Constant Current transistor circuit
There are a few wonderful internet repositories of really nice circuits that provide a useful starting point when considering a new project. For instance, maybe you want to run a motor that provides constant torque, or you have a trace of LEDs that light up in response to a sensor and as each LED "comes online" the circuit will need to keep up the juice without blowing the LEDs, or you want to make a battery charger that reliably provides a set current no matter the charge state.When looking around for a suitable circuit, and not wanting to wait quite a few months (thanks COVID-19) for a commercial solution, you turn to the buckets of bits and the internet and find that maybe the following circuit that might do the trick.
A substitution of the BD679 (hFE 750) for a TIP120 (hFE min. 1000) should be OK, and of course my favourite SS8050 comes in for the BC547. I did test with a final total of 5 LEDs and so needed to up the maximum voltage of the circuit as described from 15V to 19V. Nothing other than a few LEDs in testing blew up or became super hot, so that's a bonus. I can't imagine the little 1206 LEDs would last the distance pushed like this every day - but in this case it was just a test.
The use of simulator software was quite enlightening in how this circuit worked - check out the video to see this simulation in "action".
After playing around with the circuit for way too long I looked around for alternatives and found this possibility which seemed simpler. Again I used the SS8050 because it's OK to have favourites except maybe with your own children.
In the end I think this circuit was better for me, because it was quick to set up and quite stable when I was testing it with a large range of voltages, currents and loads. Also I didn't blow up so many LEDs testing it - wow they really smell when they go *pop*!
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