Newsgroups: sci.electronics.design,sci.electronics.components,sci.electronics.misc
Subject: Re: Touch dimmer circuit
References:  <[email protected]>  
John Woodgate  writes:

> , Tomi Holger Engdahl
>  inimitably wrote:
> >That's true. Conventional 12V transformers with halogens wired
> >to them will dim quite nicley with a dimmer which can handle inductive
> >loads.
> 
> Please explain why the dimmer must handle an inductive load. The
> resistance of the lamp(s), reflected through the transformer must
> dominate the impedance seen by the dimmer. Until all the lamps have
> failed, anyway!

You are right that the resistance of the lamp(s), reflected through the
transformer dominate the impedance seen by the dimmer. But in practical
non-ideal transformers there is always some part of the inductance
of the transformer itself seen as addition to the lamp(s) resistance.
And the cheap transformers used in many lighting applications are
anythign but ideal.

Some poor dimmer designs work well only with purely resistive loads.
When some inductance is added to the load, they can start acting strangely
at some settings. I have played with different dimmers circuits and
seen in practice that not all dimmers designed for resistive loads
will work acceptably when connected to 12V halogen lamp system
powered with traditional transformers. The dimmers designed for
inductive loads or at least withstand some inductive loads will work
without any problems. 

For example the dimmers implemented as "Very basic circuit" shown at
http://www.hut.fi/Misc/Electronics/docs/lights/lightdimmer.html#120v
do not work very well with transformers. This kind of circuits are
generally wired in series with the load, so the transformer
inductance etc. can disturb the timing of the tiggering
(can cause flickering on some dimmer settings with some loads) 
and are not particarly symmetric in their operation (can cause
some DC bias to AC which can in very bad cases cause partial
saturation of halogen system transformer). I have noticed
with some dimmers (one commercial) very poor operation
and even overheating when connected to halogen lamps with
12V normal transformer!

Better designed dimmers ment also for inductive loads perform
better in this case and can be safely used with halogen
systems powered with transformers. It is a good idea to get
a dimmer which is designed to handle well at least some
amount if inductive load, so it can be safely used and
performs well.

I have colelcted some technical information on light dimmers,
light dimmer circuits and information how to dim different loads
to http://www.hut.fi/Misc/Electronics/docs/lights/lightdimmer.html

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