Would you mind showing the circuit of the control part, MrAl? Expecting to start experimenting with that very soon.
What specs has your "ultra precision"? Could you tell range and resolution as well?
Honestly, the full circuit would be even better.
Hi there,
Actually i am happy to hear that other people are interested in these kind of circuits. Since i left the industry i still like to work with these circuits as a hobby and it's nice to see other people do too.
As far as showing the *entire* circuit however, let me think about that. I suppose i could.
It's not entirely difficult however, as the basic idea is to use an op amp to control the LM338. That way we get very precise control over the output voltage but need only one TO220 package as the pass device.
The real difference though came about through an investigation into 'fine' control techniques. It seems that there were not many solutions out there that i could find that would allow for very precise voltage settings, and the resolution of these adjustments always seemed to vary with output voltage too, which i did not like at all. For example, at 3v output we might adjust by 10mv per tenth turn of the pot (0.1 of the full rotation) but at 30v output we might only get 100mv resolution in the adjustment with 0.1 of the full rotation. What i wanted was at least 5mv resolution with maybe 1/2 turn of the pot, and i didnt want to have to use a 100 ohm pot either as the good ones are much more expensive then ordinary 1k or 5k pots.
It turns out that there is a fairly cheap and effective way to do this too, and it worked out pretty well. I needed to be able to set voltages for battery charging for different types of batteries, at least to do some testing anyway, but it's nice to have for everything else too.
So the real difference is the low cost and highly effective output voltage adjustment.
The temperature spec comes in mainly as the choice of voltage reference diode and low drift op amp. I did not expect a really huge difference in temperature indoors, but i found that the LM3xx devices had unacceptable temperature drift when used for precision bench power supplies, at least when used alone. That was because the voltage reference for these devices is located on the same chip as the voltage regulation circuit itself, which means it is subject to a large temperature rise for high input to output voltage differentials. It's amazing how bad it can get too, and i found that unacceptable also. Taking the reference off chip (away from the main heat sink) means it is always subject to room temperature, which makes a heck of a difference in the temperature spec when used indoors in a roughly controlled temperature environment like 65 to 85 degrees C.
So the main design part is just an op amp that controls the LM338, and that's not too difficult to come up with, and the adjustment comes from the pot connected to the op amp rather than the LM338.
I'll try to post a schematic by tomorrow afternoon or sooner.