How to Make a Constant Current Source using LM317

Hi friends, today we will see how to make a constant current source using an LM317 regulator and a single DC supply. You can vary the magnitude of current using the formula given below.

Circuit diagram

The following figure shows the circuit diagram of constant current generator mini project.

[caption id="" align="aligncenter" width="998"]Constant current source Constant current source[/caption]

Components required

  • LM317 voltage regulator

  • 12V DC power supply (9V can also be used)

  • Resistor (depends on your requirement)

Working principle of constant current source circuit

As shown in the figure, constant current source produces a 20mA current across its output. The voltage across output is lower than DC supply voltage by 3V. In this case, we have used 12V DC supply, therefore, we will get 20mA current with 9V across its output. If you use 9V DC supply you will get 6V across its output terminals.

[caption id="" align="aligncenter" width="183"]LM317-pinout-diagram LM317-pinout-diagram[/caption]

LM317 is a linear voltage regulator IC having following specifications:

Vout range1.25 V – 37 V
Operation ambient temperature0 °C – 125 °C
Vin – Vout difference3 V – 40 V
Minimum Load Current3.5 mA typical, 12 mA maximum
Output Imax1.5 A (with proper heat sinking)

LM317 can be used as a current regulator by replacing its low side resistor of the divider with the load itself. In our case, suppose we are using DC motor as a load. We will get 20mA current flowing through DC motor. The formula for output current is given by,

I = Vref / R = 1.25 /62 = 20mA

1.25V is the reference voltage of the LM317 regulator.

You can download LM317 datasheets from the following links:

  • LM317 Texas Instruments

  • LM317 ON Semiconductor

  • LM317 STMicroelectronics

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