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Subject: Re: Triac controlled holiday lights
References: <email@example.com> <firstname.lastname@example.org>
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X-Original-Trace: 1 Jan 2003 21:53:50 +1000, 18.104.22.168
Date: Wed, 01 Jan 2003 10:53:56 GMT
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On 2 Dec 2002 13:30:25 -0000, Steve W.EE
>In comp.home.automation Steve Uhrig wrote:
>: The triac basically is back to back SCRs.
>: An SCR works well as a diode when turned on.
>: An SCR turns off when the voltage across the anode to cathode drops to
>: zero (zero crossing on AC). With the full AC waveform, each respective
>: SCR will turn on when the voltage goes above or below zero, because
>: the SCRs are back to back (a triac).
>: If you put half wave rectified AC, or pulsating DC, into a triac, it
>: will act exactly as a single SCR. It will do its normal thing during
>: one half of the cycle, but the opposite SCR will never see its voltage
>: since you blocked it with the diode. The end result would depend on
>: how you were gating the SCR, but the max you would see out of it would
>: be half line voltage.
>: I'm not sure what you want to do. But if I understand what you
>: proposed, the above unfortunately would apply.
>: Nice to see someone experimenting and actually doing component level
>Let me repeat again what I am trying to do. I already have the
>the board built the 14 optos and triacs. It is working, switching
>on and off the AC-powered bulbs.
>What I want to do now is modify it so it will work with shorter
>strings of bulbs. I'm planning to take this to my parent's house
>in the next few days and set it up. I can't really change the
>board right now.
>BTW, the optos have a builting zero-crossing switch in them.
>Each of the 14 strings needs to have 10-20 mini lights visible,
>but the only strings I can find have 100 lights with 2 sets of
>50 bulbs in series.
>I can do one of these:
>1. Cut the strings to 25 bulbs and put a diode in series with
>each string outside the box.
>2. Same as #1 but just put one diode in series with the common
>neutral wire which feeds all 14 strings.
>3. Take the triacs out of the sockets and put SCR's in their
>place. My question is whether this would work, especially if
>the pins on the SCR's are compatible.
>4. Use some type of light bulb as a resistor in series with the
>shorter strings to drop the voltage.
>There seems to be conflicting opinions on how much the diode drop
>will cut the power/voltage. I do know that diodes are commonly
>used in series with AC-powered incandescent bulbs to cut their
>power. I know this will cut the power in half, it will make
>the bulbs flash at 60 hertz rather than 120 but the flicker won't
>be a problem. I am still not clear on how much the diode will
>drop the power/voltage on a shortened string of bulbs.
>Will placing diodes outside the box cause any harm to the
>existing triac circuits?
Why not leave your existing circuit as is? Triacs are essentially
2 x SCR's back-to-back. They're designed to to do the work of two
SCR's but in ONE package. So your problem is - how to use your
existing circuit to drive smaller strings of bulbs - right?
Well - as I see it - you have TWO options:
1. Add a little bit of extra circuitry between the opto output and
the Triac gate input to DELAY the 'turn-on' time of the Triac
during each half-cycle of the mains waveform (what the Triac was
especially designed for). This will essentially reduce the amount
of AC current to the bulbs thus allowing you to have LESS bulbs.
There will be a reduction in brightness as well but this will be
minimal - however bulb brightness will be fairly constant for all
bulbs in the same line. The DELAY circuitry should be adjustable
to enable you to 'turn-on' the Triac at "anytime" within each half
cycle of the mains. BTW - such DELAY circuitry will only allow
you to TURN-ON the Triac - it can't be used to TURN-OFF - the only
way a Triac will 'turn-off' is when the mains half-cycle reaches
the zero-crossing point. In addition to this DELAY circuitry -
you could also set up the strings of bulbs such that one set is
'turned on' by the positive half-cycle of the mains - and the 2nd
set uses the negative half-cycle ( it don't make no difference
which way the AC voltage/current is supplied to an AC light bulb -
they work just as well whichever way the polarity of the mains is.
That's what AC is all about ) - 4 amp diodes could be used as
steering diodes for this.
2. Without changing your existing PCB's - add an extra small circuit
to the output of each Triac to REDUCE the AC current being fed to
the string of bulbs - I would suggest an AC current regulator.
For either option - if you do a 'google' search on the Web you'll find
plenty of possible circuits that could be usefull to do what you want.
On another point - for all those who have 'posted' to this thread -
I'm not pointing the dreaded finger at any person(s) in particular -
BUT when I go on-line and browse through the various NG's that I'm
interested in, I really DON'T want to be wasting my time reading
messages in threads that are NOT RELATED to the actual context of the
thread in the first place - for instance in this thread alone there
were 16 (of 46 total) or so postings that REALLY didn't need to be
posted (some a little on the vitriolic side - AGAIN NOT NECESSARY).
Most of us who lurk in this NG are in the main sensible, educated,
and generally interested in PROVIDING USEFUL TECHNICAL (as well as
THEORETICAL) ASSISTANCE & GUIDANCE in the 'Electronics' field to
those of us who are in need of help and guidance - I for one don't
aspire to knowing everything about the ever changing field of
electronics - and find such NG's as this one very helpful when I'm
designing something and the old 'blank-wall' syndrome rears it's
ugly head. So please, PLEASE, on behalf of all those in this NG
who are sick and tired of the few that continually post "NEGATIVE"
(or vitriolic / bombastic / or just plain DUMB / STUPID) postings
we would MUCH APPRECIATE if you would stick to the subject of the
question / query the 'original' poster raised when sending an answer.
Maybe we should set up another NG just for bitching -
sci.electronics.design.bitching - might get rid of some of the
unecessary postings from this NG - one can but only hope!!!
That's my two bits worth.
(Retired - sometimes electronics tinkerer from down-under)
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