Inuctor and ferrite bead

Hi,

Just a question: Is ferrite bead actually an inductor? Or a filter?

Thnaks!

 

The ferrit bead is a core for an inductor, which can be used in a filter,
or as a filter.

When you send an AC signal through an inductor the higher frequencies
will be more attenuated than the low frequencies.

A loudspeaker filter often consists of an inductor and a capacitor. The
signal comes to the loudspeaker, is connected to an inductor which is in
series with the bass speaker, it is also connected to a capacitor which
is connected in series with the treble speaker.

The inductor lets the bass sounds through to the bass speaker but stops
the treble signals. The capacitor lets the treble signal through to the
treble speaker but stops the bass signals, which would damage the treble
speaker.

The inductor in this case has a core of ferrite or iron powder which
makes the inductor more effective, without the need for many more turns,
which would increase the resistance and steal power from the bass
speaker.

 

Both, really, and a bit more.

The bead is intended to form a filter, in conjunction with
the impedances surrounding it.

The bead can be modelled as an inductor coupled to
current paths within the bead itself. This makes it a
lossy inductor, which you can see if you look at the
complex impedance carefully.

 

I think it is more correct to see the bead as the core of an inductor,
the inductor is the wire going through the bead, and it all works a a
filter, stopping high frequencies and allowing lower frequencies, as all
inductors do.

That the core has a shape of a hollow bead or cylinder does not change
its function as a core for the inductor.

That the inductor in this case is a piece of straight wire through the
bead does not change its function as an inductor.

The core makes the inductance value a lot higher than in another straight
wire. Iron powder or ferrite cores always make inductance values higher.

 

Whenever an AC signal travels through a wire it creates a magnetic field around the wire, which effectively makes an inductance of
the wire.

This inductance is like many small coils along the length of the wire.

To increase the inductance we can arrange the wire in a coil, or put iron/ferrite material near it, around it, close to it, to strengthen the
magnetic field, which increases the inductance.

 

It is a one turn inductor (the wire passes once through the hole in
the core). It is usually used as part of a filter. That is, it is
used as a series impedance that passes most of the desired low
frequency part and gets in the way of or absorbs some of the high
frequency part of whatever signal enters it. The absorb comes into
play because it if a lossy inductor at higher frequencies. Often its
effect is increased by connecting a capacitor between the signal line
and ground to act as a load at those higher frequencies, so that the
impedance of the wire passing through the bead drops voltage.

Many beads have two or more wires passing through them (or whole
bunches of wire). The bead couples them all together and acts as a
filter for any high frequencys that are shared in common. Any
currents that pass one way through one wire and back out the other way
through another wire make no net magnetic field in the core, so are
not affected by it.

 

I have seen op-amp audio i/ps fed from a source wire not thread just the
once, but re-looped in and out again 2 to 3 times, through the hole of a
small ferrite bead. Does this method re-filter its unnecessary HF each pass?
And is that practice kosher?

 

A wire is an inductor in itself, a certain number of nanohenries for each
millimeter of wire. That inductance is so small that we usually ignore it
and it has no practical effect on the signal, as long as the frequencies
we are interested in is below the GigaHertz range.

If we arrange the wire in a loop, a turn, it becomes a coil, and these
small inductance values multiply and the wire attenuates frequencies in
lower ranges, like the hundred MegaHertz range.

If you make more turns, like a hundred turns, it attenuates frequencies
in the MehaHertz range or even lower, like the 100kHz range.

Putting ironpowder/ferrite material close to the wire multiplies the
inductance because it amplifies the magnetic field around the wire or
coil. This lowers the affected frequencies a lot.

A straight wire attenuates or affects the gigahertz range, put a ferrite
bead on it and it affects the hundred megahertz range.

Using both methods simultaneously, arranging the wire in a coil and
putting ferrite material close to it, around it, or inside the coil,
increases the inductance even more.

Taking the wire several times through the same hole in a ferrite bead is
to make a coil and to put ferrite around it, inside the coil and outside
it, which makes the wire into a much bigger inductance, which affects
lower frequencies.

