The thermostat takes "24VAC" from the furnace, which is really more like 28VAC. This goes through a bridge rectifier with a largish filter cap afterwords, giving around 38-39VDC as input to the (40V max) DC-to-DC converter board. The board then spits out 5.000VDC for use by the thermostat.

With a lightning strike in the neighbourhood, our house lights sometimes go brighter for 2-4 seconds, indicating a small voltage surge from the mains. I suspect this is what may have happened, just a few volts higher than normal for a few seconds, maybe a few occurrences during a storm.

Still, I wouldn't mind adding some surge protection in front of all of that. I wonder if perhaps I could just dismantle a normal AC surge suppressor and steal the relevant components for use on the 24(28)VAC input lines?

-ml