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From: "Harry Dellamano"
Subject: Re: switching regulator & EMI
Date: Sat, 19 Oct 2002 10:41:03 -0700
Organization: Posted via Supernews, http://www.supernews.com
X-Newsreader: Microsoft Outlook Express 6.00.2600.0000
"Bill Sloman" wrote in message
> "Harry Dellamano" wrote in message
> > >
> > Toroids have their flux (B field) in a closed loop as do most cores (E,
> > Pot, PQ, RM, EPC, EP, EFD) but the other cores listed do a much better
> > shielding the near field (E Field) which will be a large part of the
> > Pot cores do the best job of shielding both E and B fields. Toroids do a
> > lousy job with leakage inductance unless you Bi Filar wind, are
> > wind and gap. Other than that they are ok.
> > Just don't use an unshielded rod or drum core.
> > that should shake up the troops,
> > harry
> I'm shaken. Here we seem to have a communication from a universe where the
> physical laws are distinctly different from the ones that apply in the
> Since when has an electric field (presumably what was meant by E-field)
> a "near field" and why would a toroid be noticeably worse at "shielding"
> You have to be a bit careful in winding a toroidal coil to avoid the loop
> the plane of the toroid. Rayner and Kibble' s "Coaxial AC Bridges" (ISBN
> 0-85274-389-0) discusses this in detail in section 4.2.1 - you can
> this by looping the end of simple progressive winding back around the
> toroid, or by using the Ayrton-Perry (otherwise "bootlace") winding
> technique, or by winding a two layer coils in three stages - from the
> to half-way around the toroid, then back over the first layer all the way
> around the toroid, then back again to the start.
> It is a fairly obvious problem and was solved a long time ago
> but if you miss it you may think that toroids do a lousy job with leakage
> Pot cores are the topological inverse of the toroid, but they have slots
> let you connect your coils to the outside world, so they aren't quite as
> good at minimising external magnetic fields. RM cores are squared off pot
> cores with biger slots. E-cores are even worse - and IIRR most of Harry's
> alphabet soup are variant E-cores.
> The E-field - otherwise the capacitative coupling - from the coils is
> a problem. There is usually enough grounded metal in the vicinity to
> minimise any radiation. Picky people screen their coils with over-lapped
> electrostatic screens (avoiding the dreaded shorted turn). For low
> transformers the cores may be conductive enough to contribute useful
> shielding, but ferrite materials are pretty resistive.
> Oddly enough, Harry is right about unshielded rod and drum cores - they do
> generate significant external magnetic fields.
> Bill Sloman, Nijmegen
Thanks Bill, I knew I had something right in all of that.
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