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LM317T Adjustable Positive Linear Voltage Regulator

3 min read

Important Note

This entire repo was AI created - including all of the data within. The intent was to A) help me with my personal electronics inventory; and B) see how I could use AI to make that process a bit easier. DO NOT TRUST!

LM317T Adjustable Positive Linear Voltage Regulator

Overview

The LM317T is an adjustable positive voltage regulator from Texas Instruments that provides a variable output voltage from 1.25V to 37V. It’s one of the most popular and versatile voltage regulators, widely used in power supply design, battery chargers, and precision reference circuits.

Key Specifications

  • Output Voltage Range: 1.25V to 37V (adjustable)
  • Input Voltage Range: 3V to 40V
  • Maximum Output Current: 1.5A
  • Package: TO-220 (3-pin)
  • Dropout Voltage: 2.5V typical
  • Operating Temperature: 0°C to 125°C
  • Line Regulation: 0.01% typical
  • Load Regulation: 0.1% typical

Pinout (TO-220)

   ┌─────┐
   │  1  │ Adjust (ADJ)
   │  2  │ Output (OUT)
   │  3  │ Input (IN)
   └─────┘

Applications

  • Variable power supplies
  • Battery chargers
  • Precision voltage references
  • Audio circuit power supplies
  • Adjustable current sources
  • Precision reference voltages for audio circuits

Features

  • Excellent line and load regulation
  • Internal thermal overload protection
  • Internal short circuit current limiting
  • Output transistor safe area protection
  • Adjustable output voltage with simple resistor network
  • Low output noise (ideal for audio applications)

Output Voltage Calculation

The output voltage is set using two resistors (R1 and R2):

Vout = 1.25V × (1 + R2/R1)

Where:

  • R1 is typically 240Ω to 1kΩ (fixed resistor)
  • R2 is the adjustment resistor

Example Calculations

For common output voltages:

  • 5V: R1 = 240Ω, R2 = 730Ω
  • 10V: R1 = 240Ω, R2 = 1.8kΩ
  • 12V: R1 = 240Ω, R2 = 2.2kΩ
  • 15V: R1 = 240Ω, R2 = 2.7kΩ

Typical Application Circuit

Vin ──[C_in]── Pin 3 (Input)
                    │
                   LM317
                    │
              Pin 2 (Output) ──[C_out]── Vout
                    │
              Pin 1 (Adjust)
                    │
                   R1
                    │
                   GND
                    │
                   R2
                    │
                  Vout

Usage Notes

  • Requires adequate heat sinking for currents above 500mA
  • Input capacitor (0.33µF) recommended if regulator is far from filter capacitor
  • Output capacitor (10µF) improves transient response and stability
  • Adjustment pin capacitor (0.1µF) recommended for noise reduction
  • Minimum load current: 5mA for proper regulation
  • Excellent for audio circuits due to low noise characteristics

Inventory

  • Quantity: 5 units
  • Location: Cabinet 3, Bin 24, Section D
  • Condition: New
  • Source: Digikey

Notes

The LM317T is ideal for creating custom voltage references and adjustable power supplies. For audio applications requiring a 10V reference, this regulator provides excellent performance with low noise output. The simple resistor divider network makes it easy to set any voltage between 1.25V and 37V.

Graph View

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