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Re: AC dimmer software development question
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Many years ago, I tried a "poor mans' dimmer" by switching a diode in series
with a lamp, giving me off, 50% and 100%. I could clearly see the lamp
flicker when running on only one half of the sinewave. So imagine what
happens if you want to dim to lower intensity.

Meindert



Re: AC dimmer software development question
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... snip about selecting integral half cycles ...
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There was a time when backward areas of the world (such as
Toronto) used 25 Hz power.  The more sensitive could detect the
light flickering, especially on fluoroscents.

--
Chuck F ( snipped-for-privacy@yahoo.com) ( snipped-for-privacy@worldnet.att.net)
   Available for consulting/temporary embedded and systems.
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Re: AC dimmer software development question

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Right. I discovered this by accident. I was measuring the output of a
photovoltaic array with an oscilloscope and was observing annoying ripple
on the trace at 120 hz. it finally dawned on me that I was exciting the
solar cells with the flourescent lights in my shop. Doh!

Bob

Re: AC dimmer software development question

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Not much, there isn't. 10% power would require driving the lamp with
one out of ten half-cycles, and if you can't see flicker at 12Hz (or
10Hz where I live), then I'd strongly advise an eye test ;o)

The other problem is the somewhat dire effect on bulb life.The lamp
filament would cool significantly during the 9 unpowered half-cycles.
I'd hate to think what the lifetime of a a typical stage or
photographic lamp would be.

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But you can't GET fine adjustment with burst controllers without
significant flicker. If you disagree, please show some code to
illustrate.

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Can you point to one? All the burst controllers I've seen have been
for heating use only, and with very limited resolution.

And in any case, RFI simply isn't an issue, since filtering to any
required standard is simple and relatively cheap to achieve (phase
control messes up the power factor, of course, but that can be
corrected as well). As an example, the harmonic radiation of a typical
10kVA stage dimmer pack in a touring rack is to all intents and
purposes indetectable from more than a couple of metres away (<1dB
S/N) - and the better ones are also well screened magnetically.

In really sensitive radiation environments (operating suites,
electronics labs, munitions stores etc.), it's really best not to use
AC at all, but make sure to use a good ground shield on all cables if
you must.

--
  Max

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Some professional dimmers use a inverter to generate higher frequencies for
the AC drive for the lamp. Then burst mode control is possible, but as you
say I have certainly never seen a commercial dimmer using burst mode.

Cheers

Klaus



Re: AC dimmer software development question

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In addition to flicker, half-cycle switching sometimes can cause
audible vibration of the filament and its supports, very irritating
when it occurs.

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Cheap dimmers often do not filter RFI very well, and can be a major
source of interference to X10 communication when the switching point
falls within the signaling interval.

-- Jim McGinnis

Re: AC dimmer software development question
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My understanding is that this can happen with partial wave dimming as
well as the chokes that are used.

http://www.epanorama.net/documents/lights/lightdimmer.html#buzzing


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--

Rick "rickman" Collins

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Re: AC dimmer software development question


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Interesting document - you should read the part beginning "Other not
so good ideas for dimming".

-- Jim McGinnis

Re: AC dimmer software development question
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I did.  "unless the dimmer-driver can do some sort of dithering to
spread out the flicker spectrum".  Wouldn't that be simple for an MCU?  

--

Rick "rickman" Collins

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Re: AC dimmer software development question
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I don't think you could see flicker at 10% power and an eye test has
nothing to do with it.  Try it and you will see what I mean.  Or just
give it some thought.  


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On 6   6   6   5   6   6   6   5
Off  6   6   6   6   6   6   6

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I have never seen a cheap dimmer that did not generate RFI.  I checked
some references on the web and they indicate that RFI can be a problem.
There are also issues with harmonics that affect the power input.  

http://www.epanorama.net/documents/lights/lightdimmer.html#rfi


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--

Rick "rickman" Collins

snipped-for-privacy@XYarius.com
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Re: AC dimmer software development question

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The flicker fusion frequency varies greatly depending on the
individual, surrounding lighting and other factors. Peripheral vision
is more sensitive than central vision, and fatigue may increase
flicker perception. Even when flicker is not consciously perceived it
can have measurable negative effects.


-- Jim McGinnis

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You still have not considered the whole problem.  I can guarantee that
no one will see flicker when a 100 Watt bulb is fed 10 Watts of power
and they will not suffer any negative effects, regardless of the
frequency.  

--

Rick "rickman" Collins

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Re: AC dimmer software development question

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I assume you're implying that 10% power would result in no light
output - just heat. While that's probably true of the average domestic
lamp (particularly if only being fed from a feeble 115Vrms), it
certainly isn't true for other lamp types with low thermal mass and
higher efficiency. I have actually tried feeding 1 in 10 half-cycles
to a 2500W CP91 lamp (bog-standard for stage lighting in most West-End
theatres) as a possible method of pre-heating. The lamp gave
significant light ouput, with very clear flicker at up to about 14Hz
or so (1/7 @ 100Hz, 230V). I measured the efficiency at only about 4%
IIRC, as compared to the 20% or so at full power, but a mere 0.4% of
the full light output of a lamp like that is still a lot of lux,
especially in a darkened theatre!

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Will result in noticeable flicker with precision lamp types,
particularly at higher power levels.

Oh, and this technique is liable to generate quite large DC components
in the feed if you're not very careful, which should trip all the
breakers in any professional installation. Trying to keep a 3-phase
supply anywhere near balanced would be an absolute nightmare.

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I'd say that's simply a problem of inadequate legislation where you
live. European EMC regulations just don't allow the really cheap and
nasty designs onto the market.

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RFI is only a problem if you don't design it out. One ferrite and a
couple of caps is really all that's needed for a domestic dimmer.

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Dimmer packs for serious applications inevitably provide power-factor
correction, which solves that issue.

--
  Max

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