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From: email@example.com (Bill Sloman)
Subject: Re: Magnetically Shielded Inductors
Date: 27 Sep 2002 14:50:14 -0700
NNTP-Posting-Date: 27 Sep 2002 21:50:14 GMT
Ken Smith wrote in message news:...
> In article ,
> Bill Sloman wrote:
> >Toroids should be better. A toroidal coil has no external magnetic field,
> >even if you leave out the toroidal core. Real toroidal coils are rarely
> >perfectly uniform toroids so there is usually some external magnetic field,
> >but not a lot.
> Even a perfectly uniformly wound single layer toroid will have the an
> external field. If you hold the toroid so that you are looking through
> the hole and observe where the wire goes, you will see that there is one
> turn the size of the toroid. This can be removed by making it a 2 layer
> toroid with the second layer progressing along the core in the other
According to Rayner and Kibble in "Coaxial AC Bridges" ISBN
0-85274-389-0, in section 4.2.1, there are several ways of making an
'astatic winding'. Your prescription would seem to be the
"Ayrton-Perry" or "bootlace" technique. Rayner and Kibble seem to
prefer a slightly more complicated approach where they wind the first
layer half-way round the toroid, then wind back over the first layer
and continue past the start all the way around the toroid to the other
end of the coil where they reverse again and wind back to the start -
apparently this reduces the self-capacitance of the two-layer winding.
I've posted here on this point, but it seemed to be irrelevant to the
needs of the OP of this thread.
> I have also found that EFD cores have fairly low external field.
Compared with what?
Bill Sloman, Nijmegen
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