Engineering Tools
LM317 Voltage Regulator Calculator
Find the perfect R2 resistor value to achieve your target output voltage using the industry standard LM317 linear regulator.
The LM317 Formula
The output voltage of an LM317 regulator is determined by the ratio of two resistors (R1 and R2) creating a voltage divider at the ADJ pin.
Vout = 1.25V × (1 + R2 / R1) + (Iadj × R2)
- 1.25V: This is the internal reference voltage generated between the VOUT and ADJ pins.
- Iadj: The adjustment pin current (typically 50µA). It is usually small enough to be ignored in basic calculations.
- R1 (Program Resistor): Tied between VOUT and ADJ. Usually chosen as 240Ω or 120Ω to ensure the LM317 has its required minimum load current.
Why Minimum Load Matters
Unlike modern LDOs, the classic LM317 requires a minimum load current of 3.5mA to 10mA to maintain regulation. If your circuit draws less than this (e.g., in deep sleep), the output voltage will soar, potentially frying your microcontroller.
By using a 240Ω resistor for R1, the internal 1.25V reference forces `1.25V / 240Ω = 5.2mA` of current constantly through the resistor divider, satisfying the minimum load requirement even if your main circuit draws zero current.