Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!van-bc!ubc-cs!alberta!herald.usask.ca!ccu.umanitoba.ca!bison!sys6626!inqmind!jon From: jon@inqmind.bison.mb.ca (Jon Chesnut) Newsgroups: sci.electronics Subject: Re: light bulbs buzz when dimmed Message-ID: Date: Wed, 17 Apr 91 13:08:59 CDT References: <879@skipper.dfrf.nasa.gov> Organization: The Inquiring Mind 1 204 488-1607 > > In article <1991Apr2.162037@maximo.enet.dec.com>, ajf@maximo.enet.dec.com (Ad > > [ . . .] 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. > You're right - in low-cost dimming circuits, this can be a problem. But, as you go on to say, torroidal chokes can, for all intents and purposes, eliminate this problem. Having just completed THE PHANTOM OF THE OPERA CANDADIAN TOUR lighting and sound system supply, as well as supplying all the lighting and sound equipment for the production currently playing in Toronto, I would suggest that in the better systems, SSR's rapidly switching cause no problems whatsoever. By matching chokes to loads (as you said) the noise can be effectively eliminated. Metal dimmmer chassis also help, and shielded multi-conductor power distribution also lends a hand. I would suggest that it is only the cheapest SSR based dimmers that are prone to noise, and that even the most cursory attention to design makes these devices practically silent. Getting back to the original point, I have seen MANY mechanical dimmers that generate HUGE amnounts of noise, and especially RFI - and would go so far as to suggest that these devices are MUCH noiser than any SSR (SCR/TRIAC) setup. Jon Chesnut formerly with WESTSUN - Technical support to the entertainment industry!