Bipolar Junction Transistors wired as a current source will pass a certain amount of collector to emitter current even as a load from the positive supply to collector changes. The amount of current it passes is based on the voltage across the emitter resistor and it’s resistance. The voltage across the emitter resistor is the base voltage minus a diode drop of approx. 0.6V.

Diagram:

1KΩ emitter resistor makes the math easy. For each volt across it, it will pass 1mA of current.

You can either measure the current from Vcc to collector directly, or..

Make sure current flows through the collector while setting the current. Have the load attached, or even a direct connect to the supply voltage. Set the trimpot to zero volts and slowly raise it while measuring the voltage across the resistor. If it is 1000Ω (1kΩ) then stop when the voltage is the same as the current desired in milliamps. For example, 3V across emitter resistor equals 3mA through collector. You need to make sure supply voltage is high enough to power the load and transistor at any given amount of current.

This process is demonstrated in the video below.

Video

Quick NPN BJT current source using trimpot voltage divider circuit schematic to breadboard build



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Kit that has everything you need to learn basic electronics and Arduino.



Similar resistor kit to the one I have. Lots of values at a low price.





White version of the oscilloscope that I use in many of my recent videos.

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