>> Out of curiosity, how does a zero crossing solid state relay work? I had
>> thought it had a triac inside, but your comment about voltage drop makes
>> me question that assumption.
>
> Yes, normally they use either a TRIAC, or two antiparallel SCRs. And
> they do have a significant voltage drop. These thyristors by itself will
> always switch off on the zero crossing (of current). True zero crossing
> solid state relays have some circuitry that produces a window for the
> firing pulse that falls into a small time span just after zero crossing,
> so that switch-on is on the voltage zero crossing and switch-off of
> course remains on the current zero crossing.
A comment, in case it's not self evident - a zero crossing switch
won't give you a soft start function. It turns on at (almost) zero
voltas and stays on until the current falls to zero. To use a
triac for soft start you need to increase the conduction angle
over a number of cycles to gradually charge up the psu capacitors.
Steve
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