Re: FW: FW: Fwd: Wall switch dimmers work with 12vhalogen?

[Keith Doxey <keith.doxey@xxxxxxx>]

On Thursday, March 18, 1999 9:21 AM, Nigel Orr
[SMTP:nigel.orr@xxxxxxx] wrote:
>
> This is where it all degenerates... for your purposes,
you
> can assume that
> a transformer marked as 'suitable for dimmers', with a
> halogen light
> attached to its secondary, will look almost exactly like
a
> 240V halogen
> light as far as a dimmer is concerned.  The switch will
> see the waveform
> just as the electricity supply company sent it to you,
and
> the dimmer
> should work just fine- in the unlikely even that it
> doesn't, then you
> should take the transformer back to where you bought it,
> with the dimmer,
> and get your money back!
>
> There can be a big problem using conventional dimmers
with
> inductive loads
> (eg fluorescent lights, motors), but a lighting
> transformer with lights
> attached is _not_ an inductive load.
>
>

Just to add my bit......

The main problem in trying to dim a transformer load comes
from the dimmer itself.
Poorly designed dimmers have a non symmetrical output where
the positive half cycles of the mains are not equal to the
negative half cycles. A transformer is basically a pair of
inductors. The natural behaviour of an inductor is to
oppose any change in the flow of electricity. With AC the
current is always changing so the inductive action is
always trying to oppose the change in current. When a
transformer is fed with non symmetrical AC from a badly
designed or faulty dimmer it causes the core of the
transformer to become saturated and it stops behaving like
an inductor. At this point it becomes purely a piece of
wire. The DC resistance of a transformer primary is
normally only a few ohms and thus causes a massive current
to flow through the dimmer until something goes POP :-((

A properly designed dimmer will handle highly inductive
loads with no problem as can be seen by all the neon
lighting used in nightclubs and TV shows etc. Sadly, a
"properly designed" dimmer usually means expensive. Typi
cally a 10 amp commercial dimmer will have a triac rated at
40 amps or more and be fitted with extremely fast acting
fuses or circuit breakers. The average household dimmer
rated at 2 or 3 amps will have a triac rated at 8 amps and
a fuse (if fitted) that blows so slowly the triac will be
destroyed before the fuse blows.

Keith


Keith Doxey
http://www.btinternet.com/~krazy.keith
Krazy Keith's World of DIY Home Automation


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