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'[PICLIST] [pic] Phase control dimming on a cheap p'
2000\12\29@065703 by Richard Malcolm-Smith

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Im sure this has been asked before, but I cat find it.

Now that I have some time off work ive decided its time to make a decent
controller for the xmas lights next year.

Has anyone made something along these lines with a pic? I was hoping for
a 16f84 as I have a couple and they are easy to get, but I would get a
"Bigger" one if needed. Initially I would want to get somethign to
replace the dinky little controler box on the lights so they were all in
time with each other. My problem is I understand what is required (As
far as triacs etc) but I dont have a _clue_ where to begin coding the
thing.

Has anyone got any web resources dealing with the idea of multichannel
dimming off a pic?

Also, what is the best way to do the internal routine to fade the lights
in and out etc?

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2000\12\29@080217 by Jinx

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Each channel or group of lights has a routine to delay triggering
of the triac by whatever times you want. Perhaps use a main
counter, and split off to sub-routines (at various values of the
main counter) to alter the triggering of the triac

Either drop whole cycles (more for dimming down, less for lighting
up) or alter the point in the cycle at which you turn the triac off/on.
For best electrically noiseless results, turn the triac off/on at the
zero-crossing point

If using another method, (varying the turn-off/on point) then you'll
have to detect the zero-crossing point of the supply voltage and
take your timing from that. Note that the 0 and 1 states of an input
pin aren't symmetrical about 2.5V. For low wattage bulbs this
might not be a problem, but for heavier loads it could mean more
EMR. Use a scope to compare the actual zero-crossing point of
the input wave to the moment your s/w would turn the triac on/off
or calculate the time delays needed to synch input to ouput

I believe that the most stressful time to turn a bulb on is at peak
voltage. Cold filament - high voltage - more failures

There are details in the archives (around November 99) about this
subject, including safe ways to connect mains-derived voltage to
PIC pins

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'[PICLIST] [PIC] Phase control dimming on a cheap p'
2001\01\02@144751 by M. Adam Davis
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Jinx wrote:
> Either drop whole cycles (more for dimming down, less for lighting
> up) or alter the point in the cycle at which you turn the triac off/on.
> For best electrically noiseless results, turn the triac off/on at the
> zero-crossing point

You can go ahead and skip the idea of selectively turning on half cycles.
This results in visible flickering if you skip more than every other one.
It works for heating elements, but even incandescent lamps are fairly
responsive.

For a brief phase controlled dimming tutorial, read
http://ubasics.com/adam/electronics/doc/phasecon.shtml

It includes very simple dimming code for the 'f84.

-Adam

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2001\01\02@153328 by Bob Ammerman

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----- Original Message -----
From: M. Adam Davis <spam_OUTadavisTakeThisOuTspamUBASICS.COM>
To: <.....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU>
Sent: Tuesday, January 02, 2001 2:48 PM
Subject: Re: [PIC] Phase control dimming on a cheap pic


{Quote hidden}

For stage or home lighting this is certainly true.

But for holiday lights a little flicker can simply be taken as 'twinkling',
and the lower EMI and audible noise of zero-cross-switching make a lot of
sense. I'm guessing that heatsink and semiconductor costs can also go way
down.

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

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2001\01\02@153957 by Chris Eddy

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Fellas, I think that the best way to do this without too much fuss is the
following:

Pickup a zero cross somewhere.  I did one recently where the power supply had
a bridge rectifier, in a traditional manner, but the point on the bridge that
goes to the plus on the capacitor has an additional 1n4001 in series with
it.  That way, the point between the bridge and the diode has a rectified
sine wave shape.  I divide this down a little, buffer with an opamp, and run
it into the A/D converter.  The wave is actually more like a trapezoid, since
the bridge and cap act as a peak detector.  Nevertheless, if you convert the
input fast enough, and look for the point where the voltage is nearing zero
and turning around, then just do a comparator thing in math.  Once you know
that, then clear down a counter.

Meanwhile, in the regular timer interrupt, increment this timer.  What you
then get is a triangle wave shape in your counter, which resets on every zero
cross.  Then you simply compare the phase angle against the count, which will
need to be scaled so that the math compares for 100% on, and you have a phase
angle controller.

I just did this not for phase angle, but just to turn on and off on the
cross, to keep EMI down, as previously mentioned.

Mind you this is a voltage cross method.  This does not necessarily help with
current crossing, as a capacitive or inductive load will be out of phase.
The Key to Triac survival is in limitting the DI/DT, which may or may not be
helped by the voltage cross method.  Hmmm..


> Jinx wrote:
> > Either drop whole cycles (more for dimming down, less for lighting
> > up) or alter the point in the cycle at which you turn the triac off/on.
> > For best electrically noiseless results, turn the triac off/on at the
> > zero-crossing point

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