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'[PICLIST] [EE] question about light dimmer circuit'
2001\12\14@201058 by Eben Olson

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part 1 1999 bytes content-type:multipart/alternative; (decoded quoted-printable)


------ 2001\12\14@205640 by Augusto de Conto
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Eben

No. It is wrong.
The 1k Resistor is a current limitator.
The equation you must use is P = R.(i^2). Consequence of P=i.U and U=R.i
"R" is easy: 1k
"i" is almos equal the Triac Gate current. It depends of the Triac you choose.
   You must see in it's DataSheet. Generaly it is very low.
I am not shure, but I guess that a 1/4W will work ok.

Augusto de Conto
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 {Original Message removed}

2001\12\14@210831 by Kathy Quinlan

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Just a note, any voltage above 100VAC RMS and 150VDC  I use grater than 1W
resistors regardless of power, as they have a higher voltage rating and do
not arc over, been caught on strings of 1/4W, one fails and they all go
flash.

Just remember all good component suppliers supply the working and breakdown
voltage for their resistors, use the catalogue.....

May the Data sheet always be with you ;o) <said in best dathvader voice ;o)>

Regards,

Kat (in a silly festive mood)
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{Original Message removed}

2001\12\14@211616 by Michael C. Reid

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this can be a normal 1/4 watt or 1/2 watt resistor. The optoisolator has a
diac on the isolated output that triggers the gate of the TRIAC. There is
very little current going through this resistor.  I would recommend that you
design your dimmer using a Solid State Relay, using a random fire version.
You can get them from Digikey.  They have all the required components inside
for dimming.  You have a low voltage side and high voltage side.  The low
voltage side has a ground and 3-32vdc connection.  The solid state relay has
an internal resistor so it just connects to an output of the PIC.  You just
mount it to a heat sink and it's ready to go.  Remember that TRIAC's and
other thyristor type devices drop about 1 volt across the junction which
needs to be dissipated into some type of a heat sink.  The bigger the
better, as heat is the enemy.
 {Original Message removed}

2001\12\14@222507 by Jay.R.Vijay-Indra

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As you are detecting the zero crossing, I am assuming you are trying to
turn the Triac just after zero crossing.

The Triac operate on two basic priciple.  When its OFF, you have to supply
TRIGGER current (Igt normally about 10mA to 30mA). Once Triac is turned ON,
HOLD (Ih)current through the Triac will keep it in ON condition. If current
goes below the HOLD current (near the zero crossing) triac will turn OFF.

If the ac line current is 30v and the resistor on MOC3012 say 1k, then Igt
is 30mA and Triac should fire. At this point you can turn the MOC3012 to
OFF state and turn MOC3012 to ON state on the next zero crossing. Power
rating of the resister is 30V*30mA=0.9W.

For a design like yours, I would use say about 300R resister.
       Then the turn voltage, V=30mA*300R=9V+Vgt =10.5V

       Power rating of the Resister = 10V*30mA =300mW

If you need more info check the AppNote AN1007 at Teccor Electronics INc.

Regards,

Jay





At 20:24 14/12/01 -0500, you wrote:
>Hi all. I'm trying to build some PIC-controlled light dimmers for my
school theater. After looking at some schematics from the internet, I
pieced together the attached basic diagram. But if anyone has experience
with this kind of thing, I have a few questions. I'm very confused about
how to calculate the wattages required for the resistors. Using V=IR, and
P=VI, it seems like the 1k resistor between AC supply and the MOC triac
trigger would have to take about 15 watts. Is this correct?
>TIA,
>    Eben Olson
>
>Attachment Converted: "c:\eudora\attach\EEquesti.htm"
>
>Attachment Converted: "c:\eudora\attach\Image6.gif"
>

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2001\12\14@230324 by Jinx

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> Just a note, any voltage above 100VAC RMS and 150VDC  I use
> grater than 1W resistors regardless of power, as they have a higher
> voltage rating and do

I generally use 1W 68R or 1W 100R between opto and triac. Gate
current on the various triac types ranges from 1mA to 50mA

> ;o) <said in best dathvader voice ;o)>

Oi, it's not you been making those breathy phone calls late
at night is it ? I've got some requests

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2001\12\14@232020 by Kathy Quinlan

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> > ;o) <said in best dathvader voice ;o)>
>
> Oi, it's not you been making those breathy phone calls late
> at night is it ? I've got some requests
>


