Speaking of that, if you are using a transformer supply for the PIC and monitoring it's secondary for zero crossing, are you taking into account the phase shift when you look for peak voltage?
Thanks, the zero crossing detector comes directly from the mains.....its an EL827 dual opto, with the diodes in reverse parallel, and a series resistor, between live and neutral. (as youll know, the two transistors of the opto are connected to a pullup, also connectd to the micro.)
Attached please find the LTspice simulation of the spike generator.
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What’s worrying us is the failure of the FETs……if this is due to the surge current drawn by a dying “product under test”, then we are happy, because then we only have to use beefy IGBTs as a replacement…………but what would cause more concern, is if the high-side circuitry surrounding the FETs is coupling in some noise or pulses which is killing the FETs.
I think just mentally, the sight of that high-side circuitry sitting there precariously on the mains live just looks odd to me, and I am wondering if there is some failure mode associated with it?
We would have used a contactor to switch the mains on at mains peak, but the problem is that they bounce and that would obliterate the narrow voltage spike pulses that we can get with a non-bouncing FET switch. I do wonder if there is such thing as a cheap mains contactor or mains relay that doesn’t bounce, or at least not for more than a few microseconds?