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Re: EMI Shield Beads


From: 	Geoff Schecht[SMTP:geoffs-at-onr-dot-com]
Sent: 	Thursday, September 11, 1997 3:54 PM
To: 	Tesla List
Subject: 	Re: EMI Shield Beads



> 
> From: 	gweaver[SMTP:gweaver-at-earthlink-dot-net]
> Sent: 	Wednesday, September 10, 1997 1:55 PM
> To: 	Tesla-at-pupman-dot-com
> Subject: 	EMI Shield Beads
> 
> While looking threw an industrial electronic catalog at work today I
noticed
> something called EMI shield beads.  It had a picture and a small amount
of sales
> information about this things.  The informtion said.
> 
> EMI shield beads.  These 6 hole ferrite beads wound with various
configurations
> offer very high impedance in a small package.  Maximum suppression is
obtained
> between 25 and 250 MHz.  1.5" x 30 mm.  100 pack $17.12, 25 or more packs
$14.27
> per package.
> 
> It does not say what value can be obtained.  I was thinking these might
make
> good chokes for a TC wound with #14 THHN house wire.  Is anyone familiar
with
> these things?
> 
> Gary Weaver

Gary:

Ferrites vary widely in performance due to several factors. They are
typically Mn-Zn or Ni-Zn in composition, the former being used more for
power applications below a few MHz while the latter is used primarily in
transformers which operate at frequencies to 100Mhz. Neither of these
materials is an insulator in any sense of the word although Ni-Zn ferrites
have several orders of magnitude greater resistivity than Mn-Zn so you have
to be certain to insulate everything properly..

Those  beads are excellent as RF suppressors but they're not chokes in the
conventional sense of the word. They have a substantial resistive component
to their impedance that only kicks in usefully above 10MHz or so. That's
way above where a TC operates (at least the one's I'm familiar with :) ).
What you might want to consider is something called an impeder core (like a
Philips IMP14/6.5/200-3C85, it should be very useful at TC operating
frequencies since it's made out of 3C85 material). Impeder cores are used
as inductive ballasts in high frequency welders. They're essentially long
ferrite rods with 6 or 8 axial slots cut in them. I've wound some very good
chokes on them for radio use. 

You can also salvage a ferrite rod (a larger one is preferable) out of an
AM radio and use that as a core for an RFC. If you were considering a
toroid, I'd recommend powdered iron since ferrites saturate too easily
unless you get the really low mu variety.

Geoff