Thanks for the interesting musings. You got me thinking too...
unless i'm missing something here... When in current regulation mode, it requires a low value resistor to get any kind of reasonable current?...
eg - To get a regulated 961.588ma of current, the Resistor has to be 1.3 ohms, and at least 1.5watts to cope.
That's certainly how I read it, and that's supported by a bit of google-fu too (for example this page
So for each LM317, I might as well spend $10 on a 2watt potentiometer, just to get a steady current... I might as well just use 1.3ohm resistor
and omit the lm317 from the circuit all together!
I guess you could, but that would be a current that fluctuates with input voltage (a current limiter rather than a true regulator) so you'd still be adding some kind of voltage regulation. Besides which by my maths the LM317T & a 5W 1 Ohm cement resistor is 42c all up (using as a reference pricing from my usual, Tayda for 5W 1ohm cement resistor
) so since LM317 comes in as a winner on cost, even if you use one for voltage and one for current, why would you choose the (comparatively expensive) 2W pot?
And since your pot will be doing the current limiting directly in series with the load, wouldn't it run out of spec at 5V at 400mA (as an example) whereas at that LM317 can produce constant current at far higher voltages. In the example linked above his LEDs had a 12V supply and consumed 320mA each - 3.84W which would let the smoke out of the 2W pot alternative - or am I missing something in your design?
(Zener Diode + Resistor anyone? = Almost CC/CV lol)
Can you get a constant current circuit from just those, or constant voltage only?