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Date: 2 Dec 2002 13:30:25 -0000
From: Steve W.EE
Author-Address: stevew-ee nym alias net
Subject: Triac controlled holiday lights
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?
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