Newsgroups: rec.arts.theatre.stagecraft
Subject: Re: Lighting science
References: 
From: Tomi Holger Engdahl 
Date: 24 May 1999 22:13:54 +0300
Message-ID: 
Organization: Helsinki University of Technology, Finland
Lines: 98
X-Newsreader: Gnus v5.3/Emacs 19.34
"Frank Hebbert"  writes:
 
> Does anyone know of a good source of detailed info about phase, how dimmers
> work, 

I have written a web document on light dimmers and it is available at
http://www.hut.fi/Misc/Electronics/docs/lights/lightdimmer.html

You can find many more documents related this at
http://www.hut.fi/Misc/Electronics/lights.html#dimmer

> and similar physics behind lighting ?  

This one is taken from my document at
http://www.hut.fi/Misc/Electronics/docs/lights/lightdimmer.html

Incandescent lamp physics

A typical incandescent lamp take power and uses it to heat up a
filament until it will start to radiate light. In the process about
10% of the energy is converted to visible light. When the lamp is first
turned on, the resistance of the cold filament can be 29 times lower
than it's warm resistance. This characteristic is good in terms of
quick warmup times, but it means that 20 times the steady-state
current will be drawn for the first few milliseconds of operation. The
semiconductors, wiring, and fusing of the dimmer must be
designed with this inrush current in mind. 

Because lamp filament has a finite mass, it take some time (depending
on lamp size) to reach the operating temperature and give full
light output. This delay is perceived as a "lag", and limtis how
quicly effect lighting can be dimmed up. In theatrical application
those problems are reduced using preheat (small current flows through 
lamp to keep it warm when it is dimmed out). 

The ideal lamp would produce 50% light output at 50% power
input. Unfortunately, incandescents aren't even close that. Most
require at least 15% power to come on at all, and afterwards increase
in intensity at an exponential rate. 

To make thing even more complicated, the human eye perceives light
intensity as a sort of inverse-log curve. The relation of the the
phase control value (triac turn on delay after zero cross) and the
power applied to the light bulb is very non-linear. To get around
those problems, most theatrical light dimmer manufacturers incorporate
proprietary intensity curves in their control circuits to attempt to
make selected intensity more closely approximate perceived intensity. 


Some more documents can be found at
http://www.hut.fi/Misc/Electronics/lights.html#bulbs

> Most stuff I've seen seems to
> skip this 'difficult' stuff or gloss over it - but I want to know ! Either a
> web site or book recommendation would be great.

The page at http://www.hut.fi/Misc/Electronics/lights.html
is definately a good site to start.

> (For instance - why does a mirror ball (meant for an independent) put on a
> dimmer channel buzz continually, even when there are no channels up anywhere
> ?)

Many dimmers will put some small current out from their outputs
even when they are turned "completely off". This small leakage
current which can't put up any light to normal light bulbs
can make small transformers and electric motors buzz, even
some small strobolight to flash randomly if connected to
dimmer channel. You can get rid of this kind of poblems
typically by placing a small "phantom load" to the circuit,
like a small light bulb in parallel with the other device
connected to that dimmer channel.

The small leakage current is typically caused by the
filterign capacitors in the circuits. Typically
there is a snubber circuit in parallel with the triac
in soem circuit and it will pass some current even
then triac is off:

Basic snubber circuit between the the triac main current carrying
terminals (in parallel with the triac): 

          ||       ____
   -------||------|____|-------
          ||
        100 nF    150 ohm
        630V       1W 


Other possibility for the buzzing is that you have for some reason
enabled the preheat setting on the dirmmer. Preheat setting
means that the dimmer will pass a small power always to the
bulbs to keep them a little bit warm even when turned off
to redice the inrush current when lights are turned on again.
Some dimmers have this feature.

-- 
Tomi Engdahl (http://www.iki.fi/then/)