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'[OT maybe] Zero Crossover Light Dimmer Impossible?'
1998\04\17@153530 by Montaigne, Mike

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       I wanted to build a PIC controlled zero crossover lighting
dimmer, for our church and briefly discussed this a few months ago.  I
was using a 555 timer triggered on the zero crossover to get my delay.
The problem was I wanted 0-10 volt remote control so I could locate my
controls in the balcony and the power unit under the stage.  (The other
problem is, we have no money).

       The standard way of dimming lights is to turn on a triac some
time after the zero crossover of the ac waveform.  The problem with this
is, the triac turning on causes rf noise (which you then have to filter
with an inductor and capacitor).  By the way, the TEA1007 chip does a
real nice job for this.  See the Vellem K8003 DC controlled dimmer kit,
about $25 (change triac to 25 amp & add larger current inductor).

       I then thought, using a PIC I could hit the light with (only
one) 1/2 sine wave, say once per second, and increase the number of 1/2
sine waves every second until the light was full on.  I could then send
(serial 422 from a pc) a light number and a count of 0-256 to tell the
PIC how dim or bright.

       I tried this with a zero crossover triac and a signal generator
that I could vary the pulse period and the frequency and found that even
when I had 1/2 cycle every say 10 seconds, you could still see the flash
of the filament on a 100 watt bulb.  I know that a larger bulb, say 1000
watt would not respond as quickly, but I am trying to dim 1500 watts of
xmas tree bulbs.

       (Other than a variac) is there any (cheap) way of zero crossover
dimming for this project - am I missing something very basic?  I don't
mind using the TEA1007 phase shift solution, but I thought industry
lighting control for theater, used zero crossover, and I was trying to
understand how it worked.

tks


Mike Montaigne
Atomic Energy of Canada Ltd.
Station 18, Chalk River, Ontario
K0J 1J0, Canada

Phone (613) 584-3311 Ex. 4005
Fax:     (613) 584-4040
email:   spam_OUTmontaignemTakeThisOuTspamaecl.ca

1998\04\17@173528 by Chris Eddy

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Per your second question (from the other Email) I eagerly design products
for people all over the western PA area.  My claim to fame is re-developing
products that have become aged and parts are getting hard to find.  By this
point in a products life, the original engineers are long gone.  But in
general, I design electronics with specialties in processor design (Real big
on the PIC obviously), along with skills in analog electronics, switch mode
circuit design, and general product design.  Meanwhile, I am designing a
product or two in the background for eventual sale to the market.  Aren't
all consultants?  Three kids and a wife round out my life to a fast paced
clip.

Of course, it is only polite if you tell me what you do.

Montaigne, Mike wrote:

>         I wanted to build a PIC controlled zero crossover lighting
> dimmer, for our church and briefly discussed this a few months ago.  I
> was using a 555 timer triggered on the zero crossover to get my delay.
> The problem was I wanted 0-10 volt remote control so I could locate my
> <etcetera, etcetera>

         May I ask, tell me something about what sort of stuff you do?

>  Mike Montaigne
>  Atomic Energy of Canada Ltd.
>  Station 18, Chalk River, Ontario
>  K0J 1J0, Canada

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1998\04\17@184846 by peter

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part 0 508 bytes
Here is an alternative way of doing this, its also opto isolated.
You drive the led in the opto with a pmw signal ( of around 1kHz
should do ) you need no zero crossing with this system

Depending on your load you may need to use different transistors
ie: a TIPL 763 should be good for 1kw


Peter Cousens
email: .....peterKILLspamspam@spam@cousens.her.forthnet.gr  phone: + 3081 324450, 380534
snailmail:  Folia, Agia Fotini, Karteros, Heraklion  Crete, Greece.

