-----Original Message-----
        From:   Alexandre Guimaraes [SMTP:alexg@IIS.COM.BR]
        Sent:   Monday, 6 March 2000 15:30
        To:     PICLIST@MITVMA.MIT.EDU
        Subject:        [OT] Triac firing, zero cross detection.

            I have to control 8 triacs for dimming incandescent lamps with
the
        minimum possible cost. I have been doing some research but there are
still
        some aspects I would like if someone could help me with. Follows the
list.

            - What is the best discrete zero crossing circuit ? I have found
many,
        ranging from a single resistor, a resistor and a zener up to a full
wave
        bridge with clamping diodes and a transistor. Do they all work the
same in
        practice ? I guess they do not have to be very precise because I
would not
        be able to fire the triac exactly at zero crossing anyway ! Is the
        optocoupler circuit better ? I do not thing so, specially in my case
where I
        do not need isolation from the power line.

        Uhm, lets see, you are using a PIC computing device to fire a triac
at zero crossing but can't put a delay/advance on top of the zero crossing
you detect to get the firing exactly at the zero crossing.  Are you sure you
understand what PICs do?

            - How to fire the TRIAC without a Opto-diac ???? I have found
many
        circuits and explanations again, but none has fully convinced me !
The
        circuits range from a single resistor from the microprocessor to a
push-pull
        stage with transistors and a capacitor to the triac gate. I will not
be
        using a logic gate triac, so I know that I need more current to
drive the
        triac. Do I have to fire it with positive current in one semi-cycle
and
        negative on the other in respect to MT1 ??

        There are things out there called logic level triacs that only need
a small resistor between the PIC and the triac.  But you think they are too
small (bet you that some company out there is making just what you want), so
you need to use a resistor and a transistor to switch about 10V to gate a
power triac.

        Note the word 'trigger' - it matters not which way the cycle is
going; you just hold the trigger on for long enough for the triac to lock on
and it will stay on until current flow ceases at the next zero crossing.

        Note the phrase 'current flow ceases' - if there is inductance in
the load (or the cables leading to the load) the current will lag the
voltage and flow will not cease at the next zero crossing so the triac will
not switch off.
        You have to add a snubber circuit in real world conditions - all it
would take is for a roady to leave 2 turns unrolled in a short extension
lead to create enough inductance to keep the triac on without a snubber.

        What happens if I am using 220v
        with 2 phases instead of a phase and a neutral connection ?

        Nothing - just treat both lines as active to earth and insulate
accordingly

        My power supply
        will be a capacitor coupled supply, without a transformer.

        Now you could have problems if the power supply is drawn from a
different pair in the 3 phase cable (like 225V instead of 5V).

            Sorry to disturb with the OT question but I was not able to find
these
        answers anywhere else and I have already digged all the ap-notes
from
        teccor, motorola and many web sites. It seems that everyone prefers
to use
        opto-couplers ! It does not make sense to me if I do not need the
isolation.
        The isolation will be in the connection betwenn the controller board
and the
        power board.

        Best regards,
        Alexandre Guimaraes
        alexg@iis.com.br