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From: vermilye@Oswego.EDU (Jon R. Vermilye)
Newsgroups: sci.electronics
Subject: Re: light bulbs buzz when dimmed
Message-ID: <1991Apr4.211625.6984@oswego.oswego.edu>
Date: 4 Apr 91 21:16:25 GMT
References: <1991Apr2.162037@maximo.enet.dec.com> <1991Apr3.223655.18202@cbnewse.att.com> <7893@jhunix.HCF.JHU.EDU>
Sender: news@oswego.oswego.edu (Network News)
Reply-To: vermilye@oswego.Oswego.EDU (Jon R. Vermilye)
Organization: Instructional Computing Center, SUNY at Oswego, Oswego, NY
Lines: 31

In article <7893@jhunix.HCF.JHU.EDU> ins_atge@jhunix.HCF.JHU.EDU (Thomas G Edwards) writes:
>In article <1991Apr3.223655.18202@cbnewse.att.com> random@cbnewse.att.com (David L. Pope) writes:
>>> an annoying buzz when dimed.  Is there a better circuit for dimming
>>> the bulbs that will prevent that !@#$%^&*&^%$# buzz?
>
>>and min for the bulbs you are using). The actual cause of the buzz
>>is the length of the fillament in the bulbs, so maybe you could
>>find large diameter bulbs designed for dimming (that is, with short
>>fillaments).

The buzz is caused by the waveform the dimmer produces. When the lamp
is at full, the waveform is still close to a sine wave, and little
noise is produced.  When the lamp is dimmed, the SCR or TRIAC in the
dimmer is fired sometime during the AC cycle.  The rise time of the
switching of the SCR is limited only by the impedance of the circuit.
Some manufacturers add a choke (coil) in series with the load.  This
will limit the rise time and noise.  In most cases, noise cannot be
completely eliminated.

By the way, many theatrical dimmer manufacturers are changing from
SCR based dimmers to FET's, and chopping the sine wave on the back.
This, along with better control of rise (or do we call it fall?) time
allows for dimmers without chokes.  Since the weight of the choke
makes up about 75% of the weight of a 2.4K dimmer, this saves both 
weight & space.

 
Jon R. Vermilye                          		         315 341 2138
Department of Theatre, Tyler Hall                  vermilye@oswego.oswego.edu
SUNY Oswego                                 ..rutgers!sunybcs!oswego!vermilye
Oswego, NY 13126                                     vermilye@snyoswva.bitnet