I've been looking around for a good CCS schematic, and decided to try making a simpler one even if it means it'll behave worse. I need a 12V (would come from an ATX power supply, more than enough current from the 12V rail) circuit that can handle at least up to 1A on the load (2/3A would be ideal). Would be for general purpose, electronics testing/etc, nothing too serious or in need of high precision. Made this in a simulator, and it seems to work:
The load is represented as a pot to make things easier.
The NPN transistor is a TIP35A, power NPN. Will have a decent heatsink and a cooler pointing in its general direction.
The PNP would be a general purpose transistor, and the adjustment pot would have calculated resistors in each side to limit its throw, of course. (depending on the resulting hFE of the "darlington pair")
My question is: Am I missing something important here? I would add an LM317 as a current limit (or a high-watt resistor on the circuit at least). Would this supply vary too much with resistance changes? (It had no change on the sim, but again this app is no LTspice). I have all the components so I plan on breadboarding it to test it with low currents, but I want to be sure this can work...
One more thing: I have a (totally overkill) ST P40NF0 3L MOSFET which I guess I could use (by replacing the driver NPN with a PNP and the power PNP with this MOSFET, and connecting the PNP differently of course). This FET has a very low on resistance which I guess would make my supply more efficient. Would this be correct?