V5.0 makes only a few changes, but they're fairly substantial.

The biggest change is that instead of the primary power supply making 12 VDC and a buck converter turning that into 5V, the primary supply is 5V and a boost converter is used to make 12V. As before, the -12V supply is derived from +12V with a charge pump. One advantage of this system is that there is a use for 5V on the HV board to run the comparator that's part of the relay test. 5V used to be sent back to the HV board from the logic board for this purpose. 12V is still fed back to the power board, but there's (presently) no use for it.

Since the boost converter and the microcontroller are both fed from the 5V bus, the controller and ISP connector together form their own separate power domain fed from the 5V bus by a MAX40200 ideal diode chip. This cuts down on how much circuitry the programmer has to power.

To give the controller board a good bench test, I designed a test bench board. It has the FFC connector, LEDs to indicate the contactor state, DIP switches to simulate the AC/GMI test circuit and a barrel connector to feed either 5V or 12V power (depending on the HW version). Unfortunately, there's no safe way to test the HV board, but the point of the design is that the HV board is so simple it should just either work or not.

With the test jig, I've been able to validate the board. Tomorrow I'll rebuild my Hydra to use the new boards.

UPDATE:

I had an opportunity to complete the build with HV board 5.0. Unfortunately, it's not perfect. The contactors showed an alarming tendency to stay closed after charging completed. The good news is that the result was an "R" error, so at least it's not a silent failure. Even adding a snubber wasn't enough to solve the problem, so the SSRs, I'm sorry to say, are fired.