Timescope, thanks for an excellent description, yes I understand that clearly.
The led is the reference that the transistor uses to maintain the led's vf accross the load resistor r5 -vbe of the tranny.
I think what stopped me from grasping that in the first place was the use of the led as a voltage ref, something that I spose was popular in the early 70's, its not something I've seen a lot.
The datasheet shows pin 1 as n/c for the dil version, I think you mean the pinout for the t05 can version, pins 1 and 10 on that are the base/emitter junctions of the inbuilt current limit tranny.
So when the current limit pot is at max resistance the current limit will be at 0ma (well 1ma), and when the pot is at min resistance the imedance of the current sense resistor will take precedence and the current limit will be controlled by the sense resistor only, in this case 2a.
I'll add the mods to my supply, it only needs 3 wires connecting.
I'll put the led part of an opto isolator in series with the limit led on each supply (2 supplies are independant) then connect the phototranny collectors together to the 555 that controls the buzzer, or maybe I'll program a pic to do that yet.
Mr Al, do you mean put the current limit led transistor in series with the current limit transistor within the '723, I spose that would guarrantee that the led come on when the '723 is actually limiting, I might need another tranny to switch the led as the voltage between the current sense terminals and the + rail vary wildy being an adjustable supply.
Yes this supply is right out of the 70's, its been blown up a few times.