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'[PICLIST] theory question on phase control dimming'
2001\04\19@105500 by

Here is a question on phase control light dimming.  We use PIC's in a
lighting control system.  I have been told that dimmer manufacturers warn
against sharing neutrals in the electrical wiring if a dimmer is connected
on either phase.  Something about phase shifting of the sine wave.  My
understanding is that the sine wave is chopped and shut off to do the
dimming.  Can dimmers be used with a shared neutral, where each side is 180
degrees out of phase?

Also, does anyone out there have any cool PIC routines for dimming?

Mike

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"Michael C. Reid" wrote:
>
> Here is a question on phase control light dimming.  We use PIC's in a
> lighting control system.  I have been told that dimmer manufacturers warn
> against sharing neutrals in the electrical wiring if a dimmer is connected
> on either phase.  Something about phase shifting of the sine wave.  My
> understanding is that the sine wave is chopped and shut off to do the
> dimming.  Can dimmers be used with a shared neutral, where each side is 180
> degrees out of phase?
>
Curious statement as in a house you use a common neutral, so does that
mean if you put dimmers all over your house you going to have a problem?
However, if you use a 3 phase supply and neutral to spread the load, and
you use one PIC and one phase as trigger, yes, then you will have a
problem as 3 phases are 60 degrees out of phase from each other. You
must use a trigger for each phase.

Quentin

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You can get into trouble here because the load on one phase can shift the
apparent phase on the other side of the neutral due to current (and thus IR
loss) in the neutral conductor.

If your dimmers use a single phase reference for both phases they will get
confused by this.

I don't see how an independent dimmer on each phase can encounter this
problem.

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

{Original Message removed}
Yes, you can use a shared neutral on single phase 120/240 or three phase
Y systems. If the resistance of the neutral is too high, the switching on
one phase will cause a glitch on another phase which might false the zero
crossing detector. I have not had a problem with this, though.
Here are a couple dimming techniques we use in the Dove Systems products
at http://www.dovesystems.com .
In small dimmers (like our 4 channel "Shoebox" dimmer), the PIC does the
entire job. A pair of diodes on a center tap grounded power transformer
provide FW rectified AC. This is run thru a current limit resistor into
the INT pin on a 16c74. The INT is set to generate an interrupt on a
negative edge, which causes an interrupt a little before zero cross.
The capture/compare register is used to determine when to turn on the
triac for an individual channel. At zero cross, we quickly look thru the
four turn on times for the four channels (converted from incoming DMX and
analog levels to timer clicks 'til turn on thru a 16 bit lookup table).
The first one to be turned on is put in the compare register. When the
compare interrupt is generated, that triac gets turned on as well as any
other triac scheduled within 255 timer clicks. This prevents us from
losing one due to the time it takes to execute the ISR. The next turn on
time is determined and that value put in the compare register.
That's pretty much it. Other code is receiving DMX and putting 4
channels starting at the address on the thumbwheel switch in RAM. Other
the DMX to determine what each channel actually does. There's also code
allowing each channel to operate as a nondim.
The other approach we use is used on our larger dimmers (such as the
DM1224). The control card for the DM1224 can actually dim 24 channels on
single or three phase. Here we use a 16c74 to receive DMX and analog.
This data is run through a lookup table to give the dimming curve we
want. It is then sent to a single 12 bit D/A which is demuxed out to 24
channels. These 24 levels are compared to negative going linear ramps in
sync with the power line phases. When the demuxed level is above the
ramp, the solid state relay for that channel is turned on.
This is similar to standard analog dimmer techniques. Analog dimmers,
however, use additional circuitry to shape the curve of the negative ramp
so lamp brightness corresponds to the control voltage. We eliminated
those components, going to a linear ramp, and "predistort" the signal
compared with the ramp to yield the appropriate dimming curve.

Harold

On Thu, 19 Apr 2001 08:17:33 -0600 "Michael C. Reid"
<mikecreidQWEST.NET> writes:
{Quote hidden}

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

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Mike,

The danger with sharing neutrals between dimmers is one of wire guage.
Since you are chopping the power you cannot be assured of a ballanced load,
so the netural must be rated for all dimmers connected to it (not
completely true, but safe and easy to calculate). The mfg I used to do
field service for would require a netural for every hot on the lamp side of
the dimmer.

Electrol Eng. http://www.electrol.net has dimmers (dx series) that use pics for
control. And they do some fun things with them. (I know and like the guys
at electrol, but have no connections to the company.)

There is also an outfit in England who make some really fun things,
includeing programed microcontrolers for dimming. I can't remember name
right now but I will try to look it up soon.

feel free to email me directly, I have a fair bit of knowdlege of the stuff
out there and how to take it apart. From 0-10 vdc dimmers to top of the
line robotic lighting.  And will try to answer any questions

Sorry but I do not have the source code from electrol.

Greg
gdoylemail.com

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