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RE- Ferrite chokes & saturation - why toroids?


From: 	Robert Michaels[SMTP:robert.michaels-at-online.sme-dot-org]
Sent: 	Wednesday, November 26, 1997 3:14 AM
To: 	tesla-at-pupman-dot-com
Subject: 	RE- Ferrite chokes & saturation - why toroids?

        Good answers, Mr. Watts!! -- But I can't help adding a word
        or two (Hey, I'm like that...)

        God never intended for Man to wind toroidial cores by hand
        -- and it's against the nature of things to even try (if
        you do you'll be called to answer...)

        However, toroidial windings can be accomplished very
        proficiently by machine - faster in fact than the eye can
        see  (like the old Shell Game).

                                - - - - - - -

        The function of toroidial cores - in life - is to fully contain
        the magnetic flux within themselves.   This makes such cores
        exceedingly efficient.  There is no flux leakage to the
        outside world, whatsoever.   Hence, all windings on the core
        are cut by all lines of flux.  (Such a deal!)

        Ergo, they can be smaller, lighter, cheaper, and utilize less
        copper than any other form factor.  Ergo they are widely used
        throughout industry.

        There are also secondary advantages - like, the windings won't
        slip off the core in shipping, rough handling, high vibration/
        high G-force applications, or -- combat.

                                - - - - - - -

        The form-factors for ferrites and powdered iron include
        pot-cores.  This in addition to those mentioned in your
        post.

                                       Shuckin' it right down
                                       to the core -- in Detroit, USA

                                       Robert Michaels




TL>From:  Malcolm Watts[SMTP:MALCOLM-at-directorate.wnp.ac.nz]
TL>Subject:  Re: Ferrite chokes & saturation - why toroids?

TL>> From:   Adam[SMTP:absmith-at-tiac-dot-net]

TL><snip>
TL>> I noticed you mentioned that an iron powder toroid would need many turns
TL>> of wire for the proper inductance, due to low permeability of the core.
TL>> My question is: isn't it a real pain to wind wire through a toroid?

TL>Yes. Most small toroidal cores are used for low inductance chokes or
TL>energy storage apps (you get them in forward switchmode power
TL>supplies - iron powder because of the high DC level in the windings).
TL>Other than that, high permeability cores are used in common mode
TL>filters where cores are operated at zero core flux in differential
TL>mode.

TL>> I
TL>> have been using ferrite "U" cores taken from old B&W TV transformers,
TL>> winding each half separately, and then Krazy gluing them together (with
TL>> spacers).  Also, ferrite is usually gapped between the halves as a
TL>> measure against flux saturation at DC and very low (60Hz?) frequencies-
TL>> kind of impossible to do with a toroid (without breaking it in half :-)

TL>True

TL>> The "U" core shape definitely seems easier to wind- is iron power
TL>> available in shapes other than toroid?

TL>Rods and slugs AFAIK.

TL>> Is there a good reason for using the toroid?

TL>Not for this application to my knowledge (other than to minimize
TL>radiation). But I've used ferrite rods in chokes with no real
TL>problems. Perhaps someone who chose that core shape could answer this.