Re: Dimmers - noisy when off (CBus)

["David Buckley" <db@xxxxxxxxxxx>]

--- In ukha_d@xxxxxxx, "Paul Robinson" <ukcueman@y...>
wrote:
> I've put a multimeter across this to see what I can see, and it
> shows that there's a voltage of about 46-50V on most of my
> channels when they are off, but only two of these are noisy. Is
> this normal? Is it a fault at the CBus end for there to be any
> voltage at all? Shouldn't off *really* be off?

When dealing with semiconductor dimmers in the general case, no, they
are never off.  Triacs leak, so even when the dimmer really is
"off",
there is still a leakage current.  Most dimmers have a minimum load
requirement, for little two wire dimmers its so that they have enough
power to function, as they get their operating current by
"robbing"
power from the load.  With big dimmers, its cos they use big triacs,
so they dont blow under short, and big triacs (as a rule) leak more
than little ones.

I'm guessing here you have electronic transformers, as 40 odd volts is
a fat too high a zero voltage for a genuine resitive or inductive load
on a non-two-wire or preheat .  So my guess is that there are
variances in both the cbus triacs, and the electronic transformers.
Temporarily put a real light bulb across the dimmer output, and I'm
certain the vo;ltage will drop to very near zero, and the whine will stop.

As to solutions - nothing is broken, so its a matter of substitution
to see if its dimmer channels, type of dimmer, or specimen of
transformer that results in a whine.  If you're lucky, it'll be the
tranny, and you can swap it out for one that hopefully doesnt whine.

Again, in the more general case, some dimmers have a "pre-heat"
setting, that modifies the zero point of the dimmer to be not zero,
usually variable up to a few percent.  This keeps lamp filaments warm,
and thus they come on quicker and stress free.  Little dimmers dont do
this, but many archetectural dimmers are settable.  Using preheat
obviously delivers real rpower to the load, not just leakage.