Newsgroups: rec.arts.theatre.stagecraft
Subject: Re: Dimmer Packs and light boards
References:     
[email protected] writes:

> Tomi Holger Engdahl  wrote:
> > In reverse phase dimming in the start of the phase the
> > conducting starts at zero crossing (zero voltage and zero current)
> > which means no EMI here.
> >
> > The turn-off is in the middle of phase. Here the dV/dT and dI/dT
> > can be limited in several ways.
> 
> Please pardon me if I'm being a bit dense here, but I'm still a little
> confused...  Can you explain how the rates of change can be dealt with
> different in the reverse situation than in the traditional situation?
> 
> To me, a waveform ramping *up* from zero to a given voltage (at whatever
> slew rate the chokes will allow), should product as much EMI as ramping
> *down* from that same voltage to zero.  It's still a change in voltage over
> a fixed time period.  Is the current waveform somehow different?

If you build reverse pahse dimmer using for example FETs, you can play
with the option of using "slow turn off", which makes he change
of voltage and current "slower than immediate". The change speed
of voltage and currents can be limited with suitable capacitors and/or
inductors in the right place in the circuit. In SCR based dimmer,
the limitation could practically be done with a large coil,
with FET based reverse phase dimmer there are also other alternatives
for this.
 
> True there's no EMI on the zero crossing at the start of the waveform, but
> the same situation applies to traditional dimming at the trailing edge.
> 
> Clearly it works, because Rosco-ET, and others do it quite effectively, I'm
> just trying to understand way...

Hopefully my description and material linked to 
http://www.epanorama.net/links/lights.html
is enough to fullfill your need of information on this.

> I also found the true sine-wave dimming someome mentioned interesting.  Any
> idea how that works?  

My quess that they use a special switch mode regulation circuit.
Basically you could think the operation like a PWM circuit operating
at few tens of kHz frequency (the frequency is a quess only).
This PBM circuit chops the incoming AC on and off that the rate and
pulse the duty factor defined in PWM input signal. Then the circuit
has LC based circuits for filtering / energy storage of one PWM cycle.
This gives quite clear sinewave out with varying voltage.
That's how I have understood true sine-wave dimming. I have not
seen any practical circuit examples on this and I have not been
able to use/test any such product (you really understand electronics
thing when you can play with the device, measure it and can see the
circuit diagram of it).

> Do they run the output semiconductors in the linear
> region (similar to a class A or A/B audio amp.)?  That must produce a TON of
> heat if they do.

Definately not. This would product tons of heat. This kind of approach
would be almost as bad as ancient "series resistor based dimming".

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