Timothy...............very germane, very instructional.
Thanks,
Bob Heller
3M Product Safety, 76-1-01
St. Paul, MN 55107-1208
Tel: 651- 778-6336
Fax: 651-778-6252
==================================================================
"Christman, Timothy
(STP)" To:
emc-p...@majordomo.ieee.org
<Timothy.Christman@gu cc: (bcc: Robert E.
Heller/US-Corporate/3M/US)
idant.com> Subject: RE: Noise from
flourescent light ballasts?
09/06/2001 03:22 PM
Please respond to
"Christman, Timothy
(STP)"
Not sure if this is germane to the list, but...
Many newer fluorescent fixtures use an "electronic ballast" -- they use a
switching converter or flyback to develop the high voltage required for the
bulbs (laptop backlight elements work the same way). Apparently this
reduces the amount of iron required for the ballast transformer (and
$$$'s?), but has the unfortunate side-effect of creating a massive array of
unwanted noise sources.
Usually the switching frequency is designed to be just beyond the audible
range, which is consistent w/ your description.
As Richard noted, any switching / chopper circuit could be the culprit.
The
lights tend to pose problems for measurement/instrumentation circuits w/
high source and high receiver impedance (where capacitive coupling
dominates) -- pretty intuitive when you consider that the bulb in the
fixture has a large surface area at high potential, switching at KHz.
frequencies.
Timothy J. Christman
Test Engineer
Guidant Corporation
4100 Hamline Ave. N.
St. Paul, MN 55112 USA
www.guidant.com
Opinions are mine, not those of my employer. My employer is evidently
smarter.
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