🔧 LM317 — Basic Adjustable Linear Regulator

📘 What is the LM317?

The LM317 is a very simple linear voltage regulator. It takes a higher DC voltage (like 9–24V) and outputs a lower, adjustable voltage.

It is called linear because it works by dropping excess voltage as heat — not by switching like modern buck converters.

Output range: 1.25V → ~37V (depending on input voltage).

🔌 LM317 Pinout

The LM317 has three pins:

IN — This is where you feed the input voltage.

OUT — This is the regulated output voltage.

ADJ — This pin adjusts the output using resistors.

⚙️ Basic Adjustable LM317 Circuit

This is the simplest LM317 adjustable regulator circuit:

The output voltage is set using two resistors:

R1 — A fixed resistor (usually 240Ω)

R2 — A variable resistor (potentiometer)

Turning the potentiometer changes the output voltage.

📐 How to Calculate Output Voltage

The LM317 always keeps 1.25V between OUT and ADJ.

The formula is:

Vout = 1.25 × (1 + R2 / R1)

Example:

R1 = 240Ω

R2 = 720Ω

Vout = 1.25 × (1 + 720/240) = 5V

This is why the LM317 is popular — the math is extremely simple.

🔻 Voltage Drop (Dropout Voltage)

The LM317 needs the input voltage to be at least 3V higher than the output.

Vin ≥ Vout + 3V

Example:

Desired output: 5V

Minimum input: 8V

If Vin is too low, the LM317 cannot regulate and the output will drop.

🔥 When Does the LM317 Need a Heatsink?

Because the LM317 is a linear regulator, it burns off extra voltage as heat.

The heat is:

Power Dissipation = (Vin − Vout) × Load Current

Example:

Vin = 12V

Vout = 5V

Load = 0.5A

Heat = (12 − 5) × 0.5 = 3.5W

Anything above 2–3W usually requires a heatsink.

If the LM317 becomes too hot to touch → you need a heatsink.

🛠️ Quick Summary

LM317 is a linear regulator

Adjustable from 1.25V to ~37V

Needs Vin ≥ Vout + 3V

Output voltage set by R1 and R2

Heatsink needed if (Vin − Vout) × Iload > 2W