Attachment converted: wonderland:power.zip (pZIP/pZIP) (0000484A)

1998\04\17@194239 by Harold Hallikainen

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On Fri, 17 Apr 1998 15:32:40 -0400 "Montaigne, Mike" <montaignemspamKILLspamAECL.CA>
writes:
>        I wanted to build a PIC controlled zero crossover lighting
>dimmer, for our church and briefly discussed this a few months ago.  I
>was using a 555 timer triggered on the zero crossover to get my delay.
>The problem was I wanted 0-10 volt remote control so I could locate my
>controls in the balcony and the power unit under the stage.  (The
>other
>problem is, we have no money).


       A standard analog method of generating phase control is to
generate a negative going ramp that is reset to +10 volts at each AC line
zerocrossing.  You can then use analog comparators to compare the control
voltage with the ramp.  If the control voltage is greater than or equal
to the ramp voltage, turn on the triac.
       This technique is used in lots of theatrical dimmers.  They use a
separate ramp for each power line phase and have a comparator for each
dimmer output.

{Quote hidden}

       Yes, the turn-on can cause singing of the lamp filaments.  The
rise-time can be delayed either by adding a series inductor or by using a
slow turn on device.  Some manufacturers are using a FET with a slow turn
on, but this is substantially less efficient than using a triac (which
has a lower voltage drop than a pair of FETs and has either high voltage
across it with no current or low voltage across it with high current,
both resulting in relatively low dissipation).

{Quote hidden}

       Your're right.  The flicker is going to be horrendous.  I think
you're going to have to stick with phase control.

>
>        (Other than a variac) is there any (cheap) way of zero
>crossover
>dimming for this project - am I missing something very basic?  I don't
>mind using the TEA1007 phase shift solution, but I thought industry
>lighting control for theater, used zero crossover, and I was trying to
>understand how it worked.


       The lighting industry (at least where I work
http://www.dovesystems.com) uses phase controlled triacs (or solid state
relays) with a series choke to limit rise time.  Again, some use FETs or
IGBTs, still switching at 60 (or 50) Hz, but with a slower rise time,
making the choke unnecessary, but suffering low efficiency.  I've looked
at the possibility of high frequency switching of the AC to give a "solid
state variac", but costs are high (four FETs and associated drivers,
snubbers, and output filters per channel) and efficeincy is lower than
the same circuit with a triac (due to required diode drops, FET or IGBT
saturation voltage, etc.).
       Besides the analog comparator technique discussed above, we also
receive DMX512 and generate phase control all in a single 16c74a (see the
Shoebox dimmer at dovesystems).


Harold

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1998\04\18@072904 by peter

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Peter Cousens wrote:

SNIP



Sorry another late night slip

It was meant to go private mail

Peter Cousens
email: .....peterKILLspamspam.....cousens.her.forthnet.gr  phone: + 3081 324450, 380534
snailmail:  Folia, Agia Fotini, Karteros, Heraklion  Crete, Greece.

1998\04\18@134221 by R. & C. Gipson

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i would be interested in looking at your 0-10v scr firing circuit.


thanks,

roger

1998\04\20@141717 by Joe Little

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    I'm in the midst of a similar light dimmer project (110V, 500W, Phase
    delay triac).  I'm in a quandry over the value of the series choke.
    They're either too big and hog board space, too small (seem to have no
    effect), or too expensive.  Best so far is a home wound 35 turn on a
    surplus torroid core (From http://www.bgmicro.com).  I have no specs on the
    core and 35 turns is downright painful on the fingers. Anyone have any
    rules of thumb on how much filtering is enough???  Some vendors spec
    by the milliHenry, others publish current/rise time specs only.  Any
    suggested sources for parts?
    Joe (Atlanta)



    Big Snip----------
     lighting industry (at least where I work
    http://www.dovesystems.com) uses phase controlled triacs (or solid
    state relays) with a   >>>>>>>>>>>series choke<<<<<<<<<    to limit
    rise time.  Again, some use FETs or IGBTs, still switching at 60 (or
    50) Hz, but with a slower rise time, making the choke unnecessary, but
    suffering low efficiency.

