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Project Notes

#093 BJT CurrentSink

Test a BJT constant current sink circuit.

Here’s a quick demo of the circuit in action:



Current sources and sinks are excellently described in w2aew’s tutorial on YouTube.

This circuit uses a voltage divider bias for a BC547 NPN transistor. Assuming the current through the voltage divider is much greater than the base current i.e. I(R1) » Ib then Vb is approx 0.892V

From the BC547 datasheet, Vbe ranges from 0.55 (on) to 0.7V (saturated), so let’s take 0.6V for design purposes. So Ve = Vb - Vbe = 0.29V

Given this bias, the emitter current is determined by the value of resistor R3. And since we assume Ib is negligible, the collector current is the same. With a 1kΩ resistor, Ie = Ve/R3 = 290μA.

Actually my battery is running low and only delivering 8.41V so I’m expecting my constant current to be around: Ic = Ie = 230μA

In the test circuit, I’ve added a load of 3 green LEDS (with a forward voltage of about 1.9-2V) and a 10kΩ trimmer pot VR4.

When measure in circuit, the current is stable at 239μA as expected. This remain’s stable, even as the load changes by varying resistance VR4 or reducing the number of LEDs.



The Schematic

The Build

Credits and References

Project Source on GitHub Project Gallery Return to the LEAP Catalog

This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is just my personal collection of projects. Two main themes have emerged in recent years, sometimes combined:

  • electronics - usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting
  • scale modelling - I caught the bug after deciding to build a Harrier during covid to demonstrate an electronic jet engine simulation. Let the fun begin..
To be honest, I haven't quite figured out if these two interests belong in the same GitHub repo or not. But for now - they are all here!

Projects are often inspired by things found wild on the net, or ideas from the many great electronics and scale modelling podcasts and YouTube channels. Feel free to borrow liberally, and if you spot any issues do let me know (or send a PR!). See the individual projects for credits where due.