Starting the year with a bang!

EDIT: So I think I figured out what went wrong. I initially thought something had touched something it shouldn’t have, but the depth of the crater and the size of the bang when it went off had me thinking there might be more to it.

The chip is an ATMega328p that I was using to control voltage into my capacitor bank. I was assembling the final pieces and doing a last calibration test. Normally the unit the chip sits in is powered by a 9v battery, but I had it connected to my bench power supply because I wanted to make sure it worked with a nominal voltage.

This turned out to be a mistake. With the power supply connected, the ground in the circuit was now tied directly to mains earth, whereas previously it had been floating. As it turns out, the way my charger is wired it ends up charging the bank negatively compared to earth. So the positive side is at earth potential and the negative side is however many volts below it. Anything up to 800v below it.

So, when I started to charge it, instead of the positive side going up, the negative side was pulled down, and the ground in my circuit with it, causing my poor chip to experience a couple of hundred volts more than it’s supposed to.

This also means that the GND in my handheld unit is sitting at several hundred volts below earth potential and that’s something I have to fix. It’s well insulated, but GND should be at earth and then the most voltage it will normally see is the 9v from its own battery.