Linear regulator (78xx, LM317)

Makes a steady output voltage from a varying input.

Schematic letter: U / IC
Unit / key rating: Output voltage (V), current (A)

Pinout

78xx: IN, GND, OUT: Left to right, viewed from the front.
LM317: ADJ, OUT, IN: A different pin order from the 78xx!

Real photos

What is it for?

A linear regulator holds its output steady by turning the excess input voltage into heat. A 7805 gives a fixed 5 V, a 7812 a fixed 12 V; an LM317 is set with two resistors.

It's simple and quiet, but runs very hot when the input–output difference is large: power dissipated = (input − output) × current.

Things to watch out for

⚠ The input must be at least 2–3 V above the output.
⚠ Put small capacitors (0.33 µF / 0.1 µF) on the input and output, or it may oscillate.

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