How do I build a 10uA current source/limiter?
Something like this will give you a fairly constant current. It's only (as in not quite as) stable as your 12V supply, but it has quite good compliance &amp; output resistance and reasonable temperature stability for normal room temperature type changes (maybe +/- a couple % for +/- 10°C change). Ideally, put the BJTs face-to-face with a touch of thermal grease between them and shrink wrap around them. 

One can do much better with an op-amp and a proper reference, but this will work and meets your stated requirement of using only discrete components and no ICs. 







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