You happen to have any insights what to look for when trying to find similar example circuits / projects?
Yes, I am thinking about a high-power current mirror.
So here is my solution - totally unproven: use two p-channel mosfets to form a current mirror, with one of the mosfet in parallel with the led string. The other mosfet's drain goes to ground through a current sink controlled by your mcu. This current sink could be a constant current source with a voltage input (typically done by an opamp + npn).
I did some more research on this topic, and found a nice summary on current sources/mirrors by Rod Elliott
mainly taken from "Designing Analog Chips
" by Hans Camenzind. So there I picked up the Four Transistor Circuit Mirror circuit (Figure 11b):
And then tried to find a transistor array with all the four transistors thermally coupled but ran into some problems as most components seemed to be for low current applications (e.g. That 300 series
, 20 mA),and the high current ones were in mA-range for example at 100 mA (Intersil CA-3083
). The ULN2075B
Quad NPN Darlington switch could take 1.5 A with the downside of slowing down the response times a bit due to the Darlington design.
So when not finding really high current (more than 1 A) PNP arrays (or P-channel MOSFET arrays) I decided to try to build the idea around the ULN2075B, however I was not quite sure about how to implement the constant current part for the current mirror branches. I would not like to use some fixed resistor values as at least LED current should be able to be varied from the constant current source, thus changing voltages across the transistors and ideally the circuit would not be that picky in regard to voltage supply if I want to put more LEDs in series for example.
So would the low-cost constant current circuit of the following Instructables
work? A bit like replacing the resistors from Recom's example sheet using current mirror to balance current for parallel LED strings (Figure 7 on Page 9
So my bare design without the constant current blocks looked like that:
And if I add the constant current parts with a limiting resistor of 0.5 ohms (R6 and R8) for 1 A then the circuit would look like that:
The ~V_in taken from the emitters of the Darlingtons would correspond to the supply voltage of the Instructables that was allowed to vary between 2 and 18 volts in the Instructables still getting the constant current operation.
So as a summary, do you think dhenry
that this would be a feasible alternative to what you proposed or did I miss something and the circuit would not work as designed while waiting to get the transistor array?