Electronics · Free Calculator
LM317 Voltage Regulator Calculator
Set the output of an LM317 adjustable regulator: find Vout from the two resistors, or R2 for a target voltage.
Vout = 1.25(1 + R2/R1)R2 for targetPower dissipationAdjust current
V
The regulator dissipates 4.98 W and needs a heatsink. Use the linear regulator and heatsink calculators.
Output voltage5.036 V
Regulator dissipation4.982 W
Dropout headroom9.96 V
How this result was calculated
- Vout = 1.25 × (1 + R2/R1) + Iadj × R2 = 5.036 V
- P = (Vin − Vout) × I = 4.982 W
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LM317 Output Formula
Vout = Vref × (1 + R2 ÷ R1) + Iadj × R2
Vref = 1.25 V, Iadj ≈ 50 µA
Vref = 1.25 V, Iadj ≈ 50 µA
R1 is normally 240 Ω (or 120 Ω) so that at least the minimum load current flows even with no external load.
Worked example
R1 = 240 Ω and R2 = 720 Ω give 1.25 × 4 + 0.036 = 5.04 V. For 12 V, R2 = 10.75 ÷ (1.25/240 + 50 µA) = 2.04 kΩ, so fit 2.05 kΩ (E96).
Key insight: a 10 µF capacitor from ADJ to ground improves ripple rejection by about 15 dB, which is an easy upgrade on any LM317 supply.
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Frequently Asked Questions
What is the LM317 output formula?
Vout = 1.25 × (1 + R2/R1), plus a small term from the 50 µA adjust current.
Why is R1 240 Ω?
It draws about 5 mA, which keeps the LM317 in regulation with no external load.
What is the maximum output current of an LM317?
1.5 A in the TO 220 package, limited further by heat dissipation.
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