> To accomplish this, the zero crossing point need be precisely derived, as
> this is the reference from which the phase switching is determined.
> The actual switching is handled with a triac (BT139 to BTA 25/26 dependin=
g
> upon the load requirements) which in turn is driven by a random phase
> opto-coupler with triac output (MOC3020)
>

Perhaps I'm missing something... How are you turning _off_ a triac _during_
the phase? Does not a Thyristor continue to conduct until the zero
crossing? Did you by-chance fall into the same automatic brain substitution
I did?



> A good, simple DIY zero crossing circuit I have applied in the past can b=
e
> found here:
>
> http://www.dextrel.net/diyzerocrosser.htm
>
>
Hmmm... this seems... needlessly complex. My preferred solution has been
simply a PS2502 optocoupler (AC input/ two reverse parallel LEDs) with the
necessary current limiting resistors directly across the line. The
resulting waveforms are exactly the same shape, though the exact timings
have some dependency on the resistors chosen and exactly when in the cycle
enough forward current is present to turn the output on. In practice a
micro is constantly timing the overall width of the ZC pulse and deriving
the true zero mathematically.

-Denny
--=20
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