One sort of core is called a pot core, because it looks like a pot where
the coil is put inside a pot of ferrite/iron material. You can put
another pot core on top of it, like a lid of a jar, so the coil is
completely surrounded by magnetic material, both inside and outside the
coil.

http://www.elexp.biz/cmp_core.htm
http://www.zmanmagnetics.com/e_core_pot_core.htm

 

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The wire threaded through the bead (either once or a number of times)
becomes (or already is depending on frequency) an inductor.

Inductors in the presence of AC current through the wire have reactance
(often designated as Xl) which is measured in ohms. The higher the
frequency the greater the reactance.

In one sense an inductor alone is not a 'filter'. (Draw a line across piece
of paper and put some coils/squiggles in the middle of it to indicate
inductance and it should demonstrate that is it not 'filtering' anything in
particular)?

Filters are traditionally thought of as circuits designed to;
a) Allow all signals/voltages above a certain frequency - High Pass.
b) Allow all signals/voltages below a certain frequency - Low Pass.
c) Allow all signals within a certain bandwidth - Band Pass.
d) Prevent or bypass a certain specific frequency signal/voltage which is
desired or causing a problem etc. - Often called by some such name as a
'Stop' or 'Spike' or 'Notch' Filter.

True 'filters' require design of all the components and impedances involved.
Filter circuits can be very simple comprising little more than one inductor
and/or one capacitor, or very complicated with many components.

However if the requirement is to stop or attenuate higher frequencies while
allowing lower ones to pass an inductor including one using a ferrite bead
as the magnetic core could be considered to 'filter out' those higher
frequencies.

Thus a ferrite bead on the AC power lead of a computer monitor would allow
the 60 hertz AC power to go through unimpeded. But high frequencies which
might be causing interference to/from nearby equipment could be greatly
attenuated.

Inductance 'L' is measured in Henries. Reactance is measured in ohms.
Frequency is measured in hertz.
Reactance Xl = 2 x pi x frequency x L.
Thus Xl at 1000 hertz for 1 henry inductor is; 2 x pi x 1000 x 1 = 6284 ohms
(Inductive reactance).
Xl at freq. 20,000hz for a 0.01 henry inductor is; 2 x pi x 20,000 x 0.01 =
1256 ohms
At very, very high frequencies the inductive reactance of the wire itself,
without any extra inductors (beads etc.) can be most significant.

Any help?

 

If I read you correctly, your correction would not be
necessary if I had said "The two terminal device consisting
of a short length of wire going thru the bead can be ...".
While I would be hard put to deny the correctness of that
correction, I doubt that anybody failed to understand what
object my "can be modelled" statement applied to.

That is a correction I cannot accept. In response to the
question, "Is ferrite bead ... a filter?", I wrote:
The bead is intended to form a filter, in conjunction with
the impedances surrounding it.
Your addition of the usually expected gross frequency response
goes beyond the OP's question. I don't dispute that it usually
forms a LPF, but that new issue is not "more correct".

[More new and uncontroverted facts cuts.]

 

Passing a wire more than once through a core multiplies the effective
inductance produced by approximately the turns count squared. So 3
passes gives almost 9 times the inductance and so, 9 times the
impedance for all frequencies that the device acts as an inductor.
Unfortunately, it also increases the shunt capacitance that jumps
signal around the inductor at high frequencies and lowers the
frequency of self resonance, at which the device ceases to act like an
inductor. So it helps improve noise rejection or whatever inductive
effect you may be after at the lower end of the spectrum, while giving
up lots of effect at the high end.

 

First question to ask is, "What do I mean by ferrite bead?"

Ferrite bead is a generic term applied to donut shaped pieces of
SOME material. If you dumped a bunch of electronic componetry on the
bench and asked a dozen people to pick out the ferrite beads, you'd
probably get very good agreement among the group.

But if you aske those same people about the characteristics of those
devices, you'd get a lot of disagreement.
The correct answer is, "Where's the spec sheet for this device?"

www.amidoncorp.com
has some spec sheets.
Also ran across this interesting article
http://www.uska.ch/emv/de/pdf_dat/amidon_d2.pdf

mike



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