Newsgroups: alt.engineering.electrical
Subject: Re: variable transformer for light
References:    
"TimPerry"  writes:

> [email protected] wrote:
> > On Sun, 8 Oct 2006 11:29:19 -0400 TimPerry
> >  wrote:
> >> [email protected] wrote:
> >>> Does anyone make wall mount light dimmers of the variable
> >>> transformer type (as opposed to the usual type of on-off current
> >>> limiting)?  it would not need to support very many watts, so I
> >>> would be hoping such a think could be made to fit in a standard
> >>> wall switch box.
> >>>
> >>
> >> autoformers are made in all sizes
> >> http://www.variac.com/staco_Variable_Transformer_Map.htm .
> >>
> >> however a product for this application would have to be well fused
> >> as the user could decide to screw in a large lamp and overload the
> >> device.
> >>
> >> a solid state dimmer will be much less expensive and handle much
> >> larger loads for its size.
> >
> > Do you know of any that have a true sine wave output?
> >
> 
> it would be pretty wasteful and expensive to use for example a 100W
> amplifier to power a light bulb.

The new wave of sine wave output dimmers has taken a different
approach, using the switch-mode technology widely used in devices as
diverse as the ballasts of many discharge luminaires and the power
supplies of virtually all computers and lighting consoles. There are
few companies which work on sinewave dimming field.Dynalite has been
shipping the SVC (Sinewave Voltage Converter) dimmer since
1998. Bytecraft VST (Variable Sinewave Technology) won an award at
PLASA 98. Jands Electronics has announced the development of a
prototype of its SWDim sine wave dimmer. The principle behind all of
these sine wave dimmers is very simple: The incoming mains is switched
on and off between 600 and 1,000 times per mains cycle (30-50kHz) with
the on time (width) of each pulse being proportional to the required
output power from the load, a method known as Pulse Width Modulation
(PWM). The very finely chopped output is then filtered back into a
continuous waveform through an inductor similar to the choke on a
phase control dimmer, but very much smaller, as the pulse frequency is
higher. The shape of the filtered output waveform is almost identical
to the input waveform, only its amplitude is different, precisely what
happens in a resistance dimmer.

> while i have in the past run into cases where the RFI/EMI from a wall dimmer
> causes a buzz in audio equipment i find that most TRIAC dimmers these days
> employ zero crossover switching and RFI/EMI filters.

How can a TRIAC dimmers employ zero crossover switching ? 
A triac dimmer in order to operate needs to turn the triac 
on in the middle of the mains phase to do dimming (and then 
keep conducting to next zero crossing). RFI/EMI filters can 
reduct the noise caused by the quitc turn on. 

There are dimmers based on other semicondictors (power FETs etc.) 
that turn on ath the zero crossover and then more or less 
"slowly" turn off in the middle of the mains phase as needed. 
When properly implemented this type of dimmers cause less 
noise to mains then the old triac dimmers and need less filtering. 


>
> i dont know what you application is, however, you might find something here
> that you can adapt. http://www.littlite.com/main.php
> 
> 

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