Hi,
There is the current feedback, and phase angle thyristor driver drawings.
When the machine switches on U14 figures out the phase rotation, and whether a star star or star delta transformer is being used. As you can see the only connection to the pic's is the primary and secondary windings. There is no connection to neutral or earth. This works fine in most cases, but with certain mig welders nearby the current loses control, & during the weld some thyristors seem to stay open for a cycle, at random intervals. This got me thinking how does the pic detect the zero crossing point when the phase inputs are not refferenced to the pic power supply, I am wondering how it ever worked at all.
Bob...
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