Newsgroups: sci.electronics.design
Subject: Re: Need a schematic for a 12 VAC dimmer halogen lamps
References: <[email protected]> <[email protected]>
From: Tomi Holger Engdahl 
Organization: Helsinki University of Technology, Finland
Lines: 60
X-Newsreader: Gnus v5.3/Emacs 19.34
Date: 13 Jun 1999 23:45:01 +0300
Message-ID: 
"Ing. Franz Glaser"  writes:
> If you want to avoid using a mains transformer you could use a
> primary switched power supply, it has a much smaller transformer.

There this kind of devices for making 12V AC (not 50 Hz output)
for powering low voltage halogen lamps and those are typically
called in lighting industry as "electronic transformers".
Some of those products are deisgned so that you can connect
them after a traditional mains voltage light dimmer without
any problems (everythign works nicely). If the "electronic transformer"
is not desiged to work correctly when dimmed, then the result might
be unsatisfactory and can even cause damage to the 
"electronic transformer" electronics and/or light dimmer.

> You can also make the output of the power supply variable. It is not
> necessary to power the lamp with AC.

Some example circuit for dimming 12V halogens from 12V DC source:

http://ourworld.compuserve.com/homepages/Bill_Bowden/page6.htm#dimmer.gif
http://www.darkportal.com/cc/circuits/circ0270.htm
http://www.wenzel.com/pdffiles/dimmer.pdf
http://user.aol.com/cybercir/cir13.htm
http://dustbin.virtualave.net/circuits/dimmer.html
http://www.saunalahti.fi/~eeroalk/himmenni.htm

> But seriously, the principle of halogene lamps and their long life is
> the minimum heat of the filament. They need it to make the metal
> steam come back to the filament. So I would never try to dim such
> a lamp except with a black shield mechanically.

You can usually dim halogen lamps without much problems.
When the light is dimmed and the temerature is redices so much
that the halogen cycle does not work then the filament temperature
is so much reduced that the does not cause much problems
even though the metal steam is not brought back to filament.
In long run the halogen bulbs will blacken somewhat in this
kind of use, but tuning them to full for some time the black
dissapears quite quicly.

DImmed low voltage halogen lighting is used in quite many places 
(bars, reastaurants etc.) without much problems and sold/installed 
by many light companies, so there must not e much problem.

I have one dimmable 230V 300W halogen lamp at home and I use it
often at laf power or less and there as not been any problems
in it. No blackening problems because I soemtimes turn it to full
power when I need lots of light. Dimmin 230V halogen bulbs is
a standard practice in stage lighting for years and no serious
problems there either.

If you can withstand the fact that the bulb might blacken a bit
sometimes and this can be vsolved by turning the light to full power
for few minutes there is no problems in generally dimming halogen bulbs.

-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web pages at http://www.hut.fi/Misc/Electronics/