LM317 Calculator

The LM317 is a popular adjustable positive voltage regulator that maintains a constant nominal reference voltage of 1.25 volts between its output and adjustment terminals. This internal reference voltage forces a constant current to flow through resistor R1, which in turn flows through resistor R2 to ground. The standard governing equation is: Voltage = 1.25 × (1 + (R2 / R1)). Note that this formula calculates the ideal theoretical output; real-world circuits must also account for a tiny quiescent adjustment pin current (typically around 50 microamps) flowing out of the adjust terminal, though it is often negligible for standard resistor values under 240 ohms.

Suppose you are designing a variable power supply and you select a standard value of 240 ohms for resistor R1 to ensure a minimum load current. You want to find out what output voltage you will achieve by using a 1,200 ohm (1.2 kΩ) resistor for R2. First, divide R2 by R1: 1,200 / 240 = 5. Next, add 1 to that result: 1 + 5 = 6. Finally, multiply by the 1.25V internal reference voltage: 1.25 × 6 = 7.5 volts. Therefore, your circuit will output a stable 7.5 volts.

Always keep resistor R1 relatively small—typically 240 ohms or less—to maintain sufficient load current and stabilize the regulator. Ensure your input voltage is at least 3 volts higher than your desired maximum output voltage to allow proper headroom for the regulator to operate efficiently. Finally, always attach an appropriately sized aluminum heatsink if your circuit handles significant current drops, as thermal dissipation can cause the device to enter thermal shutdown.