Just a quick test and a quick solution
Had a few minutes to just check up on why the load would change when the voltage does. A simple test of trying a higher voltage power supply seemed to do the trick. Stayed within 10mA of where it should, and probably did better than that, but my multimeter can’t measure that. The new power supply is a 200mA 12V wall wort which is very unregulated. I measure 16V off the bugger with no load. Makes me wonder if what is printed on the box is merely a suggestion. Still, nice to have the headroom. Now the opamps have enough swing to really kick that MOSFET into regulating the current flow reliably.
Another point which has come up, which adds weight to why Dave Jones chose a 1Ω shunt, is that he can read 1mV = 1mA whereas I have to read 100µV = 1mA. Bit of a pain in the butt in terms of panel meters. I always like to cram as much functionality as I can into my projects, so I had intended to add a switchable range feature on my panel display. So it can read 200mV full scale or 2V full scale. Sort of annoying that I will have to switch ranges to exceed 1.999mA (would read as 199.9mV, I’d change the decimal manually) but such is life. I will definitely need a four digit panel meter, the last digit – the 0.1mV – is essential so I can read down to the mA. Adjusting to the mA is hilarious though, lightly brushing the knob within half a bee’s dick of it’s life to get it spot on. It can do it, it’s just fiddly. Definitely using the largest knob I can find for maximum precision turning.
If I manage to pull this off and it works well, I will probably adapt the current limiting on the power supply to use a 0.1Ω shunt since finding 1Ω ones is truly difficult! I do have appropriate 1Ω ones picked out on Digi-Key though, and since they are only a few bucks I might as well go for them, but sheesh, you’d think this stuff would be more common. Maybe I’ll rummage around again at A1 when I go for the panel meter and see what shunts I can find. Not that I need much of an excuse.
Construction of the board will commence shortly, or as soon as I get off my lazy butt. Not feeling it just this moment and I usually like to be in a good headspace when firing up the iron.