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13.8V 20A power supply-Looking to add adjustable current limit?

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floobydust:
NPN emitter-followers as pass transistors have a voltage gain of ~1 and thus the highest speed.
PNP Darlingtons are the slowest possible pass transistor configuration. You are using them with voltage gain, looking at the big (voltage) feedback loop including the op-amp. A few mV change at the base can cause many V swing at the PSU output.
I have stability issues with them because hFE can vary from 750 to 4,000 with 18,000 given as max. hFE sags under load, changes with temperature and the loop gain is all over the place. Stability can be difficult.

LM324 output stage is Darlington, you lose several volts for VOH. I don't think it can actually go high enough to shut off the 2N6051's.
You might need a start-up resistor, I'm not sure what the LM324 will do with both inputs at ground on start-up.

MarkF:
I saved this from somewhere.  Maybe it will help.

   

Xnke:
Worked out a trade with a local to obtain a few LM723's and 2SD111 transistors, that will pretty well push this design off the table. Started the metal work on the cabinet tonight, cardboard aided design is wonderful for sheetmetal cabinets.

Now the only issue involves how to quiet down this power transformer. It has a bit of hum when drawing 40A DC off of it, however it does not get hot...I can keep my hand on it indefinitely at that power level. I think it's up to the task. It's about the same size as a decent old "1000 Watt" microwave oven transformer (not one of these puny new ones...) that weighs 20lbs.

floobydust:
I mount noisy power transformers on rubber isolation mounts. The vibration doesn't get coupled to the chassis. This is for 1000VA up. Very quiet afterwards.

not1xor1:

--- Quote from: Xnke on January 27, 2019, 08:38:12 pm ---Putting them on the low side of the supply is fine-I can rework this as an emitter follower, but is the only reason to do so because "emitter followers are more stable"?

I'll get paid Tuesday or so, I could just wait and order some NPN pass transistors. But making 68$/week that is a bit of a superfluous expense. ;)

I could add a second transformer for control voltage, that's no big deal. The parts bin is *very* heavy, and is about 10 meters x 20 meters. Same with big heatsinks...I sort big heatsinks by the pound, not by dimensions.

Now the last question about the PNP low side regulation-Will this (seeing as I need a negative ground) cause issues with other connected equipment by virtue of having a "moving" ground?

--- End quote ---

There is another reason why it is better to use the positive rail as common (ground, not earth): single supply opamp like LM358/LM324 inputs and outputs can get very close to the negative rail, but not to the positive one.
In any case you can use simple charge pump (2 diodes + 2 caps per rail) to get two other rails and so overcome all opamp input/output limits.
The messy circuit below needs to be revised (it was a quick copy/paste from pieces of other circuit simulations), but shows:
- 1 more negative and 1 positive rail via charge pump
- adjustable voltage and current and fast current limit
- leds displaying CC/CV mode
- slow turn-on


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