Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!pilchuck!dataio!fnx!del
From: del@fnx.UUCP (Dag Erik Lindberg)
Newsgroups: sci.electronics
Subject: Re: light bulbs buzz when dimmed
Message-ID: <960@fnx.UUCP>
Date: 26 Apr 91 18:39:23 GMT
References: <7893@jhunix.HCF.JHU.EDU> <1991Apr4.031246.14045@athena.cs.uga.edu> <1991Apr22.161357.23840@syssoft.com>
Organization: I/Ovations    Kirkland, WA
Lines: 30

In article <1991Apr22.161357.23840@syssoft.com> tom@syssoft.com (Rodentia) writes:
>To summarize:  If dimming by chopping cycles or half cycles of the 60Hz
>waveform causes audio noise based on these 60 or 120 Hz fundamentals, 
>couldn't the noise be made inaudible by doing the chopping at some
>inaudible frequency such as 20KHz?

Yes

>  Would this be less efficient or
>more expensive than the other (transformer based, I believe) approach
>mentioned in this forum?  Or would it just not work?

Well, it wouldn't work using the inexpensive technology most of the dimmers
available are based on.  That being the SCR or TRIAC.  These devices can
be triggered, or 'fired' at an arbitrary time, but can not be turned
off, period.  They are essentially 're-armed' when current stops flowing
in them, or every time the AC reverses direction.  This means that no
matter what frequency you modulate the triggering, they can only be
turned off 120 times a second.

In order to do what you want, you would need to use high voltage power
transistors, at considerably greater expense, not only for the transistors
themselves, but for the triggering circuitry.  The triggering circuitry
is not inherently more expensive, but high volume demand exists for SCR
trigger circuits and a wide variety of packaged solutions exist at low cost.


-- 
del AKA Erik Lindberg                             uunet!pilchuck!fnx!del
                          Who is John Galt?