mike wrote:


If you don't turn it on/off at zero crossings, the saturation
state of the core can make a great difference in the next weld.


** Zero crossing switching is the worst possible for creating large in-rush surges and hence fully magnetising the core of an AC supply transformer.

Switching on at a voltage peak, while under load, is the best option.

A triac switch will always go off near zero current, which will be nowhere near zero volts.


Well, I don't recall mentioning volts at all.

** You did, cos "zero crossing" means at zero voltage crossings.


Yes, triacs switch off at the zero crossing of the current.
Not much you can do about that.


** Then why did you say it was needed ??


I question not turning it on at zero volts???


** Really ? Did you try Googling the topic?


Please explain the large in-rush current when there's zero volts
on the transformer primary?


** The surge current peak comes a little after the first voltage peak.

Consider that it takes half the time for the applied voltage wave to first average zero if you switch at a peak - half a cycle later instead of a whole cycle - minimising the low frequency component of the wave.


.... Phil