🚀 go-pugleaf

rando@skipper.dfrf.nasa.gov (Randy Brumbaugh) writes:
  
> 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?

I've been interested in digitally controlled light dimmers for a few years.
I worked for a New Zealand company a couple of years ago, and actually got 
around to MAKING one.  The unit we made was based on the 80C552 which is
a derivative of the 8051.  The first version was just 4 channels with
256 light levels.  The software I wrote allowed 3 phase operation, and
the plan was to make a 12 channel 3 phase unit, but it got vetoed by the
boss.

I had the microprocessor trigger the triacs directly, just as you are
thinking of.  I had a lookup table in EPROM consisting of 256 16 bit
values, where the 16 bit values were reloads for the timer counter,
and the 256 values were the desired brightness levels.  To tell you
the truth, we have only ever used one curve - linear brightness of the
lamp.  This is what the operators wanted and what the control desks put
out, so it is what we delivered.  All adjustment for the non-linearity
of the bulb and triac triggering time was done in the dimmer.
To generate the look-up table to produce the linear brightness curve
I wrote a flash basic program that worked out the amount of energy 
left in each half cycle at each point in time ('area under the curve')
and then allowed for the basic non-linearity of the bulbs - when you
give them just a small phase angle they glow red, but produce little
useful light.

The work we were doing needed 256 light levels.  You can get away with
64 and still have a professional product, as long as you don't intend
to 'Fade' the light - with just 64 levels, it seems to 'Jump' on slow
fades.  On 10 minute fades with 15 watt bulbs in a dark room near the
lower light levels, my boss noticed that my 256 levels 'Jumped' so I
had to redo my software for 1024 levels in the end (by that stage the
boss had also got me to change to a 68705 CPU because it was $1 cheaper
than the best 8051 price at the time).  Soon after that I left the
company because I felt the boss was wasting my time!

Anyway, I wouldn't advise people to try skipping half cycles to dim
lamps - unless the bulb has got a high thermal inertia, it looks
really bad.  All the professional dimmers have good, big torroids for
filtering, too.  A 500us time constant is fantastic - you can get a
good quality system with just 140us torroids.  We used to use EI type
chokes, but they are more than twice the weight and size.  Also, if
you want to drive halogens (through transformers usually) you have
to ensure both positive and negative half cycles have the same trigger
point - different by more than a couple of percent and your transformers
will be very unhappy indeed.

DMX-512 is neat, but the 250kbaud rate makes the software a mite tricky,
especially if you intend to be doing real time triac triggering as well!

 - Regards
   Steven Murray

-- 
Steven Murray
uunet!slxsys!stevem  stevem@specialix.co.uk
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