Introduction

The LM317 is an adjustable voltage regulator that can be configured to supply a variable output voltage, making it ideal for use in a DIY adjustable power supply. This experiment will show how to build a variable power supply using the LM317.

Materials Required

• LM317 Adjustable Voltage Regulator
• Potentiometer (5kΩ or 10kΩ)
• Resistors (R1 = 240Ω)
• Input power supply (12V or higher)
• Capacitors (0.1µF and 1µF)
• Breadboard and jumper wires
• Multimeter (to measure output voltage)
• Load (such as a small motor or LED)

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Circuit Diagram

The following diagram shows how to configure the LM317 to create a variable power supply:

Pin Configuration of the LM317

• Pin 1 (ADJ): Adjustment pin, connected to the potentiometer for controlling the output voltage.
• Pin 2 (VOUT): Output pin, connected to the load or output terminal.
• Pin 3 (VIN): Input voltage pin, connected to the power supply.

Steps for the Experiment

1. Place the LM317 on the breadboard.
2. Connect the positive terminal of the input power supply to Pin 3 (VIN) and the negative terminal to ground.
3. Connect a 240Ω resistor between Pin 1 (ADJ) and Pin 2 (VOUT).
4. Connect the potentiometer between the ADJ pin and ground to adjust the voltage output.
5. Connect a 0.1µF capacitor between the input and ground, and a 1µF capacitor between the output and ground (optional, for stability).
6. Connect the load (motor, LED, etc.) between the output pin and ground.
7. Turn on the power supply, and adjust the potentiometer to change the output voltage. Use a multimeter to measure the output voltage and verify that it varies smoothly as you adjust the potentiometer.

Explanation

The LM317 is configured to provide a variable output voltage that is controlled by the potentiometer. The adjustment pin (ADJ) regulates the output voltage according to the following formula:

V_OUT = 1.25V × (1 + R2/R1) + I_ADJ × R2

In this formula, R1 is the fixed 240Ω resistor, and R2 is the variable resistance provided by the potentiometer. By adjusting the potentiometer, you can control the output voltage between 1.25V and the input voltage minus a small drop (typically 3V).

Results and Observations

After completing the circuit and powering it, you should observe:

• A stable, adjustable output voltage that can be controlled with the potentiometer.
• The output voltage smoothly adjusts from around 1.25V to a maximum of approximately the input voltage minus 3V.
• The load (e.g., an LED or motor) responds according to the output voltage.

Applications of the LM317 Variable Power Supply

This circuit can be used in several applications, such as:

• Building a simple adjustable power supply for electronics experiments.
• Providing a variable power source for testing different components like motors, LEDs, etc.
• Adjustable battery chargers or regulated power supplies for specific applications.

Conclusion

The LM317 is a versatile regulator that allows you to create a simple, reliable variable power supply. With just a few components, you can build a power supply that is adjustable over a wide range of voltages, making it a valuable tool for electronics enthusiasts.