Re: Battery tag spot welder.
I've repaired quite a few commercial spot welders over the years and for repeatable results, they generally use a phase-angle controlled triac on the primary of the transformer and a timer which is based on counting mains cycles. The ones we used to get were programmed with thumbwheel switches and used a postcard-sized CMOS board.
The sequence was Jaws Shut>Preheat Current On, time off. Fusing Current on, time off. Cooling period, release jaws.
Just in case anyone wants to make on with an arduino etc!
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Kevin
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