I have bought two of your PCBs from repairman2be (rev. 8.), awaiting them any day now. I have studied the schematic pretty thoroughly and tried to read up on the threads.
Very interesting, and good to have something educating to do with my time :-). So far I have one question that I can't find a comment on in the forums.
First, let me introduce myself. I'm a Swedish 30 year old mmWave/μWave/RF engineer (read: allergic to noise) and I just had a second son so I had to find something to do all the nights I stay up taking care of him. Designing a PSU seems like a good choice!
It started with me realizing that I today financially can handle some of the ideas I had when I was 20, now time is more of an issue instead. But hey, it doesn't matter if it takes time to reach the finish line.
I always wanted to build a boombox. I thought a good place to start was to design a PSU (to not waste battery when a socket is available) and a relay-circuit. I read up a lot on audio PSUs and actually designed a linear 10-18V, 10A supply, and hand drew the breadboard and everything. When I saw the end-price I reconsidered. This PSU is kinda useless for anything but the amplifer, it's better to have a 0-30V supply I can use for lots of things. Especially considering I have no use for or place to store a boombox. The reason I'm saying this is that I might be tainted by audiophiles in my way of thinking :-). That I'm tainted by my RF-world at work I don't see as a problem, RF is probably what I'm going to use it for.
Then I found this design and got interested. But I still wanted it to be able to run the amplifier I had in mind for the boombox, which is something like 14V/6A. I considered paralleling two of these or use a LM723 regulator. I ended up with the idea of two 0-15V LM723 PSUs which I can parallel connect for 0-15V/0-10A or series connect for 0-30V/0-5A. I was pretty happy with this idea and started designing. However, I couldn't get the current limit to work as nicely as I desired for a bench PSU.
Then I came back to this design, and here I am now.
I'll start with building your 3A design pretty much identical to yours. To be able to do some measurements and see how it behaves before I try to improve it :-). After that I will start designing my own board. I'll probably stick to 3A or 5A for a while, if I need an amplifier PSU I'll design it for that purpose.
My ideas for improvements is to reduce the voltage drop a little, have a look at the negative supply (it hurts my eyes that it doesn't load both lines symmetrically). If I reduce the voltage drop and feel that I can't find an optimal transformer I might reduce the output voltage a little. I feel 30VAC is a little too high and 24VAC apparently is not enough so there is some unused power wasted there. We will see how it goes.
For later I might also take up the idea of LT1236 as reference. We will see. First I believe I have some startup transients to deal with!
So, now to my question:
Your R9 (original R17), is 68 Ohm. The original design says 33 Ohm but you changed it to 68 Ohm. Why did you change that? Have you found some issues with stability if it is too small? The reason I'm asking is that if I lower the current sense resistor I might want to increase this. Maybe I'm doing this wrong. I calculate the original design to give a minimum current limit of 10mA (while it says 2mA in the description), and your gives 20 mA.
Edit: I found the change, it was just to get the voltage divider to give the correct minimum voltage. However I'm still confused, but this can probably anyone answer. It should be simple. I don't get the formula for the current limit.
If we have a minimum of 5mV at the non inverting U3 input, that should allow for a 5mV drop over the current sense resistor right? That's 10 mA... Not 2..
Edit2: LTSpice seem to agree with me...
I might have more questions but I'll search and think a little more before I speak :-)
Ps. Thanks for the pdf with QA and other useful info. Would have saved me some time if I read that before I started study the circuit. But imI might learn it better by figuring things out on my own.