Introduction

The LM317 adjustable voltage regulator can also be used as a constant current source by configuring it with a resistor that sets the current. In this experiment, we will explore how to create a constant current source for driving LEDs or charging batteries.

Materials Required

• LM317 Adjustable Voltage Regulator
• Resistor (R1 = 1.2Ω for a 1A current)
• Power supply (5V to 35V DC)
• Load (such as an LED or a small battery)
• Breadboard and jumper wires
• Multimeter (to measure current)

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Circuit Diagram

The following diagram shows how to configure the LM317 as a constant current source:

Pin Configuration of the LM317

• Pin 1 (ADJ): Adjustment pin, connected to the current-setting resistor.
• Pin 2 (VOUT): Output pin, connected to the load.
• Pin 3 (VIN): Input voltage pin, connected to the power source.

Steps for the Experiment

1. Place the LM317 regulator on the breadboard.
2. Connect the power supply’s positive terminal to Pin 3 (VIN).
3. Connect Pin 2 (VOUT) to one side of the load (LED, battery, etc.).
4. Connect the other side of the load to the ADJ (Pin 1) through a current-setting resistor (R1).
5. Connect the other side of the resistor to ground.
6. Use the formula I = 1.25V / R1 to calculate the desired current (e.g., for 1A, use a 1.2Ω resistor).
7. Turn on the power supply and measure the current using a multimeter.

Explanation

When configured as a constant current source, the LM317 maintains a voltage of approximately 1.25V across the resistor (R1) connected between the ADJ pin and ground. The current through the resistor is determined by the value of the resistor and the 1.25V reference voltage, as given by the formula:

I = 1.25V / R1

By selecting an appropriate value for R1, you can set the constant current. For example, using a 1.2Ω resistor results in a constant current of approximately 1A (1.25V / 1.2Ω ≈ 1A).

Results and Observations

After setting up the circuit and powering it, you should observe:

• A steady, constant current flowing through the load, as measured by the multimeter.
• The current remains stable even as the input voltage or the load resistance changes.
• For loads like LEDs, the brightness will remain consistent as the LM317 ensures a constant current.

Applications of the LM317 as a Constant Current Source

The LM317 constant current source is useful in applications such as:

• Driving high-power LEDs that require constant current.
• Charging batteries, where constant current charging is needed in certain stages.
• Providing a stable current for testing circuits or components.

Conclusion

This experiment demonstrates how to configure the LM317 as a constant current source. The circuit is simple and highly effective for driving current-dependent loads like LEDs or charging applications. By adjusting the resistor value, you can set different constant current levels.