Re: Re: Dimmers - noisy when off (CBus)

["Paul Robinson" <ukcueman@xxxxxxxxxxx>]

Thanks Rob & David.

I think I'll get a couple of those osram units to play with. Good to know
this is not
entirely abnormal :-)

(and yes, these are electronic dimmers, made by Aurora)


----- Original Message -----
From: "David Buckley" <db@xxxxxxx>
To: <ukha_d@xxxxxxx>
Sent: Sunday, October 03, 2004 7:59 PM
Subject: [ukha_d] Re: Dimmers - noisy when off (CBus)


>
>
> --- 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.