Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!uakari.primate.wisc.edu!ames!skipper!rando
From: rando@skipper.dfrf.nasa.gov (Randy Brumbaugh)
Newsgroups: sci.electronics
Subject: Re: light bulbs buzz when dimmed
Summary: dimmers that don't buzz
Message-ID: <879@skipper.dfrf.nasa.gov>
Date: 16 Apr 91 00:45:10 GMT
References: <7893@jhunix.HCF.JHU.EDU> <989VZ1w163w@k5qwb.UUCP>
Organization: NASA Ames-Dryden FRF, Edwards, CA
Lines: 31


In article <1991Apr2.162037@maximo.enet.dec.com>, ajf@maximo.enet.dec.com (Adam J Felson) writes:
> [ . . .] annoying buzz when dimed.  Is there a better circuit for dimming
> the bulbs that will prevent that !@#$%^&*&^%$# buzz?
>  
The buzz is caused by the SCR/TRIAC type dimmers which are found in
many low cost dimming circuits today.  These work by switching the
AC waveform on at some point in the cycle.  This creates a nasty
high frequency spike which is both audible and can cause interference
for nearby audio equipment.  This is especially bad for theatre light
dimming, which have many dimmers and lights over the audience.

Most dimmers incorporate a torroidial coil at the output, which slows
the rise time and helps some.  The coil must match the load for best 
results.

Several manufacturers are selling non-SCR dimmers which gradually 
ramp the voltage up and down, completely eliminating the buzz.
There are several circuits for doing this.  I've seen these demo'ed
and they are impresive.  Some (Kliegl) are also short circuit
protected - you can short the output with a crowbar and not blow
the supply circuit breaker. The current islimited by the dimmer 
(of course the voltage drops a lot!).  

For a quick overview of these dimmers see the latest Theatre Crafts
magazine.  There is a good article in it (April, I think).

These new dimmers are still more expensive, but it seems a safe bet 
that the new technology will replace the SCR before too long.

Randy Brumbaugh   rando@skipper.dfrf.nasa.gov