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'[OT] Triac firing, zero cross detection.'
2000\03\05@233244 by Alexandre Guimaraes

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   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.

   - 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 ?? What happens if I am using 220v
with 2 phases instead of a phase and a neutral connection ? My power supply
will be a capacitor coupled supply, without a transformer.

   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
spam_OUTalexgTakeThisOuTspamiis.com.br

2000\03\06@010058 by Gennette Bruce

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       -----Original Message-----
       From:   Alexandre Guimaraes [SMTP:.....alexgKILLspamspam@spam@IIS.COM.BR]
       Sent:   Monday, 6 March 2000 15:30
       To:     PICLISTspamKILLspamMITVMA.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
       .....alexgKILLspamspam.....iis.com.br

2000\03\06@012005 by Dwayne Reid

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face
<x-flowed>At 01:30 AM 3/6/00 -0300, Alexandre Guimaraes wrote:
>     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.

What you need to evaluate is how noise near the zero crossing will affect
you.  Phase control dimmers are inherently noisy and the glitches caused by
very low or very high phase angles (firing near the zero crossing) will
seriously mess up many zero cross detectors.  Then you can decide what
suits your purpose.


>     - 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 ?? What happens if I am using 220v
>with 2 phases instead of a phase and a neutral connection ? My power supply
>will be a capacitor coupled supply, without a transformer.

I generally fire direct coupled triacs in quadrants 1 & 2 (positive gate
drive for both positive and negative load current.  How you drive the
triacs depends upon your operating temperature (triacs at -40 require MUCH
gate current) and hou much load you might be connecting (low current loads
may require continous gate drive).

My standard light chasers use a picket fence type pulse stream at about 250
mA but with a fairly low duty cycle (100 uSec pulses).  This works reliably
in the cold.

Hope this helps.

dwayne



Dwayne Reid   <EraseMEdwaynerspam_OUTspamTakeThisOuTplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax

Celebrating 16 years of Engineering Innovation (1984 - 2000)

* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
Do NOT send unsolicited commercial email to this email address.
This message neither grants consent to receive unsolicited
commercial email nor is intended to solicit commercial email.

</x-flowed>

2000\03\06@014123 by Alexandre Guimaraes

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>         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?

   I am quite sure I understand what PIC's do. I have been working with
microprocessors for more than 15 years :-) What I meant is that the
detection would not be exactly at the zero crossing because you would always
have some difference induced by inductive loads and noise. I guess it would
happen in any one of the circuits. That could not be compensated in software
because it would not be constant.

>         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.

   The problem with logic level triacs is the price. They are nice for a
hobby project but not for production where price is a major problem.


>         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

   I do not have a earth connection !!! Is is not mandatory to have earth
connections in houses installations in Brazil !! The outlet output may have
just 2 connections with 2 power phases. I hate it but it is true.

>         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).

    That is not going to be the problem. It is going to be connected to the
same phases of the load always.

Thanks for the fast answer.

Best regards,
Alexandre Guimaraes
alexgspamspam_OUTiis.com.br

2000\03\06@110913 by Wagner Lipnharski

picon face
Hi Alex, following some european tradition for residential
installations, Brazil implemented the Grounded-Neutral solution, not
that it is perfect, but it works with restrictions.  Since the Neutral
Wire current in residential installation is not that big, and does not
generate several volts drops at the wire, then grounded-neutral works as
acceptable.  In south of Brazil, we already use the 3 pole outlet (with
ground pin), since it offers much better ground safety, please observe
that the words "mandatory" and "safe" aren't the same.  Even the
smallest computer (PC's) room (in the south) use the regular 3 pin
(ground, phase, neutral).  Of course, some cities still using those ugly
old 220V installations with only Phase-Phase, have a floating neutral
(non existant), so there is not a polarized pin at a 2 pin outlets.

Alexandre Guimaraes wrote:
>     I do not have a earth connection !!! Is is not mandatory to have earth
> connections in houses installations in Brazil !! The outlet output may have
> just 2 connections with 2 power phases. I hate it but it is true.

2000\03\06@122827 by Edson Brusque

face
flavicon
face
Hello Alexandre,

>     - 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.

   It really depends on your application, if you give more datails, I could
help you better. I've already made some prototypes using just a 2M resistor
between the phase and the pic input, and it worked very well.

>     - 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 ?? What happens if I am using 220v
> with 2 phases instead of a phase and a neutral connection ? My power
supply
> will be a capacitor coupled supply, without a transformer.

   Sorry, can't help you much here because my circuits are copyrighted. I
think the optos are unnecessary.

   Best regards,

   Brusque

2000\03\06@174457 by hl

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Look at application notes microchip many there
Bruce

2000\03\08@070003 by rubenj

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<x-rich><color><param>0100,0100,0100</param>This is what I have found from the web:


SC03_TECHN_PUBS_1.pdf from philips.


1863.pdf - Aplication note; MICROCONTROLLER AND TRIACS

ON THE 110/240V MAINS from SGS Thomsom (ST).


<color><param>7F00,0000,0000</param>>     I have to control 8 triacs for dimming incandescent lamps with the

{Quote hidden}

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AB Liros Elektronik
Box 9124, 200 39 Malmv, Sweden
TEL INT +46 40142078
FAX INT +46 40947388
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