Shucks caught out again ;o)

It is actually quite funny, as I have chronic obstructed Asthma (and spend a
fair bit of time in hospital) the nurses come into my room and think they
have just walked into an obscene phone call ;o)

Regards,

Kat.
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2001\12\14@233559 by Jinx

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> It is actually quite funny, as I have chronic obstructed Asthma
> (and spend a fair bit of time in hospital)

Sorry to hear that - I wish you well

> the nurses come into my room and think they have just
> walked into an obscene phone call ;o)

Lucky old nurses. Small compensation for the rotten pay

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2001\12\15@030805 by chris

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 I use 1W on 120V & 2W on 240. These make for nice wide gaps between the
leads, bigger than the UL ratings for spacing at line voltages. It's best to
take the min Igate from the data sheet and multiply by 5, or as much as 10
for high di/dt or low-temperatures are expected. If your 120 mA Igate is
being applied to a device with a min rating of 20mA, I'd say it'll work
well. As for the sizing, Who wants to figure out the real power for that
resistor, not I for sure. Consider that you are firing through it for how
many usec during each 8.3 msec period? add in the currrent lag time, and you
may find that after an hour or two an 1/8 W resistor works on paper, but
will make smoke 50% of the time for some of the reasons already mentioned.

 Another reality is to forget the MOC301X's. Again, smoke all too
frequently. Go with series multiples of MOC306x's (at least a pair). The
redundancy is important when you consider the failure mode, which will leave
your load full on if a single device shorts.

 Chris

  Hi all. I'm trying to build some PIC-controlled light dimmers for my
school theater. After looking at some schematics from the internet, I pieced
together the attached basic diagram. But if anyone has experience with this
kind of thing, I have a few questions. I'm very confused about how to
calculate the wattages required for the resistors. Using V=IR, and P=VI, it
seems like the 1k resistor between AC supply and the MOC triac trigger would
have to take about 15 watts. Is this correct?
 TIA,
     Eben Olson

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2001\12\15@031226 by chris

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BTW, Eben, do you have code for the 628 & this circuit to share?
I would love to play along.....I have done this for SCR pairs with an 18C442
in asm, and it was way too much work. AC power control is sort of a hobby
for me....

CL
 {Original Message removed}

2001\12\16@173017 by Alexandre Domingos F. Souza

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       Meow,

>Just a note, any voltage above 100VAC RMS and 150VDC  I use grater than 1W
>resistors regardless of power, as they have a higher voltage rating and do
>not arc over, been caught on strings of 1/4W, one fails and they all go
>flash.
>Just remember all good component suppliers supply the working and breakdown
>voltage for their resistors, use the catalogue.....

       Never seen this happens, even in noisy lines (subject to high voltage spikes). But nice to know ;o)

>May the Data sheet always be with you ;o) <said in best dathvader voice ;o)>

       (inspire)...(expire)...(inspire)


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Alexandre Souza
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2001\12\16@203326 by chris

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I have worked with AC Power control for about 15 years, and have seen all
kinds of components fail that were spec'ed well according to datasheets.
Smoke, charring, and ozone effects are pretty common. Often, post-mortems
have shown that the devices' ratings mean very little when after years of
use, contaminants of all sorts coat component surfaces, or high humidity has
broken down a dielectric coating. They almost always do what they're
supposed to when they're new and clean. Real-world is another story.

Chris

> {Original Message removed}

2001\12\17@085346 by Roman Black

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Chris Loiacono wrote:
>
> I have worked with AC Power control for about 15 years, and have seen all
> kinds of components fail that were spec'ed well according to datasheets.
> Smoke, charring, and ozone effects are pretty common. Often, post-mortems
> have shown that the devices' ratings mean very little when after years of
> use, contaminants of all sorts coat component surfaces, or high humidity has
> broken down a dielectric coating. They almost always do what they're
> supposed to when they're new and clean. Real-world is another story.


