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From: Mark Lloyd
Subject: Re: Triac controlled holiday lights
Date: 2 Dec 2002 11:07:15 -0600
X-Newsreader: Forte Agent 1.91/32.564
On 2 Dec 2002 01:10:23 -0000, Steve W.EE
>I have been building a circuit which will flash
>holiday lights. It has 14 pairs of optocouplers/
>triacs to control 14 strings of lights. So
>far this is working well but I'm considering
>a last-minute redesign.
>Each letter in the display has from 10-20 lights
>in it. In the past I was able to get strings
>of 100 lights which were wired in 5 circuits of
>20 bulbs. We'd put the bulbs we want in front
>then hide the remainder of the 20 in the back.
>Problem now is that all the strings of 100
>mini-lights are wired in 2 groups of 50 bulbs
>The idea occurred to me if I could drive the bulbs
>at half voltage I could cut them into segments of
>One obvious solution would be to put a diode in
>series with each string of bulbs. I don't see
>a problem with this does anyone know why having
>a triac switch a recitifed load would be bad?
>Would there be any advantage in alternating the
>polarities of the diodes on each string of
>What about a single diode in series with the common
>return wire, of a suitable amp rating like 5A, which
>would rectify all the loads?
>Another option would be to pull out the triacs and
>pop SCR's in their place. They are in sockets but
>the board cannot be rewired. The triacs in TO220
>have gate on one side going to the opto thru a 1K,
>MT2 in the middle going to hot AC, and MT1 on the
>other end going the load. The other side of the
>goes to the MT2/hot AC.
>Are SCR's in TO220 pinned out so they could work in
>the same socket and also rectify the AC?
>Any other ideas. I think using high power resistors
>might be messy but thought of using larger bulbs like
>1-watt as reisitors. Of course the larger bulb would
>not be fully heated
Yes it would. Half the string (25 bulbs in a string of 50), will be
operating on half the voltage applied to the whole string. Half the
string connected to 60V will be using the same voltage per bulb as the
whole string connected to 120V. You can measure this if you want to be
sure. Note that is takes awhile for the temperature of the filament to
change, so a half-wave-rectified 120V heats the same as 60V. Also,
don't forget the fuses. They're important with these lights.
>so it's resistance would have to
>be experimented with.
How about using a single 2:1 step-down transformer and operating the
entire AC portion of the circuit (including TRIACs) on 60VAC? You
should be able to use one of the same transformers used for foreign
travel, as long as it can provide enough current (around 1.4A). This
would also make it safer.
You could even to 30V (assuming you could find the proper transformer)
for operating 12-14 bulbs instead of 25. How about something like
--------------) || (---------*------------------ 60VAC
) || ( |
120VAC ) || ( \----)||(---------- 30VAC
) || ( )||(
--------------) || (---------*----)||(---\
Both transformers are 2:1 units.
23 days until the winter solstice celebration
"There's no more proof for the existence of God than there is for the existence of the
Easter Bunny. That's right. The Easter Bunny."
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