LM317 Calculator — Adjustable Voltage Regulator
Calculate the output voltage of an LM317 adjustable linear regulator from R1 and R2, or solve for the R2 value needed to hit a target output voltage.
Calculate
Ω
Ω
V
Vout = 1.25 × (1 + R2 / R1) + Iadj × R2
- 1
Resistor ratio (R2 ÷ R1)
720 ÷ 240 = 3 - 2
Base voltage (Vref × (1 + ratio))
1.25 × (1 + 3) = 5Dominant term — VREF × (1 + R2/R1) - 3
Output voltage (+ Iadj × R2)
5 + 0,036 = 5,036
Hvordan fungerer denne lommeregner?
Vout = 1.25 × (1 + R2 / R1). Set R1 = 240 Ω (recommended), then choose R2 to hit your target voltage. For Vout = 5 V: R2 = 240 × (5/1.25 − 1) = 720 Ω. Input must be at least Vout + 3 V. Output range: 1.25 V to 37 V (LM317 datasheet values).
Formel
How this is calculated
The LM317 is a three-terminal adjustable linear voltage regulator that holds a constant 1.25 V (the reference voltage, Vref) between its OUTPUT and ADJ pins. A resistor divider made of R1 (connected from OUTPUT to ADJ) and R2 (connected from ADJ to ground) sets the output voltage: Vout = Vref × (1 + R2 / R1) + Iadj × R2, where Iadj is the small current (~50 µA typical) flowing out of the ADJ pin. Because Iadj is tiny, the simplified form Vout ≈ 1.25 × (1 + R2 / R1) is accurate to within 1–2%.
Texas Instruments and ON Semiconductor datasheets recommend using R1 = 240 Ω so that the Iadj × R2 error term stays small and the quiescent current through R1 is large enough to keep Vout stable over load changes. The output is adjustable from 1.25 V to 37 V. The input voltage must exceed Vout by at least the dropout voltage (typically 3 V at full load, 1.5–2 V at lighter loads). Power dissipated by the IC is approximately (Vin − Vout) × Iout — at high input-to-output differentials and high load currents, the LM317 may need a heatsink.
Standard resistor values (E12 or E24 series) are usually chosen for R2 since exact values are rarely available; a small trimmer pot in series with R2 allows fine adjustment. For precision applications, a stable supply reference and careful PCB layout minimise output noise.
Ofte stillede spørgsmål
With R1 = 240 Ω, the current through the divider (Vref / R1 = 1.25 / 240 ≈ 5.2 mA) is much larger than the ADJ pin current (Iadj ≈ 50–100 µA), so Iadj contributes less than 2% error to the output voltage. Smaller R1 wastes more quiescent current; larger R1 makes the output more sensitive to Iadj variation with temperature and load.
The LM317 requires Vin to exceed Vout by at least the dropout voltage — typically 3 V for full-load operation (up to 1.5 A). For a 5 V output you need at least 8 V input. At lighter loads the dropout can be as low as 1.5 V, but 3 V is the conservative, reliable minimum.
The IC dissipates P = (Vin − Vout) × Iout as heat. At 1 A load with a 9 V input and 5 V output, that is (9−5) × 1 = 4 W — the LM317 in a TO-220 package can handle about 1 W in free air without a heatsink. For higher dissipation attach a heatsink (thermal resistance ~5 °C/W is common) and ensure the junction temperature stays below 125 °C.
Også kendt som
TG we-Calculate Editorial Team. (2026). LM317 Calculator — Adjustable Voltage Regulator [Online calculator]. TG we-Calculate. https://we-calculate.com/da/calculator/lm317-calculator
TG we-Calculate Editorial Team. "LM317 Calculator — Adjustable Voltage Regulator." TG we-Calculate. 2026. https://we-calculate.com/da/calculator/lm317-calculator.
TG we-Calculate Editorial Team, "LM317 Calculator — Adjustable Voltage Regulator," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/da/calculator/lm317-calculator
@misc{wecalculate_lm317_calculator, title = {LM317 Calculator — Adjustable Voltage Regulator}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/da/calculator/lm317-calculator}}, year = {2026}, note = {TG we-Calculate} }
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