Newsgroups: rec.arts.theatre.stagecraft
Subject: Re: Dimmer Packs and light boards
References:  
"Timothy Creswick"  writes:

> Could build a system for dimming / on-off quite easily.
> Just need some opto-isolators and triacs to get the job done.  If you're not
> proficient with electronics, I recommend that you don't try this (120/240v
> has the potential to really screw things up).  If you are interested, run a
> search on google or similar for "dimmer circuit" or "triac dimmer circuit".

You can find a document written my me on light dimmer and dimmer circuits at
http://www.epanorama.net/documents/lights/lightdimmer.html

> Depending on how many channels you want, you may as well construct the
> control system as a line for every channel, rather than digitally
> manipulating the data with multiplexers.  This would work by run a 10-core
> cable between the controller and the powersupply.  One pole would be the 0v,
> or 10v or -10v or whatever you choose, the other 9 would be variable for
> each channel.  Obviously, if you want more channels, use more control
> cables.  It is critical with this setup that you COMPLETELY isolate the
> control circuit from the mains voltage for obvious safety reasons.  

That's critical.

> If you
> only want on/off capabilities, use opto-isolators (uses optics to form a
> high-voltage relay which physically isolates both circuits).  if you want
> dimming capability (it's not much more work), then you need TRIACs, and
> check them for safety.  

A small opto-isolator is not ehough for switching. Optoisolators
are generally low output current devices not directly suitable
for driving any mains load. Optoisolators are suitable for
driving TRIAC which then controls the load.

A simple component to control light on/off is a relay. 
If you do not need to swithc often, then elctromechanical
relay is the cheapest alternative. If you need to switch
often or for some other reason, then select semiconductor
relay (SSR). Those semicondictor relays are basically
just an optocoupler, TRIAC and a zero-cross detector
integrated as one components (inside on package). 
You can find more information on SSR:s (ncluding how to make
your own using TRIAC and optocoupler) at my document published at
http://www.epanorama.net/circuits/semiconductor_relays.html

> They will cost you a lot more though.  You will also
> need to think about dissapating the heat to make the unit safe.  I recommend
> that you have a casing made, or that you buy a ready made housing for this
> project and you have it tested for electrical safety (insurance/litigation
> etc) so that you can't be held responsible for any accident.
> 
> Hope this helps,
> 
> Tim
> 
> 

-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/