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From: email@example.com (Bill Sloman)
Subject: Re: Magnetically Shielded Inductors
Date: 27 Sep 2002 15:09:37 -0700
References: <firstname.lastname@example.org> <3D9450FE.email@example.com>
NNTP-Posting-Date: 27 Sep 2002 22:09:38 GMT
Fred Bloggs wrote in message news:<3D9450FE.firstname.lastname@example.org>...
> Bill Sloman wrote:
> > "John Devereux" wrote in message
> > news:email@example.com...
> >>Which have lower magnetic emissions, unshielded toroidal or
> >>"shielded" bobbin core inductors?
> >>I am looking at inductors for a 500kHz buck SMPS and want to
> >>minimize the local magnetic interference.
> >>For example,
> > 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.
> > Theoretically, a pot core is topologically equivalent to a toroid and
> > should have the same zero emission, but real pot cores have slots to allow
> > connections from the coils inside the pot core to the outside world, and
> > these slots do allow some leakage flux - more than you get from a toroid.
> > The cylindrical coil in a pot core is a lot easier to wind than a toroid,
> > which is why the "shielded" bobbin core inductors are so much more popular.
> > ------
> > Bill Sloman, Nijmegen
> I am sure that you are confusing far field from near field with this.
IIRR - and I'm remembering stuff from a long time ago - there was no
suggestion of a "near" field or a "far" in the discussion. You
integrate the effects of all the wires in the toroidal coil, and they
came out zero anywhere outside the coil.
Hence Rayner and Kibble's reference to "astatic" windings - see my
other posting this evening.
I'll be delighted if you can prove me wrong - I do hate "the things I
know that ain't so".
> The toroid will have a severe near field compared to a shielded
> component. He will therefore have to watch its placement relative to his
> other circuit components, as well as their layout.
I don't think that this is true - as I've posted here before, the
standard non-contact conductivity probe is a pair of astatic toroidal
cols stacked one on top of the other. Fully immerse both of them in a
conducting fluid and the fluid going through the central hole and
round the outside couples one coil to the other. This could scarcely
work if there were significant "near field" effects.
Bill Sloman, Nijmegen
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