In article ,
Bob Larter wrote:

On 11/12/2009 7:49 PM, Al wrote:
On Dec 11, 6:37 pm, wrote:
On Dec 10, 6:10 pm, wrote:





Hi all,

I have here a Ferrite bead. Its from inside a laptop and is the first
component before the DC cable even hits the motherboard.

Its got about 2.5 turns, ferrite outside dia approx 5mm and length
approx 10mm and looks very similar to :

http://newzealand.rs-online.com/web/...Action.html?me...

It has no part numbers on it whatsoever.

Looking at the RS-Online website there seem to be about 5/6 "possible"
ferrite beads it could be. All these suspect beads have Resistance @
25Mhz and 100Mhz listed as attributes.

I assume that I can workout which bead my unknown one is out of the
laptop by putting a 25Mhz sine wave signal through the bead and using
a formula to work out the resistance?

If so, what is the formula?

Cheers

-Al

You'll need to know what ferite material is used. This may give you
some ideas.

http://www.fair-rite.com/newfair/materials.htm

G²

Thanks for that link - reading it now. Since I first made the post I
have noticed two things:

1. Almost all the ferrite beads with 2.5 turns that I have seen on
component websites seem to have spec sheets that show freq response
from 1Mhz through to 1000Mhz with corresponding resistance vales
typically in the range 100-700Ohms.

I assume therefore that if this ferrite choke is anything like the
ones I have found, it is not so much for suppressing mains hum but
more likely high freq rf interference and the like? (whats more there
is another choke on the dc side of the ac adapter for the laptop which
looks more geared to mains hum)

2. If you know the:
Length
Diameter
Turns
Wire size
of a given bead, and the above can be measured, can you calculate the
Impedance @ a Frequency (F)?

I realise not knowing the exact ferrite material may require some
generalizations in any equation related to the above.

Its prob not ideal, but I am beginning to think I will get away with
any wide band ferrite bead in a similar packaging may do the trick?

Ah. If you just want to replace a broken one, any similar-looking one
should do the trick.

Actually, if it's broken, you an probably just forget about it, unless
you see evidence that the box is causing interference to other gear
nearby.

Isaac