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LM317, LM350, LM338 voltage regulator or current limiter calculatorDesigning a reliable linear power supply, a battery charger, or a constant-current LED driver requires precise resistor selection. This calculator quickly computes the resistor value required to use the LM317 / LM350 / LM338 as a voltage regulator circuit or current limiter circuit. LM317, LM350, LM338 are adjustable three-terminal positive-voltage regulators. They require two external resistors to set the output voltage in voltage regulator circuit or one external resistor to set the output current limit. Voltage Regulator ModeIn standard adjustable voltage mode, the regulator maintains a constant nominal reference voltage (Vref = 1.25 V) between its Output (Vout) and Adjustment (Vadj) terminals. By placing resistor R1 between these pins, a predictable current is established. Resistor R2 then programs the final output voltage. For voltage regulator circuit enter the target output voltage (Vout) and the value of R1 resistor to calculate the value of R2 resistor. Current Limiter ModeTo configure the LM317 / LM350 / LM338 as a precision current limiter, a single resistor (R1) is tied directly between the OUT and ADJ pins, and the load is moved to the ADJ pin. The regulator dynamically alters its internal resistance to maintain exactly 1.25 V across R1, creating a stable constant current output (Ilimit). For current limiter circuit enter the target output current to calculate the value of R1 resistor. Choosing Between LM317, LM350, and LM338While all three integrated circuits (ICs) share identical pinouts and use the same 1.25 V reference voltage math, they differ drastically in their continuous current handling and thermal capabilities:
This tool also displays resistor color coding and SMD resistor code. Frequently Asked QuestionsWhat is the ideal value for resistor R1 in an LM317 voltage regulator circuit? For standard LM317 circuits, the recommended value for R1 is 240 Ω (or 120 Ω for the LM350 / LM338 series). The regulator requires a minimum load current (typically 3.5 mA to 10 mA) to maintain stable voltage regulation. Using a 240 Ω resistor across the 1.25 V reference pin ensures a constant 5.2 mA draw, satisfying the minimum load requirement under all conditions.
How do you calculate the resistor values for a specific LM317 output voltage?To calculate the required R2 value for a targeted Vout while using a standard 240 Ω resistor for R1, rearrange the standard formula to: R2 = R1 × (Vout / 1.25 – 1) For example, if you need a 5 V output rail: R2 = 240 × (5 / 1.25 – 1) = 720 Ω. You would select the closest standard EIA precision resistor or use a variable potentiometer. How do I calculate the resistor for an LM317 constant current LED driver? When using the calculator in current limiter mode, only resistor R1 is used to program the target output current. The formula is R1 = 1.25 / Ilimit. If you need to drive a high-power LED array at a constant current of 700 mA (0.7 A), the required resistance is 1.25 / 0.7 ≈ 1.78 Ω.
Why is my LM317 or LM338 regulator getting extremely hot during operation? Linear regulators act like variable resistors; they drop excess voltage by converting it into pure heat. The power dissipation (Pd) is calculated as: Pd = (Vin - Vout) × Iload. If you drop 12 V down to 5 V (7 V drop) while drawing 2 A using an LM338, the IC will dissipate 14 W of heat. Without a substantial aluminum heatsink, the chip will quickly hit its internal thermal shutdown limit to prevent burnout.
Can I substitute an LM317 with an LM350 or LM338 using the same resistor values? Yes. Because the LM317, LM350, and LM338 are topologically identical and all use an internal 1.25 V reference voltage, they are drop-in replacements for one another regarding calculation math. You can swap an LM317 for an LM338 to upgrade a circuit's current capability from 1.5 A to 5 A without changing your existing R1 and R2 resistor configuration.
When should I use the calculator in Voltage Regulator Mode vs Current Limiter Mode? Use Voltage Regulator Mode when powering digital microcontrollers (like an Arduino or ESP32), sensor modules, or logic boards that require a strict, steady operating rail (e.g., 3.3 V or 5 V). Use Current Limiter Mode when driving high-power LEDs, charging battery packs (like NiMH or Lead-Acid), or running electroplating tanks, where an uncontrolled spike in current would cause catastrophic thermal runaway.
Can this calculator process a combination of both simultaneous voltage and current regulation? No, because a standard single-chip LM317 circuit cannot regulate both parameters at the exact same split-second. It operates either as a pure voltage source or a pure current source. If a load demands more current than the current limiter setting allows, the regulator drops the voltage down to save the circuit. If you require simultaneous independent clamping of both maximum voltage and maximum current, you must cascade two separate IC stages (an LM317 voltage regulator feeding into an LM317 current limiter).
What are the input voltage rules for both modes on this calculator?Linear regulators suffer from a "dropout voltage" penalty (typically around 2 V to 3 V).
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