LM317 Voltage Regulator Pinout, Circuits and Equivalents

Back to Basics: LM317 Regulator

The LM317 is one of the most widely used adjustable voltage regulators in electronic circuits. Whether you’re working on DIY projects or designing complex power supplies, understanding how the LM317 works is essential for any electronics enthusiast or engineer. In this article, we’ll explore the LM317’s pinout, working principle, applications, circuit design, and more. Let’s dive into the essentials of this versatile voltage regulator.

What is LM317?

The LM317 is an adjustable three-terminal voltage regulator that maintains a steady output voltage. It can supply an output voltage in the range of 1.25V to 37V, with a maximum output current of 1.5A, making it highly versatile. It’s commonly found in adjustable power supplies, battery chargers, and LED drivers.

One of the main advantages of the LM317 is its simplicity and ease of use. Unlike fixed voltage regulators, the LM317 allows you to set the desired output voltage with just two external resistors, offering a flexible solution for various applications.

Key Features of LM317

Here are the standout features that make the LM317 a go-to choice:

1. Adjustable Output Voltage: 1.25V to 37V.

2. Output Current: Up to 1.5A.

3. Wide Input Range: Accepts up to 40V input.

4. Built-in Protections: Thermal overload, short-circuit, and current limiting.

5. Stable Performance: Excellent line and load regulation.

6. Simple Configuration: Just two external resistors needed to set output voltage.

7. Package Options: Available in TO-220

These features make the LM317 suitable for a broad range of regulated power supply applications.

LM317 Pinout

The LM317 has three pins that are fundamental to its operation:

Pin	Name	Description
Pin 1	Adjust	Adjusts the output voltage
Pin 2	Output	Provides the regulated output voltage
Pin 3	Input	Receives the unregulated input voltage

How Does the LM317 Work?

The LM317 regulates voltage by adjusting the potential across external resistors connected to the adjustment pin. When a higher voltage is applied to the input, the LM317 maintains a constant output voltage using the following formula:

V_out=1.25V×(1+R2/R1)+I_adj×R2

Where:

• V_out is the output voltage.
• R₁ and R₂ are external resistors that set the output voltage.
• I_adj is a small current (typically negligible) flowing into the adjustment pin.

This simple equation gives users precise control over the output voltage by selecting appropriate resistor values.

Common Applications of LM317

Thanks to its adjustability and robust features, the LM317 is widely used in numerous applications:

1. Adjustable Power Supplies

Allows for custom output voltages tailored to specific devices or testing scenarios.

2. Battery Chargers

Ideal for charging batteries like lead-acid or NiCd by providing precise voltage control.

3. LED Drivers

Ensures LEDs receive a constant current to operate safely and efficiently.

4. Audio Equipment

Provides clean, stable power to low-voltage audio systems, minimizing noise.

5. DIY Electronics

Popular in hobbyist projects due to its low cost and easy integration into various circuits.

LM317 Circuit and Diagram

Designing an LM317 voltage regulator circuit is simple. Here’s a basic setup:

Components:

• R1 = 330Ω (fixed resistor)
• RV1 = 10kΩ (potentiometer to adjust output voltage)
• C1 = 0.1µF (input bypass capacitor)
• C2 = 1µF (output capacitor for stability)
• Vin: Unregulated input voltage (e.g., 9V battery)
• Vout: Adjustable, regulated output voltage

Working Principle:

Turning the potentiometer RV1 changes the resistance ratio, thereby adjusting the output voltage. The capacitors help smooth out voltage fluctuations and improve noise immunity.

Power Dissipation in LM317

Power dissipation is a key factor to consider during design. It can be calculated with:

P=(V_in−V_out)×I_load

Where:

• V_in is the input voltage.
• V_out is the output voltage.
• I_load is the current drawn by the load.

To avoid thermal shutdown or damage, adequate heatsinking or other thermal management techniques are essential.

LM317 Equivalents and Alternatives

There are several equivalent or replacement options for the LM317, offering similar or enhanced features:

Picture	Model	Output Voltage Range	Max Output Current	Dropout Voltage (typ)	Package Options	Notes
	LM317	1.25V to 37V	1.5A	~2.0V @ 1A	TO-220	Baseline standard
	KA317	1.25V to 37V	1.5A	~2.0V @ 1A	TO-220	100% compatible
	LT317	1.2V to 37V	1.5A	~1.8V @ 1A	TO-220	Improved dropout
	NTE956	1.2V to 37V	1.5A	~2.0V @ 1A	TO-220	General-purpose replacement

Low Dropout (LDO) Alternatives

These aren’t drop-in replacements but offer improved efficiency and lower dropout:

Picture	Model	Output Voltage Range	Max Output Current	Dropout Voltage (typ)	Package Options	Notes
	LT3080	0V to 36V	1.1A (single)	~0.35V @ 1A	TO-220	Ultra-low dropout, can be paralleled
	MIC2941A	1.25V to 26V	1.25A	~0.5V @ 1A	TO-220	Excellent for automotive or battery-powered designs

LM317 Package

TO-220 (LM317T)

The TO-220 is the most commonly used package for the LM317. It supports higher currents and features a metal tab for heatsink mounting.

Features:

• Through-hole, 3-pin package
• Excellent thermal dissipation
• Ideal for power supply modules and voltage regulator circuits

Conclusion

The LM317 remains one of the most versatile, reliable, and cost-effective solutions for regulated power supplies. With its adjustable output, built-in protections, and easy configuration, it’s ideal for DIY builders, engineers, and educators alike. Whether you’re designing a power supply, driving LEDs, or charging batteries, the LM317 can handle it—all with just two resistors and a bit of planning.