Newsgroups: rec.arts.theatre.stagecraft
Subject: Re: An Odd Idea
References:  <[email protected]>  <[email protected]>
"Nigel Worsley"  writes:

> > the right people) that is ALL the front end control you need. Then use
> > solid state relays like opto 22 or crydom to control the loads. These
> > This is exactly what MOST major dimmer manufactures use for their
> > dimmer racks. BUT...this case we are not shitching them on -off 120
> > times a second, we are just turning them on constantly.
> 
> Just because it looks like an SSR doesn't mean that it is. Most of them
> will only switch on at the zero crossing, to reduce inrush currents in the load,
> and would be useless in a dimmer. 

That's true that most SSRs only switch on at the zero crossing. 
But there are also SSR that can activated at any angle. 
Most SSR manufacturers have both kind of SSR on their selection of 
product they offer. The ones that can be turned on at any angle 
can be used to make a dimmer, and those are used in real life 
to make different kind of phase control applications (light dimmers, 
AC motor controllers etc.)

> It is far more likely that a dimmer would
> use a module with a pair of thyristors ( or even IGBTs ) in it, with the isolation
> and drive circuitry provided externally.

All those appriaches are used. 

A pair of thyristors with external isolation was the traditional 
way to build dimmers originally, and still to my knowledge used 
on very high power dimmers. 

Most low to medium power dimmers nowadays use triacs as the 
switching component. Majority of light dimmers in use are 
built using triacs (used from those wall switch dimmers 
to theatrical dimmer racks). 

There are also some dimmer applications that use SSRs that 
can turn on the power at any range. This kind of SSRs are 
generally used in quite low power (up to few amperes per 
output current rating) applications. 

Some modern dimmers use other controllign elements like 
power MOSFETs and IGBTs to control the mains voltage. 
This kind of control elements have ability to turn on and 
off the power at any range. They can for example turn 
the power on at the zero crossing and stop the current 
flow smoothly in the middle of the phase, thus making 
a dimmer that does not need filtering coils to smooth 
the power output waveform.

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