Thanks for the support, especially cocoonut.
I'm an EE by trade, power supply designer by experience, and I've been busying myself designing a power protection front end for a replacement - if i can find one. As some background, I've been able to crudely trace the insides of the various corpses. Of the several I've had die across the years, all but one were with visible charring on the incoming power supply section. The input of 24VAC nominal power comes off the two prongs of the connector, runs right to what is almost certainly a surface mount MOV or TVS clamp, then right into a four-diode full wave bridge rectifer, and then into the filter caps that make DC for the microprocessor that runs things and the various relays and such.
The charred components included the MOVs and diodes a couple of times, and some times just one or the other. Even the un-charred components were either fully shorted or fully opened even in the cases where there was no PCB charring. Re-building the power parts of the PCB with new MOV, diodes, filter caps and regulator for the microprocessor did not revive the corpses I tried this on.I suspect that the controller was fried, which you'd expect if its regulator died.
So yeah, a MOV. Big sucker, probably recessed into the wall. I am also qualified to kibitz with HAI's power supply design. Looks like the fuse they put in the power path is AFTER the MOV on the board, so frying the MOV then fries the rectifier diodes without opening the fuse. The fuse was opened only in the one case where the lighting stroke also shorted the 24VAC control transformer from primary to secondary. That was before the house surge suppressors.
My current (!!) thoughts on a front end for this includes some inductance on both the incoming leads to the 'stat, then a fuse in each line, then a big-sucker MOV, Maybe even a TVS clamp to a dedicated safety-ground wire just ahead of the 'stat power connector.
You have to be careful with MOVs. MOVs lose a little bit of their "break over" voltage each time they are broken over and clamp overvoltage. How much they lose depends on how much heat is generated in the MOV during the transient. The original AC power line MOVs were rated at 130VAC. they would sustain some hits, their voltage would drift down until little blips up on the AC power line would trip them, then they'd drift down more, and finally, when the MOV started breaking over at normal AC line peaks, they would rapidly overheat and die, sometimes spectacularly. This discovery led to the idea that you never use a MOV without a current limiting device, like a fuse, ahead of it on the AC power line.
It's accepted practice to NOT use MOVs less than 150Vac on normal AC power line applications for this reason. On a nominal "24Vac" control line, the actual AC voltage is often 28Vac-30Vac if they're lightly loaded. By the time you add onto that AC line surges, you probably need a 45-50V MOV on the output of the control transformer to keep the MOV from dying soon. TVS devices may or may not have a similar vulnerability. I'm in the process of refreshing myself on them.
In any case, I really do want to find a plug-in replacement if I can. I'd like to keep the re-building the world to a minimum.
