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[filmscanners] Re: Power Line Filter

A few applicable thoughts--

There's a wide variety of electrical noise junk that CAN be on a
powerline, and there are a wide variety of things that CAN (depending
on how they're designed) put noise on the line, including: electric
toothbrushes, compact flourescent light bulbs, fan speed controllers,
light dimmers, power tools, arc welders...

Manufacturers of sensitive electronic equipment (such as scanners)
should routinely design their power supplies to filter out anything
that might make their equipment misbehave, but SOMETIMES they're
cheap enough to cut corners or are not savvy enough about how rough
life can be in the real world and don't do a good job.

The cheapest UPSes only sit on the line and watch for a power outage,
and thus do nothing about electrical noise and junk.  Some "line
conditioners"  just compensate for varying line voltage and do
nothing about noise.

Sometimes both of these have filters to filter out some noise.
Filters can range from a single capacitor to a complex network of
components, and thus can do a better or worse job of handling
whatever is, from time to time, on your powerline.

When UPSes are running off batteries, most of them deliver square
waves (rather than true sine waves) which are full of harmonic noise
that CAN screw up sensitive electronics.  The ones that generate true
sine waves (with low harmonic content) are more expensive.

The "the $16 surge protector/noise filter" that Harald described is
typical of many such units from a variety of brands.  Almost always
upon disassembly you'll find the surge protection provided by some
MOVs and the filtering provided by a simple capacitor across the
line.  The capacitor CAN be perfectly effective in eliminating your
problem IF your problem noise is at a high enough frequency that the
capacitor's most effective.  If not, it can have virtually no effect.

Regarding specific products:  I use APC SmartUPS UPSes which claim to
produce a true sinewave, and whose output looks pretty good on an
oscilloscope, but do have about 100 millivolts peak to peak noise at
about 100 kilohertz (all very approximate).  Tripp-Lite power strips
have multi-stage filter networks (the outlets farther away from the
power cord have the best filtering) that are more sophisticated (and
presumably better) than a simple capacitor.

So if YOUR film scanner is one of the ones that IS sensitive to HIGH
FREQUENCY powerline noise, and if you actually HAVE such noise and it
IS the source of your problems, then any cheap, capacitor-based noise
filter COULD eliminate your problem.  However this is only one of a
continuum of possible scenarios...

Of course the above discussion only applies to electrical noise
CONDUCTED along the powerline.  It is perhaps more likely for
sensitive electronic equipment to be affected by noise that is
RADIATED from nearby (say within six feet?) objects such as monitors,
stereo speakers, etc., but then that's a whole other discussion isn't
it ;-).


At 10:40 AM -0500 30-3-02, Some Bozo wrote:
>I didn't think that question would initiate such debate.  I have to admit
>though, Owen's original post was hilarious.
>The answer here may not be entirely clear, but let me ask this.  IF
>interference caused scanning noise would the $16 surge protector/noise filter
>which 'filters the electromagnetic interference and radio frequency
>interference (EMI/RFI) coming through electrical lines from outside sources.
>It states that it filters to 40dB (100kHz to 100MHz)'
>be sufficient to eliminate the problem.?....One post suggested not because
>the cutoff starts too high.


Bill Fernandez  *  User Interface Architect  *  Bill Fernandez Design

(505) 346-3080  *  bill_sub@billfernandez.com  *  http://billfernandez.com

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