Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!elroy.jpl.nasa.gov!ames!ginger!rando
From: rando@ginger.dfrf.nasa.gov (Randy Brumbaugh)
Newsgroups: sci.electronics
Subject: Re: Light Dimmer
Summary: Misc info on light dimmers
Message-ID: <713@ginger.dfrf.nasa.gov>
Date: 28 Nov 90 00:52:17 GMT
References: <2570007@hppad.HP.COM> <3450@exodus.Eng.Sun.COM> <1990Nov27.201653.4865@hayes.ims.alaska.edu>
Organization: NASA Ames-Dryden FRF, Edwards, CA
Lines: 47


I also have done some work with light dimmers, mostly trying to 
fix them.  (This newsgroup was very helpful).  Anyway, with
all this talk of digitally controlled light dimmers, I thought
I would throw in a couple of things:

1)  There is a standard for serial, digital dimmer control,
 USITT (US Institute for Theatre Technology) DMX-512.
 It is a 250kbit/sec RS-422/485 serial link.  I have a copy and
 will elaborate if there is any interest.  A light board which
 uses this protocol will control most dimmers on the market, and 
 a dimmer can be controlled by most light boards.

2)  Consider the dimmer curves.  There are four, I think:
 - The control position vs digital output value (In the control
   board).  I think this is usually linear. (i.e. halfway up = 
   128 if the range is 0-255).
 
 - The digital input to the dimmer control circuit vs. the firing
   of the SCR/TRIAC.
 
 - The firing of the SCR vs. average voltage output.  The voltage 
   varies throughout the AC cycle, so this is not as straightforward
   as it might seem.

 - The average voltage to the lamp vs. light output.  This is 
   surprisingly non-linear.  Something like a 10% voltage drop
   gives a 50% light output decrease.  I can look up the exact
   curves if anybody is interested.

 ( I guess there is also light output vs. perceived brightness
   in the human eye, which is not linear.)

 There are several "standard" curves - "linear voltage", "linear
 light", "linear brightness", etc.  Some are preferred by stage, 
 some by TV.
  
 My point is, if you are building a dimmer to mate with an existing
 control board, you can only control the second curve.  Maybe 
 some kind of EPROM, table-look-up would be a good idea.  Maybe a 
 switch to select from 2-3 curves.  Maybe using a timer/counter
 directly happens to work out to a reasonable curve?

  Does anybody have any experience in this area?

  Randy Brumbaugh
  rando@skipper.dfrf.nasa.gov