Wow, thanks, that's really helpful! I didn't think of using those little relay modules because I think of those as an arduino project part (I was thinking of trying to do this with an arduino a while ago but I don't think it's worth the complication), and also because I wanted to use DPDT relays to also control signals. But for the price of those modules I can afford to use 2 relays per block.
A few more questions:
It looks like the rectangular blue 10A Songle relays are only rated for 3A or 5A for inductive loads. Does this mean I would be better off with the 30A (non-rectangular) relays to make sure they last a long time? Or should I just use a TVS diode to protect the contact side of the relay and use 10A relays? Because the diode on the relay boards just protects the coil, right?
That buck converter looks very handy for powering the relay boards. Could I also use a second buck converter (or some other DC supply) to "power" the isolated track sections rather than connecting the isolated rail to my AC accessory terminal and then using a bridge rectifier for each relay input? I guess the real question is can the DC negative output of the buck converter be "phased" with my transformer by connecting the negative output to common, since that would have to be the return path though the non-isolated outside rail.
How much current should I expect a relay board to draw from the control inputs? Without a schematic (and without a general understanding of how one would design an optocoupler input for a relay) I don't have any intuition about that.
Thanks again!
