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PICList Thread
'Zero Crossing, Phase Control and Interfacing Triac'
1997\05\27@140519 by David Dobbs

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Hi,
       We are working on a lighting controller but we need some info and /
or code on Zero Crossing, Phase Control and Interfacing Triacs. We would
prefer code for the 16C84 but that doesnt really matter as we can convert it.

                               Thanks from
                                       David Dobbs


               ////////////////////////////////////////////////////////////////
///
               Just to let you know we have a web page at
                     ' http://www.innotts.co.uk/~dobbs/ '
               ////////////////////////////////////////////////////////////////
///

1997\05\27@154502 by rtracy

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The Motorola Optoelectronics Device Data book has a very good section on
triac interfacing.  Page 8-24 - Applications of Zero Voltage Crossing
Optically Isolated Triac Drivers.  It has all of your basic logic to
triac opto to power triac circuits.  In some "circuit cookbooks" there
are circuits for direct logic drive of power triacs.  Unless the triac
is designed to be logic controlled do NOT direct drive.

Rob Tracy
FotoShield Controls
RnD

David Dobbs wrote:
>
> Hi,
>         We are working on a lighting controller but we need some info and /
> or code on Zero Crossing, Phase Control and Interfacing Triacs. We would
> prefer code for the 16C84 but that doesnt really matter as we can convert it.
>
>                                 Thanks from
>                                         David Dobbs
>
>
///////////////////////////////////////////////////////////////////
>                 Just to let you know we have a web page at
>                       ' http://www.innotts.co.uk/~dobbs/ '
>
///////////////////////////////////////////////////////////////////

1997\05\27@164131 by Harold Hallikainen

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On Tue, 27 May 1997 19:03:09 +0100 David Dobbs <spam_OUTdobbsTakeThisOuTspamINNOTTS.CO.UK>
writes:
>Hi,
>        We are working on a lighting controller but we need some info
>and /
>or code on Zero Crossing, Phase Control and Interfacing Triacs. We
>would
>prefer code for the 16C84 but that doesnt really matter as we can
>convert it.


       I can give you some comments, but my employer paid for the code!

       We used a 16c74a in a 4 channel dimmer (the Dove Systems
Shoebox... see http://www.dovesystems.com/dove ) .  It works like this..

       Zero-crossing detection is done by hanging a current limiting
resistor between one end of the power supply transformer secondary and
the INT pin.  The INT pin is set to watch for a negative edge, which
generates an interrupt a little before the negative zero-crossing.  The
positive zero-crossing is determined by timing from the negative one.
       Phase control is done by using the compare mode of the capture
compare register.  On zero crossing, we look through a table of how far
into the half-cycle we want to turn on the triac, finding the earliest
one.  We put that in the compare register and return.  On getting the
compare interrupt, we turn on that triac and look up the next one.
Actually, we also turn on any others that have a turn on time within 256
clocks of the current timer value to avoid missing any due to interrupt
response time.  This results in a VERY minor interaction between
channels.  I can see it on the scope when running the simulator at 4 MHz,
but can't see it in the product running at 16 MHz.
       Another interrupt is receiving the DMX 250 Kbps signal and
stuffing the appropriate bytes into an array where a table lookup then
determines timer values for the compare register (above).  The main loop
polls the A/D and reads in 4 channels of 0 to 10 volts control, if
present.
       The triacs are driven by MOC3010N opto triacs.  The optos are
driven directly by PIC output pins.

Harold

1997\05\27@180059 by rtracy

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>
> Rob:
>
> Just wondering, what disasters happen if you direct drive? Do you mean
> through a series limiting resistor as opposed to via an opto-isolator?
>
> Regards,
> Reg Neale

Yep... Thats what I mean.  I have tried it just to see how well it would
work.  Some triacs will switch just fine, but others will only switch
the positive cycle of the AC wave.  These triacs can be forced into
conducting the full wave by driving the gate well over the recomended
current.  Since you are using DC this is expected.  I have also found
that most of the time the triac will not switch at all.... Unless you
are totaly insane and reference your logic ground to AC N.  If you do
this it will switch but all of the wonderful noise, surges and spikes
are now on your ground plane... big mistake... :(  Anyway... all of this
has to do with my 12C509.  In a product we sell... a MID400 AC detector
is used along with a MOC3055 drive for a power triac.  The triac is used
to switch an inductive load at 1A.

Rob Tracy
FotoShield

1997\05\28@020031 by Andrew McDonald

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On Tue, 27 May 1997 19:03:09 +0100, you wrote:

There is an excellent article called 'Driving Triacs with the PIC
micro-controller' written by Bart Trepak in the March 1996 issue of
Electronics Today International magizine (UK) pgs 38-46.

It covers what you are interesting in, along with some flow charts,
assembler code & cct diagrams for swiching & dimmer control via the
PIC 16Cxx to get you on your way.

If you have trouble locating this article email me and i'll send you a
copy by post. Recently sent a copy to an American guy who was
interested in the same thing.

{Quote hidden}

.....maccoKILLspamspam@spam@ihug.co.nz
Andrew McDonald
Wellington
NEW ZEALAND

1997\05\28@133150 by Bruce Cannon

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I have tied the PIC ground to neutral, and an EXTINT pin to hot through a
4.7M resistor, and gotten good zero cross detection.  I have then timed
over for the desired phase angle, and pulsed a logic triac directly from
another io pin.  Works great, few parts, spiffy.  Mchip has an app note on
the zero cross method, and as someone else mentioned, motorla's data book
has good info on triac driving, also teccor's had good tutorial.

>         We are working on a lighting controller but we need some info and
/
> or code on Zero Crossing, Phase Control and Interfacing Triacs. We would
> prefer code for the 16C84 but that doesnt really matter as we can convert
it.
>
>                                 Thanks from
>                                         David Dobbs
>
>
>
///////////////////////////////////////////////////////////////////
>                 Just to let you know we have a web page at
>                       ' http://www.innotts.co.uk/~dobbs/ '
>
///////////////////////////////////////////////////////////////////
>

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