Re: Controlling Holiday Lights

[sylvan butler <ZsdbUse1+noZs_0611@xxxxxxxxxxxxxxxxxxxxx>]

On Fri, 03 Nov 2006 21:44:31 GMT, Max <nospam@xxxxxxxxxxx> wrote:
> I assumed he was talking about some device other than a triac that
> could be switched at a much higher frequency on and off during any
> point in the ctcle.   I

That gets a bit ugly (and more expensive) if you intend to control
120vac.  (Of course, one could rectify the 120vac to create ca. 170vdc
and use MOSFETs...)

> imagine this would work to give you some lesser power out.  All you

Yup.

> would be doing is chopping the AC into really small chunks and they
> would average over the entire wave(s).

that's pretty much how it would work.  But it is a lot easier with D.C.
You would have to be really motivated to do it with A.C.

> I think the problem would be noise and heat.

Heat is not much of a problem compared to older dimmer technologies.
Switching on/off means the switch is either conducting (very low volt
drop hence power loss) or not conducting (essentially no leakage
current, thus no power loss).  I do have some 40amp SSRs that require a
pretty substantial heat sink for full power.  But you figure even a
small voltage drop at 40amps means a lot of waste power.  Running 20amps
and mounted to a metal box they don't get noticably warm.

noise, OTOH...  Electrical noise can be a problem if switching large
loads, inductive or capacitive loads, or attempting to pass fast pulses
thru long wires.  As marc pointed out, with triac switching 60hz A.C.,
it just isn't that fast.  However, if switching large loads you prefer
to not "dim" them, and to avoid electrical noise (and sometimes even
bigger problems) you usually sychronize the turn on and turn off at the
zerocrossing.  But even a typical home-use dimmer for a few hundred
watts will generate electrical noise.  The standard practice is to put
in a small choke (inductor) to quiet it down.

Of course, if you meant accoustic noise, well, mechanical relays don't
switch that fast either.  :)   More seriously, sometimes you can find a
resonant frequency of the filaments in the lights and hear a pretty good
hum.  In practice it doesn't happen very seriously, very often.

sdb

--
Wanted:  Omnibook 800 & accessories, cheap, working or not
sdbuse1 on mailhost bigfoot.com