Triac control for a Step Down Transformer ?

[schmitt trigger]

If you ever decide to one day investigate thyrystors, (SCRs and Triacs) there are three key fact to understand them:

-The thyristor turns off whenever the current drops below the holding level. Note that I mentioned current, not voltage. This is quite important because in the vast majority of cases, the current waveform will be lagging several milliseconds behind the voltage's zero crossing.

-Another "black art" issue is their maximum dV/dt rating. Meaning that, if the Triac turns off, and as a consequence the voltage across MT1 and MT2 rises faster the critical dV/dt value, the Triac will retrigger itself. This is taken care by the snubber circuit, but for peace of mind select a Triac with a ruggedized dV/dt rating, the so called "Alternistors".

-Lastly, the thyristors are not fast by any stretch of the imagination. They will work at powerline frequencies. There are specialized "inverter grade" thyristors, which can be switched to a few kilohertz. The switching frequency falls within the audible range, and have been mostly superseded by IGBTs.

Thyristors are still found in ultra-high power control, there are some hockey-puck monsters capable of controlling 2.1 kiloamps at 2.5 kilovolts.