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Re: EMI Shield Beads


From: 	Phil Chalk[SMTP:philoc-at-ozemail-dot-com.au]
Sent: 	Thursday, September 11, 1997 4:43 AM
To: 	Tesla List
Cc: 	vince-at-oznet11.ozemail-dot-com.au
Subject: 	Re: EMI Shield Beads

Tesla List wrote:
> 
> 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

Gary, Hi,

Re these beads, if they _really_ are 1.5" x 30mm (bit of a mix ?), then
they may be of some use.  How big are the six holes ?  Price seems good
- but then ALL of your prices seem good when you're 'down under'.

Over here there's a commonly used 6-hole (F14) ferrite bead, sometimes
supplied wound with ~ #26 tinned Cu wire and then known colloquially, at
least amongst the hams, as a 'VK200' choke.  The VK is our Ham call
prefix, and the 200, I think, is uH. They are used, eg for power supply
decoupling on PCBs etc, and typically rated around 1A, & self-resonant
around 60 MHz.  However, these things are about 6mm x 10mm (say 1/4" x
3/8")

Trouble is, ferrite, at the frequencies & esp. voltages we're talking
about, looks like a pretty good conductor.  You'd have to use well
insulated wire - again, how big are the holes ?

I wound chokes for my NST on 10mm x 100mm ferrite rods.  First about 5
layers of 4mil LDPE, then first layer of #23 enamelled Cu. wire.  Then
another 4 or 5 layers 4mil LDPE, 2nd layer of wire, more LDPE, 3rd layer
wire, then 3 layers of heatshrink over whole thing, extending about 10mm
beyond the rod at each end. About 400 turns total & bang on 6mH.  Before
long they started arcing internally - hard to say exactly where.  Soaked
them both in polyurethane varnish overnight, then filled up voids in end
of heatshrink with same, and been hunky-dory ever since.

My point, in a long winded way, is that if you just use enamelled wire,
you're sure to have arcing problems.  I think you'd need a fair bit of
plastic insulation on your winding wire.  I think you'd end up with
inductance in the few hundred microHenry range, but that'll depend on
the ferrite material, etc.  The one's I'm familiar with are often
materials most suitable for use in the several MHz range, and not
necessarily so effective in the low 100s of kHz or so.  Due to it's
physical size, this may well turn out to be a material suitable for
lower frequencies.

I reckon if you post fuller techo details of the beads, there are people
on this list who could tell you a lot more.

There also seems to be a preference amongst many for the use of
air-cored inductors, instead of, or as well as ferrite ones.  I'm sure
you'll hear much more.

Cheers,

Phil Chalk.

> 
> 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