PS:

On ( another :p ) side note on the subject of thyristors vs triacs, I'd lik=
e to
point out an experience I made.. triacs are much harder to use than SCR's, =
because
the latter can switch off much more easily, as for half a cycle they won't =
conduct
no matter how you drive the gate. Triacs instead will be able to unlatch on=
ly when
zero crossing the AC waveform,

I mean, it's a very short moment and this has some implications. For exampl=
e I don't
think you can use triacs easily at more than line (50-60Hz) frequences.

Also, both triacs and SCR's if turned on at zero crossing, the gate must be=
 kept on
for a relatively long time, because at zero crossing it's hard to keep the =
device
held in latched state, and if you turn off the gate too soon, the device wi=
ll unlatch.

and since current and voltage aren't in phase in
some cases, you will do it when there's a non-zero voltage accross the tria=
c (this
for example has EMI implications).

The OP had a good intuition using two SCR's, as a way to gain more control =
on the
heaters (e.g. also allowing one semi-wave (alternating please!) to limit po=
wer).

On a final note, SCR's, unlike triacs, CAN be switched off even when curren=
t is
passing through them, by sending a powerful enough negative pulse on the ga=
te.
There are special SCR's (gate turning off SCR's are called, if I recall wel=
l) that
are specifically designed to switch off faster than a SCR driven with a - g=
ate.

Just sharing the very little that I know, as what I learnt on this list is =
still
much more than what I could ever give. (unfortunately)

Cheers,
Mario=20
--=20
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