout | Error |
|---|
LM317 basics
Key specifications
- Output range: 1.25V – 37V
- Max input: 40V
- Max current: 1.5A (with heatsink)
- Dropout: ~3V (Vin must be Vout + 3V)
- Reference voltage: Vref = 1.25V
- Adj current: Iadj ≈ 50 µA (negligible)
Formula
Vout = 1.25 × (1 + R2/R1)
R2 = R1 × (Vout/1.25 − 1)
P_lm317 = (Vin − Vout) × Iout
Typical: R1 = 240 Ω
Minimum R1: 120 Ω
Use 0.1µF on input, 1µF on output
Heatsink guide
- < 0.5W — no heatsink needed (TO-220)
- 0.5–2W — small heatsink recommended
- 2–5W — medium heatsink required
- > 5W — large heatsink + thermal paste
- Max junction temp: 125 °C (θjc ≈ 5 °C/W)
- For high dissipation, consider a buck converter instead
Good to know
- Add 10µF capacitor on ADJ pin for better ripple rejection
- Add output diode (1N4001) from output to input to protect against reverse current
- LM317 efficiency = Vout/Vin — at 5V from 12V it's only 42%!
- LM338 is pin-compatible, handles up to 5A
- LM317T (TO-220) vs LM317L (TO-92, 100 mA max)