Jim Lesurf wrote:



Just to let people know that I have just put up a couple
of new pages that
explain the basics of how mains RFI filters work. The aim
being to allow
readers to decide which type of filter - if any! :-) -
might be most
appropriate if they have a problem with RFI.

The first page is at

http://www.audiomisc.co.uk/mains/filters1.html

and that has a link icon at the end to take you to the
second page that
deals with ferrite and VDR devices and their use.

I know you wanted to keep it simple, but I wonder if you've
missed the big issue?

If you consider real components rather than idealised
inductors and capacitors, then the picture has another
dimension. The range of frequencies that can be considered
to be RFI is so great that parasitics may be just as
important as the nominal component values.

Consequently, a real filter will be effective at some
frequencies, but not others, across a broad spectrum. You
don't show that what rolls off at some frequency may well
roll back on higher up...then off, then on again, etc.

As a result, every RFI filter has a particular profile of
protection. Every consumer mains outlet has a particular
profile of threat, which will change over time. It's like
fitting a key to a lock, when the keys all look the same and
the lock is unknown and frequently changed. Keeping *all*
noise out is too unlikely to consider...perhaps the best you
can hope for is, by chance, to defeat particular problems
with your particular equipment in your house.

Ian