Re: Controlling Holiday Lights

[Marc_F_Hult <MFHult@xxxxxxxxxxxxxxxxxxxxx>]

On Fri, 3 Nov 2006 16:05:12 -0700, sylvan butler
<ZsdbUse1+noZs_0611@xxxxxxxxxxxxxxxxxxxxx> wrote in message
<slrneknip8.p3a.ZsdbUse1+noZs_0611@xxxxxxxxxxxxxxx>:

>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.

There you go again ;-)

Powering lamps designed for 120VAC with rectified 170 VDC (= 170 VRMS) as you
suggest is problematic because if the circuit were to ever stop chopping, the
lamp would burn out right quick.

So when MOSFETs are used as the dimming control elements for lamps with an AC
power source, the lamps are powered by AC, *not* DC. (and the circuit is
elegant, not "ugly", in my personal opinion).

It isn't any harder to do it right; it requires the essentially the same
components as what you suggest and so is not intrinsically "more expensive" as
you claim.


... Marc
Marc_F_Hult
www.ECOntrol.org