Searching \ for '[EE]: Low Voltage AC Dimming and Reverse Phase Con' in subject line. ()
Make payments with PayPal - it's fast, free and secure! Help us get a faster server
FAQ page: www.piclist.com/techref/power.htm?key=voltage
Search entire site for: 'Low Voltage AC Dimming and Reverse Phase Con'.

Exact match. Not showing close matches.
PICList Thread
'[EE]: Low Voltage AC Dimming and Reverse Phase Con'
2001\09\27@214858 by Josh Koffman

flavicon
face
Greetings all! I'm back again. I used to be a semi regular participant
using the address spam_OUTjoshyTakeThisOuTspammb.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

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email .....listservKILLspamspam@spam@mitvma.mit.edu with SET PICList DIGEST in the body


2001\09\27@225030 by Michael C. Reid

flavicon
face
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 removed}

2001\09\27@230909 by Josh Koffman

flavicon
face
Thanks for your response. I was thinking of putting the dimmer on the
12V side of the transformer because I would be using this dimmer for
these lights only. I would build it into the transformer package and
just take a control signal in to control the lights. I know you can dim
transformers, I'm just trying to get around the associated problems.
Also, I'm trying to minimize losses. Plus, I'm getting into designing
dimmers, and working with the 12V should be a bit safer than the 120v
(and don't worry, I know it's still dangerous). This way I'd feel a bit
safer testing out my designs before transfering them to 120V. I guess to
summarize, I know I could dim the transformers, but I'm seeking a way to
just dim the lower voltage.

Thanks!

Josh Koffman

"Michael C. Reid" wrote:
>
> 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 removed}

2001\09\28@084842 by Olin Lathrop

face picon face
> So I was thinking, what is different about chopping a
> 12VAC wave that makes it different that chopping a 120VAC wave?
> Nothing...

True, in theory.  If you're using a triac or something, keep in mind its
voltage drop.  This can usually be ignored at 120V, but might be significant
at 12V.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olinspamKILLspamembedinc.com, http://www.embedinc.com

--
http://www.piclist.com hint: To leave the PICList
.....piclist-unsubscribe-requestKILLspamspam.....mitvma.mit.edu



'[EE]: Low Voltage AC Dimming and Reverse Phase Con'
2001\10\03@143934 by Harold M Hallikainen
picon face
       There should be no problem doing phase control at 12V (after a
transformer) using triacs. The triac voltage drop is now a larger
percentage of the line voltage, so you have to keep that in mind. The
current limit resistor on the MOC triac will also probably have to be
reduced to get adequate trigger current.
       Triacs CAN drive transformer loads, it's just a little tricky. One thing
to watch out for is that the phase control is very well balanced between
positive and negative half-cycles. Any DC imbalance shows up as DC
voltage across the transformer primary, leading to core saturation and
high currents. Careful design of the phase control circuitry can minimize
this, however.
       You can also use "solid state transformers" after a phase control dimmer
to drive low voltage lamps. These are small high frequency switching
power supplies. The input AC line goes through a full wave bridge to a
power oscillator that drives a transformer. The high frequency AC drives
the lamps directly. These are available as a consumer item at pretty low
cost. Since they have no (or minimal) filter capacitor after the
rectifier, the phase control "passes through" the device.
       Another approach (we did a 72 channel DC dimmer this way) is to have a
big 12V transformer running off the line. Full wave rectify the
secondary, grounding the negative side of the bridge output. Run the
positive side through the loads to FETs to ground. The FET gates can be
directly driven with a phase control signal (from a PIC or other phase
control generation circuitry). This approach is nice in that the FETs do
not have to deal with a negative voltage on the drain (as they would if
we were to phase control the AC directly). Phase control of AC would
either require two FETs, two IGBTs, two SCRs, or a single triac.
Rectifying the power early saves us from this hassle. Also, the lack of
filtering on the output of the rectifier makes it so standard dimmer
curves still work (it's still a sine wave, but the negative half is
switched back up to the positive side). We also avoid the cost and space
requirements of a large capacitor to filter the DC to handle the lamp
loads. Finally, the RDSon of the FETS is very low, so minimal heat
sinking is required. The only real voltage drop (thus requiring heat
sinking) is in the bridge rectifier. You can reduce this drop by using a
center tapped transformer and two diodes (instead of the four). We used a
12V 50A secondary toroid transformer driving a bridge rectifier module in
our 72 channel dimmer.
       I have not done anything with reverse phase control, since most of our
circuits use triacs or SSRs. I have not seen any economical method to
move away from triacs or SSRs in higher power circuits. Using FETs or
IGBTs in AC circuits requires two per channel along diodes to steer the
current to the appropriate device. Coming up with isolated gate drive
also seems to be a problem. By the time you add the steering diodes to an
FET circuit, the voltage drop approaches that of a triac, so we don't get
any efficiency gain by going to the more complex circuitry. IGBTs have an
even higher voltage drop. I tend to look at them as like a Darlington
pair with the first transistor being a FET. Turning on the FET shorts the
base to the collector. However, since it takes 700mV to turn on the base,
the collector cannot drop below 700mV, while the saturation voltage on a
BJT is down in the 300mV area. If we just used a FET, we'd be dealing
with RDSon instead of the saturation voltage, and have higher efficiency
due to the lower voltage drop.

Harold

FCC Rules Online at http://hallikainen.com/FccRules
Lighting control for theatre and television at http://www.dovesystems.com

________________________________________________________________
GET INTERNET ACCESS FROM JUNO!
Juno offers FREE or PREMIUM Internet access for less!
Join Juno today!  For your FREE software, visit:
dl.http://www.juno.com/get/web/.

--
http://www.piclist.com hint: To leave the PICList
EraseMEpiclist-unsubscribe-requestspam_OUTspamTakeThisOuTmitvma.mit.edu


More... (looser matching)
- Last day of these posts
- In 2001 , 2002 only
- Today
- New search...