LM317 Voltage Regulator Resistor Calculator

How it works

Vout = 1.25 V × (1 + R2 ÷ R1) + I_ADJ × R2 (I_ADJ ≈ 50 µA); constant current: I = 1.25 V ÷ R; dissipation = (Vin − Vout) × I

The LM317 holds exactly 1.25 V between its output and adjust pins, so the current through R1 is fixed at 1.25/R1 and flows on through R2 to ground, lifting the adjust pin — and therefore the output — to 1.25 × (1 + R2/R1). The classic 240 Ω for R1 keeps at least 5 mA flowing so the regulator stays in regulation with no load; the 50 µA adjust-pin current adds a small error that matters only with large R2. Because it is a linear regulator, everything above the output voltage is burned as heat, and at half an amp from 15 V to 9 V that is 3 W — a heatsink is mandatory. Wire the load from the adjust pin instead of ground and the same 1.25 V reference across a single resistor makes a rugged constant-current source.

Worked example

Target 9 V with R1 = 240 Ω: R2 = 1,474 Ω → 1.5 kΩ gives 9.14 V. From a 15 V input at 500 mA the LM317 dissipates about 2.9 W and needs a heatsink.

Frequently asked questions