LM317 Voltage Regulator Calculator

The LM317 is a three-terminal adjustable linear voltage regulator commonly used in bench supplies, battery chargers, prototypes, and low-noise analog circuits. It maintains about 1.25 volts between its output and adjustment terminals. Two external resistors set the output: R1 connects output to adjustment, and R2 connects adjustment to ground. This calculator solves the standard design equation for R2 after you enter the target voltage and chosen R1. Ignoring the small adjustment-pin current, output voltage equals 1.25 multiplied by one plus R2 divided by R1. Rearranging gives R2 equal to R1 multiplied by output voltage divided by 1.25 minus one. A 240-ohm R1 is a traditional choice because it draws enough current to help satisfy the regulator's minimum-load requirement. Other values may be suitable, but the data sheet for the exact device and manufacturer should guide the final design. A linear regulator turns excess voltage into heat. Its approximate dissipation is input voltage minus output voltage, multiplied by load current. Converting 12 volts to 5 volts at half an amp therefore produces about 3.5 watts inside the regulator. That is substantial heat for a small package and generally requires a heat sink selected from ambient temperature, thermal resistance, airflow, and the maximum permitted junction temperature. The displayed ideal efficiency is output voltage divided by input voltage; quiescent and resistor currents make real efficiency slightly lower. The input must stay above the requested output by the regulator's dropout allowance, commonly around 2 to 3 volts depending on current and operating conditions. Ripple troughs also matter: an unregulated rectifier supply that averages high enough can still fall below regulation between charging peaks. Add input and output bypass capacitors according to the data sheet, use short wiring, and include protection diodes when large capacitors or external voltages could discharge backward through the device. Calculated resistance is an exact target, not necessarily a stocked component. Choose the nearest preferred resistor or combine values, then recompute the actual output and account for resistor tolerance, reference tolerance, and adjustment-pin current. Confirm that input voltage, output current, power dissipation, and safe-operating-area limits all remain within the specific LM317 variant's ratings. This tool supports preliminary circuit design, but physical power-supply work should always be checked against manufacturer documentation and measured safely.