Re: C-Bus and Wire-wound transformers - HELP

["David Buckley" <db@xxxxxxx>]

--- In ukha_d@xxxxxxx, Bruno Prior <bruno@r...> wrote:
> I'm using C-Bus for control and have gone for the 2A per circuit
> dimmers, so they ought to be able to handle a 300VA load just
fine. But
> the wire-wound transformers make the most god-awful racket when
the
> circuit is dimmed

Yes, they will.  With standard dimmers (ie triac based) there is a
nasty inrush current at the point when the triac turns on at 100
times a second, and thats where the buzz comes from.  Its the
mechanical components of the transformer getting a mechanical shock.

The only guaranteed way to reduce transformer buzz (and harmonic
nasties introduced into the phones and hi-fi at the same time) with
conventional triac dimmers is effective control of risetime.

The way this is usually achieved is by using an inductor (a choke)
in the dimmer.  The risetime of a dimmer is almost a quality
measure; the very best dimmers have many hundreds of microseconds of
risetime.  Lesser dimmers have very short risetimes.  Laws of
physics come into play here; a long ristime needs a big
(electrically) choke, which is (physically) large and heavy.  This
is why "professional" dimmers are big and heavy.  Have a butchers
at
http://www.mode-lighting.co.uk/Products/Architectural.htm
for a
compromise example.

Some companies (such as Lutron, with their Lamp Debuzzing Coil, see
http://www.ylighting.com/ln-ldc-10-tcp.html,
dont know if there is
240v version) sell an add-on gizmo which is effectively a choke in a
box.  The choke increases the risetime, which is good.  But what
this will do is to transfer the noise from the transformer (which
will be quieter) to the choke (which will now make the racket)

Not good.

There are only a small number of solutions to this problem.

a) an appropriate electronic transformer.  But I would caution that
having two six meter antennas a few inches apart with the best part
of 20A flowing being switched through them at something over 20KHz
may cause interference to sensitive electonics...

b) Use different dimnming technology. Sinewave dimming.  s'not
cheap, but is silent.  Will also need to be integrated into your
CBus system, which may or may not prove a challenge.

c) Go DIY - for the adventurous only - even better than sine wave
dimming for genuine magnetic transformers is to use a variac.  The
motor driven variac and controller will set you back about $1000 new
(www.variac.com), but this stuff often apears on the s/h market for
very little money.....   When I was a kid I had variac dimmed lights
in my bedroom, using meccano components to add the control
capability!