These are power supplies for some ID-20 RFID readers; the installation is in a model railroad system where I have a 14V AC line running around the layout.
Or switch to using a switching regulator (pun intended) and get 85%-95% efficiency.
I used linear supplies (a) for the obvious simplicity of building one, and (b) I seemed to be having some issues with interference from another switching power supply connected to the system, so I wanted to avoid the noise of a switcher.
Why do you need two supplies not just one?
I actually tried powering both readers from the same 5V supply, but for some reason it didn't work. I had built a module that provides optically isolated serial ports and a 5V supply to avoid the noise problem mentioned above. It works with either reader plugged in, but with both plugged in the readers don't work - not even activating the LED that registers a card read.
to work with just the module's power supply - it's a 1.5A TL7805-5 with the proper capacitors, and I think that should be ok for two readers. But I threw together another 5V supply, powered the second reader with it, and everything seemed happy. I know I should try to figure out why that's the case, but the ID-20 data sheet has no info on power requirements, and there is already a lot of voodoo involved. I had to optoisolate them because the read range would drop to nothing if a monitor's power supply was also connected to the system.
I'm pretty sure that can't work without shorting the AC input directly across two diodes. A full diagram would be needed to check.
I don't think it shorts; the power supply rectifiers are simply in series:
AC ----- ~rectifier~ ------ ~rectifier~ ----- AC
How about just converting it to a half wave? Can't you just pull out some of the diodes or replace them?
I'm not entirely clear on this.... if the filter capacitor on the input of the regulator is big enough, don't you still wind up charging to peak voltage?
So.... still haven't figured out my original question: is there any drawback/advantage to doing it this way? Yes, I could built a 9V supply for each if I was worried about heat, but to my simple-minded way of looking at it, it seems more efficient to put the supplies in series than to put them in parallel, each with an extra board to dump more heat.