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Constant Current Source

Schematic of an LM317 voltage regulator configured as a constant current source.

 This simple circuit uses the LM317 voltage regulator to provide a constant current output to a ground referenced load.

  The value of Rc should not exceed 120Ω. Above this value there is an insufficient amount of current available for the LM317 to operate properly.

  The wattage rating of the resistor Rc can be a potential issue. All of the current flowing from the LM317 output pin passes through Rc. As a consequence, the power dissipation in Rc can climb quickly for higher value currents. The power dissipation can be found using the Ohms Law solver with the value of output current and the resistance Rc.

  See the “Voltage to Current” circuit snippet for a differential amp based solution that is voltage controllable.

The output current, Iout, is found by:

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