LM317

The LM317 is an adjustable positive voltage regulator integrated circuit widely used in electronics. It provides a stable output voltage that can be set with two external resistors. The regulator maintains approximately 1.25 volts between its output and adjustment terminals, known as Vref. The output voltage is determined by the formula Vout = Vref (1 + R2/R1) + Iadj × R2, where R1 is between output and adjust, R2 between adjust and ground. Vref is about 1.25 V and Iadj is a small bias current, typically around 50 µA. With adequate heat sinking, the LM317 can deliver up to about 1.5 A, and the maximum input voltage is around 40 V. The dropout voltage is typically 2–3 V at moderate load and rises with higher current.

Pin configuration varies by package, but in the common TO-220 package (front facing you), its pins are, from left to right: adjust, output, input; the metal tab is connected to the output. Other packages may differ in pinout.

Applications and usage: the LM317 is widely employed in bench power supplies, hobbyist projects, and electronic devices requiring an adjustable DC output. It is used as a linear regulator, and can be part of current-regulation schemes or battery charger circuits when paired with appropriate resistors and protection components. Thermal management is important at higher currents, as excessive heat reduces regulation accuracy or causes protection to trigger.

Variants include the LM317L (a low-current version, typically up to about 100 mA) and other package styles. The LM317 family is complemented by related parts such as the LM337, a negative regulator for complementary applications.