I am in the lighting control industry and our triac and dual scr dimmers are
used to dim 12 and 24 VAC light via transformers all the time.  All you have
to do is take into account the loss associate with the tranformer and the
high inrush current when turing on the transformer.  Most triac dimmers are
set up to drive inductive loads without any problem.

-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Josh Koffman
Sent: Thursday, September 27, 2001 7:50 PM
To: PICLIST@MITVMA.MIT.EDU
Subject: [EE]: Low Voltage AC Dimming and Reverse Phase Control


Greetings all! I'm back again. I used to be a semi regular participant
using the address joshy@mb.sympatico.ca. I really enjoy the list, but
unless I keep up, the volume of messages just gets away from me.
Hopefully I will be able to cope, and I'll be on for much longer. I
really do appreciate the help that this list provides.

And on that note, my problem du jour. I'm going to be experimenting with
some 12VAC lights, and I'd like to dim them. I can't dim them with a
120VAC dimmer, because they'll be running off a transformer, which is an
inductive load. So I was thinking, what is different about chopping a
12VAC wave that makes it different that chopping a 120VAC wave?
Nothing...so I think. So my question is, will this work, and what should
I use as the triac? I was thinking of detecting the zero crossings with
a circuit similar to that seen at
ftp://ftp.unina.it/pub/Other/electronics/ftp.armory.com/CCTS/SSRelay.cct
(you can view it with your web browser, you don't need to ftp), and
triggering the triac with another circuit using a MOC3012, also shown in
the above mentioned link. Will the triacs that they suggest work at only
12V? I'm guessing I'll need to change the value for the resistor in the
zero crossing detector. Anyway, if anyone has any ideas, I'd really
appreciate hearing them.

I'm also interested in developing some circuits using Reverse Phase
Control (chopping off the second half of the AC wave rather than the
first half). The most common way to do this commercially seems to be by
using IGBTs (Insulate Gate Bipolar Transistors). I'd like to try this
approach at both 12VAC and 120VAC. Unfortunately, I don't know where to
start. I don't even know any part numbers, so I can't look for data
sheets. I also have no idea how to trigger these beasts. Any info or
pointers to part numbers would be muchly appreciated. Also, if anyone
knows where to get samples or order these in low quantities (like 3) I'd
appreciate the tips. I can't spare too much money on this research.

Anyway, I appreciate any help you can give me.

Thanks!

Josh Koffman

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