1998\04\20@193945 by paulb

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Harold Hallikainen wrote, quoting Mike Montaigne:

>> I tried this ... and found that even when I had 1/2 cycle every say
>> 10 seconds, you could still see the flash of the filament

> Your're right.  The flicker is going to be horrendous.  I think you're
> going to have to stick with phase control.

 This is exactly what might be predicted.  Obviously you (Mike) have
never tried experimenting with transmitting sound using a common torch
bulb.  Quite surprisingly, it really does work, and up to quite a few
hundred hertz, perhaps even a thousand or so.

 OTOH, you are going to be amazed how short you can make your lamp life
your way (pulsing them at full voltage from cold) due to inrush current.

 One of the most useful aspects of phase control is the ability to set
a minimum or "start up" brightness level where lamps are pulsed with the
last part of the cycle only, say 10 volts, which limits the current
*even* with a cold filament to no more than the nominal bulb rating.  At
this power level you can barely see the filaments glow, yet they will
have raised their resistance to most of its full value.

 Of course, using a PIC controller, you may implement either a minimum
power level, an obligatory "warm-up" time during which the phase is
restricted to this level, or a "smart" algorithm which performs both;
i.e., keeps bulbs warm for a preset period since last use during which
time they may be brought to (near- or) full- brightness immediately,
after which they will be shut off and when next commanded on, will be
limited to the warm-up level for the pre-determined period.

>> is there any (cheap) way of zero crossover dimming for this project -
>> am I missing something very basic?

 In a sentence; Yes, you do *not* use zero-crossing for any device that
doesn't like being repeatedly turned on and off.

> The lighting industry (at least where I work
http://www.dovesystems.com) uses phase controlled triacs with a series
choke to limit rise time.

 I'd pay heed to Harold; to those that have done it before.  They would
be using the cheaper way already *if it worked*!  The use of series
inductors is most elegant, they should be selected for a rise time of a
millisecond or so in combination with the load resistance to be
controlled.

 There should also be a snubber capacitor, *rated for the load current*
across the combination of Triac and choke inductor where it interrupts
the load circuit.  Note that 1} there must *not* be a capacitor directly
across the Triac. 2} The inductor actually protects the Triac from
surges (such as that due to the snubber capacitor), keeping it within
its current ratings. 3} As the thyristor switches itself off at near-
zero current, negligible "kickback" is generated by the inductor.  If a
much smaller capacitor is provided to absorb this, it has a series
resistor to limit surges. 4} Most interference generated by the Triacs,
if they, the inductors and snubber caps are in a shielded enclosure,
will be at audio frequencies.  It is picked up by microphone cables and
coupling transformers by mutual induction from the mains wiring, and is
more audible simply because the higher frequencies are coupled more
efficiently.  It may also be picked up through un-shielded power
transformers in the audio equipment.  Most importantly, it can *NOT* be
removed by bigger choke inductors in the dimmer, but only by a well-
designed and installed audio system.

 Do not be told otherwise!

 Cheers,
       Paul B.

1998\04\21@111521 by Harold M Hallikainen

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       It's kinda tricky.  Ideally the choke would be rated in mH with a
maximum current limit (to avoid core saturation).  However, core
saturation doesn't seem to be a real concern here.  The chokes for our
dimmers were selected before I got there, so I have not seen the design
criteria.  The ones we are using definitely could not be board mounted.
They're too big.  Ideally, you could do a simple time constant
calculation (T=L/R), but this assumes constant L (which actually varies
as the current varies due to core nonlinearity).  A higher power lamp
actually requires less inductance since the load resistance is less.
However, you need a larger wire size and sitll have so many ampere-turns
to deal with in terms of core saturation.
       You might want to call Lyle Nighwonger at Clearwater Technology
magnetics division in San Luis Obispoi, CA (area code 805).

Harold


On Mon, 20 Apr 1998 11:42:00 -0500 Joe Little <EraseMEJoe.Littlespam_OUTspamTakeThisOuTSCIATL.COM>
writes:
{Quote hidden}

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