> I didn't look carefully enough to see to what extent you MUST limit the
> isolator output current, and I don't have any relevant experience.  But
> I'll bet 5.6k is way too big...  (This implies that triac circuits
> normally
> waste quite a bit of gate current.  Interesting.)

No - not much "waste" current, especially with a zero crossing driver as:

- When the triac is off no current flows.
- When the triac is on there is minimal voltage drop across the triac so no
drive voltage so essentially no current flows
- When the opto driver turns on at zero crossing the voltage STARTS to rise
across the triac until it reaches turn on point, at which stage we return to
the condition above and essentially no current flows.

The gate resistor needs to be small enough with the zero crossing driver
that enough current can flow to trigger the triac while the driver still
considers you are at or near the zero crossing point. Depending on driver
design, the driver may turn off again if the voltage rises  too high without
the triac firing.


        Russell McMahon

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