I've used tantalums in projects for 20+ years,
and never had one fail. I also have replaced
10+ electro caps in every TVs we repaired and
never a tantalum. I'll go tantalum anyday provided
the voltage ratings are respected. Electro caps
are little chemical time bombs full of acid and metal,
I really hate the things, especially in Australia
where we get erratic mains voltages and high
temperatures. If I have to use electros in
something sensitive I use the 105'C electros
and overrate voltage at about 3x and mount them
away from heatsinks and big resistors. Some TV
makers deliberately put small electros next to
large heatsinks. Bastards, they're not stupid. :o)

One good trick with any PSU cap is to put a
resistor in series between the retifier bridge
and the cap. This reduces startup surge current
and increases the diode conduction time, both good
things. Most of the commercial manufacturers use
resistors anywhere sensitive, mainly because they
can then use cheap weak parts and still get away
with it. But if you use the safety resistors and
GOOD parts you get a product that runs for 20
years. :o)
-Roman

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2001\12\17@093523 by M. Adam Davis

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I've read that electrolytics actually degrade faster when they are not
used close to their voltage rating, ie, a 35v cap on a 5v line - though
I can't remember where...

-Adam

Roman Black wrote:

>temperatures. If I have to use electros in
>something sensitive I use the 105'C electros
>and overrate voltage at about 3x and mount them
>away from heatsinks and big resistors. Some TV
>

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2001\12\17@130505 by Harold M Hallikainen

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       A quick look at the circuit...  The 1K only has full voltage across it
for a very short time. Once the triac fires, the voltage across the triac
drops to about 1.5 volts. A 500mw resistor works fine here (unless the
triac blows open, then the resistor will blow).
       I don't see why the 10K to ground driving the opto is required. The PIC
output goes all the way to ground, so the pull down is not needed.
       The bridge and opto zero cross detector could be replaced with an AC
opto. In dimmers where we use this (typically our three phase dimmers),
we use an H11AA1. We also put a resistor across the LED here to adjust
the width of the zero cross pulse. In other circuits (such as our DM406),
we have a separate full wave rectifier on the secondary of the power
transformer that we use for zero cross detect.
       For receiving DMX, we use the MAX 488, which has a higher ESD rating
than chips in the 75 series. Also, when used as a DMX transmitter, it is
slew rate limited, which reduces reflections in poorly terminated lines.
       Finally, the DMX standard calls for a 5 pin XLR connector, not a BNC.

Harold

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

On Fri, 14 Dec 2001 20:24:43 -0500 Eben Olson <banielspamspam_OUTCROSSWINDS.NET>
writes:
{Quote hidden}

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2001\12\18@182947 by Peter L. Peres

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> I've read that electrolytics actually degrade faster when they are not
> used close to their voltage rating, ie, a 35v cap on a 5v line - though
> I can't remember where...

Aluminium electrolytics rely on aluminium oxide dielectric formed between
the aluminium electrode and the electrolyte. This oxide can disappear if
the caps are not used properly (or stored for too long), causing leakage
or a short. It can also break down at a lower than designed voltage.
Reversing the polarity will eat away the oxide ...

Older large caps (ca. before 1970 I think) used to require a 'forming'
time, where you'd connect them to a current limited PSU and have them at
1/3Vcc for a few hours, then 2/3Vcc for more hours, and the at full
voltage for some more time. This re-forms the oxide. The method is still
valid when restoring very old equipment. It can save you a big bang and
smoke.

New caps don't really need this, and they are not expected to last that
long in the first place (try to imagine some all-plastic contemporary
equipment in 10 years).

Most tantalums used in consumer equipment are (wisely) not employed as
supply filters, but as bias decoupling and as coupling caps or timing etc
caps, and are not subjected to voltage and heat abuse. They last forever
(>30years in correctly designed equipment in my experience). More recent
miniature equipment uses lots of tantalums (or other solid dielectric
types) in SMPSUs in addition to other roles but they are of special type
(ESR and voltage and current ratings etc). They do not cause problems
afaik.

Peter

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2001\12\20@200030 by Alexandre Domingos F. Souza

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>One good trick with any PSU cap is to put a
>resistor in series between the retifier bridge
>and the cap. This reduces startup surge current
>and increases the diode conduction time, both good
>things. Most of the commercial manufacturers use
>resistors anywhere sensitive, mainly because they
>can then use cheap weak parts and still get away
>with it. But if you use the safety resistors and
>GOOD parts you get a product that runs for 20
>years. :o)

       Don't count on that! The small kind of slot machines we found in Brazil (is called "hell fire") uses a switched PSU that fails A LOT. Of course, the culprit is always the (_#%**(_ safety resistor between the mains and rectifier. EVERY time I get one of these to fix, the first thing I do is to change the input resistor. I tried to put a resistor in series between the caps and bridge, but nothing better - it just fails in about a month. Bad PSU, Bad PSU :o(


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