In article , Ian Bell
wrote:
Ian Bell wrote:
Jim Lesurf wrote:
Hi,

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.

From endless searching through catalogues, it seems to me that most
commercially available mains filters are aimed at trying to stop
equipment *transmitting* crap onto the mains rather that stopping it
getting in to equipment

I'd agree that dealing with problems by 'silencing' the source is - where
possible - probably better that having to protect each individual 'victim'.


though I guess reciprocity applies to at least some degree.

You sometimes have to be careful with reciprocity when people start playing
with ferrite devices. :-)

An advantage of fitting suppression at (or near) the source is that it
helps cut down on the number of possible routes by which interference might
then distribute itself.

Alas the problem in practice may sometimes be from sources outwith your
control or even being able to locate them.

I should also have said that my experience with linear supplies is that
there is a huge amount of crap on the mains from laptop SMPSUs and the
like that falls squarely in the audio band. RFI filters are not
effective in the audio band of course.

Yes, although that does depend to some extent on the type of filter and
RFI. So for example the use of ferrites as common mode chokes can work down
to relatively low frequencies as they discriminate in a way that lets them
pass the wanted mains (or loudspeaker signal) more easily. But that doesn't
help much if the interference isn't common mode. You may also need to use a
loop of ferrite as a 'cm choke' rather than a simple clip-on chunk so as to
get a large enough inductance.

Snag here is the absence of useful measured data on the details of how much
any given commercial filter rejects.

I didn't comment on approaches like 'coating the wires with ferrite' as I
think some cable makers say they do. That struck me as a perhaps dubious,
and probably pointless exercise. Too little ferrite and too easily
saturated by the mains power, I suspect.

Must admit I also wonder about using VDRs as it occurs to me that in some
cases they may *draw* current to them which otherwise may have gone
elsewhere.

Unfortunately, I've not found a lot of reliable measurements on these
matters that show what tends to happen, and how often, in domestic UK
situations. I did find some data on wall socket impedances, but with no
guide to how consistent this was from one house to another!

Slainte,

Jim

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