LM317T Adjustable Positive Linear Voltage Regulator IC (TO-220)

Overview

The LM317T is the gold standard for adjustable linear regulation. Whether you need a specific 3.3V for a microcontroller, 9V for a guitar pedal, or 4.2V for a charging circuit, this IC allows you to tune the output with precision.

Housed in a TO-220 package, it can be easily mounted to a heatsink for high-power applications. It is significantly more flexible than fixed-voltage regulators (like the 7805), allowing your power supply to grow and adapt to your project's specific needs.

Technical Specifications

Basic Application Circuit

To get the LM317T running, you only need a few external components. The following diagram shows the standard configuration for a variable power supply:

Component Selection

• C1 (Input Capacitor): 0.1μF ceramic capacitor to filter input noise.
• C2 (Output Capacitor): 1μF tantalum or 10μF electrolytic for improved transient response.
• R1: Typically a fixed 240Ω or 330Ω resistor.
• R2: A potentiometer (often 5kΩ or 10kΩ) to adjust the output voltage.

Pinout Configuration

Holding the flat side with text facing you:

1. ADJ (Adjust): Connects to the junction of R1 and R2.
2. VOUT (Output): The regulated output voltage.
3. VIN (Input): The unregulated input supply.

⚠️ Warning: The pinout is NOT the same as a 7805 regulator. Pin 1 is Adjust, not Input. Always double-check your connections before powering up.

Calculating Output Voltage

The output voltage is determined by the ratio of the two resistors ($R_1$ and $R_2$). Use the following formula to calculate your target:

$V_{OUT} = 1.25V \times (1 + \frac{R_2}{R_1})$

Common Setups:

• For 3.3V Output: Use $R_1 = 240Ω$ and $R_2 = 390Ω$
• For 5V Output: Use $R_1 = 240Ω$ and $R_2 = 720Ω$
• For 12V Output: Use $R_1 = 240Ω$ and $R_2 = 2.06kΩ$

💡 Design Tip: Linear regulators generate heat based on the voltage drop ($V_{IN} - V_{OUT}$) multiplied by the current. If you are dropping 12V to 3.3V at high current, a TO-220 Heatsink is mandatory.