Re: AC dimmer software development question

From: Klaus Vestergaard Kragelund (klauskvik_at_hotmail.com)
Date: 02/18/04 

Date: Wed, 18 Feb 2004 21:59:33 +0100

"Max" <mtj2@btopenworld.com> wrote in message
news:rre730pmr9jnjqpskm44uubgcstk4u4bel@4ax.com...
> On Wed, 18 Feb 2004 01:41:28 -0500, rickman wrote:
>
> >A lamp is excited by 120 Hz, not 60 (power waveform vs. voltage) and the
> >filament of a typical light bulb does not go out during cycles. So
> >there is considerable room for pulsing half cycles.
>
> 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.
>
> >The brightness of
> >an incandescent lamp is not linearly proportional to the power that is
> >applied, so you need fine adjustment, but not all the way to zero
> >power. Instead if you experiment, you will find that you can provide
> >full range dimming with a much smaller range of power.
>
> But you can't GET fine adjustment with burst controllers without
> significant flicker. If you disagree, please show some code to
> illustrate.
>
> >I have seen dimmers implemented this way when minimum RFI is important.
>
> 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.
>

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

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