By all means, build a variable PS with an LM317, or two... The ATX looks a bit dangerous for the projects as it can source quite a bit of current and blowing fuses testing projects is not my thing. It is useful to power a few things and I have a bunch I use every now and then when I need to test something, like feeding the input of your variable PS to test it. As Plazma just said, the "LAB SMPS" option is now available looks great and are cheap, with that and a laptop power bank you get quite a bit out there, I'm considering to get those but as SMPS doesn't quite get to a linear one I'm still with my project, even if I do get some of those.
The circuit with 2 LM317 with a switch for selecting a few different current limits seems nice, I never build one but I wish I did, now I'm trying to get a more advanced one as I see I should be to the task.
The frankie with Daves approach tends to be unstable, Dave stabilized it making it really slow to climb and fast to go down, with some big capacitance on the Vadj pin. The voltage being FF shouldn't overshoot which is nice. When he shows the Vadj pin on his you can see quite some oscillations going on in a few different situations. CC is one of them and you might get a noisy output there, put the shunting transistor to some abuse or be too slow and have quite a big current overshoot, you choose.
Now, after my last post one idea came to mind that looks promising at a first glance in the simulator, more testing should be done there before addressing the real world design, ideally my design should be scalable to higher voltages and currents. Decent currents could be archived with a single big mosfet without worrying much about redesign other than stability tweaks to compensate for gate capacitance, for an extended voltage range the opamps rails becomes a problem, like 0-30V range you want the opamps to be able to run a bit wider than the range, and low dropout from the main supply for efficiency and better range from the same pass device. I'd like to run all that from a single transformer, which combined with the last few points makes for the need of some higher rail likely using a multiplier, to archive a higher voltage rail for the opamps. Last but not least, I want to be able to control it with a µC. On top of all that I want to use all standard components, nothing fancy or special. All of that put a lot of restrictions, I'm not to fancy about precision and stability, to makes things easier but that mostly comes to the µC side of things, where you could choose better reference and converters to improve on that, as well as opamps if going to the limit.
Just for kicks, the difference in the new approach is using what Dave shown on the first video designing a lab PSU, in EEVBlog #221 as the constant current control, a conventional approach to control the voltage using a second opamp and a diode to select between both. As I have the current sense resistor in the high side after the pass device for the current mode it makes easy to stabilize the voltage mode under different loads without introducing new losses and keeping a good load regulation. An N-channel pass transistor looks like the more promising device but as has been discussed around here getting a proper transistor to work in this kind of circuits inside the SOA and not shortening it's life is a harsh problem, it looks like I can get an IRFP064 like the one in the Peter Oakes's design for about $3 which seems like a suitable part for a nice range, not needing to go for IXYS which could cost many times that. I'd probably start with some smaller IRF part to test around as I've been doing with my dummy load, if all comes together I'd get a few of those so shipping doesn't cost more than the devices.
Going too long with this, I just hope to find some time for it and finally get to something, I want to get this right and test it to then share it with all of you, with a flexible design that anyone can build with it's own specs, right now I'm moving the lab in a new room in my house so it's all been quite messy plus some big projects on my day job eating all my time. I need this supply for myself too, I'm setting up the lab quite a bit lately and if I could save the $ for the supply I'd like to have and settle with this the $ could go to some other toy as I'm collecting quite a few lately and building some others.
JS