Uses for LM2596 DC-DC Buck Converter

I use them to provide controlled current to DC operating accessories from constant source wall warts, much like your lighting idea

Some accessories require AC, clearly this DC supply wouldn't be suitable.  You'll need an input source of DC to feed the module.  Also, to utilize this for any significant current, you'll also need to add bulk capacitance to any rectified DC.

Yeap GRJ, so far I use a seperate feed from my xformer accessory connection for any AC voltages required in the 12-18VAC range.  Right now everything I have that takes AC operates in the 12-15VAC ranges.  I have found that 12VAC runs about everything I have in the AC requirements bucket.  As I add AC accessories if I find that I need significantly varying AC voltages, I'll address it then.

My goal is to figure out how to run/wire all my DC accessories to operate off a single ATX power supply.  The challenge of course is the 3VDC, 4.5VDC, 6VDC, 9VDC and 12VDC variations between lights, and animated accessories running into a fuse box then power distribution buss.  And then of course there's the AMP math.  Between house wiring, boat wiring and layout wiring, it seems I spend more time thinking about amps than anything else.

Well, the ATX supply should power everything you might need unless you really have a ton of stuff!   For that use, the buck module looks OK, but one issue is the max of 12VDC output.  That only allows you about 10 volts DC maximum from the buck modules.

I expect that I will use buck modules on where I don't have a regulated feed from the ATX, things like 3.0, 4.5, 6 and 9VDC.  I'll need to draw from the 3.3 and 12VDC outputs respectively and dial it down.

As to fuses, I''m thinking that I'll put those in after the buck modules but before the load, - going to run a common ground back to the ATX P/S

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