Inuctor and ferrite bead

Hi.

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.

--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.

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.

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.

--
Roger J.

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.

--
Roger J.

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.

--
Roger J.

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.]

--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.

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?"

has some spec sheets. Also ran across this interesting article

mike

--
Return address is VALID but some sites block emails
with links.  Delete this sig when replying.
.
Wanted, PCMCIA SCSI Card for HP m820 CDRW.
FS 500MHz Tek DSOscilloscope TDS540 Make Offer
Wanted, 12.1" LCD for Gateway Solo 5300. Samsung LT121SU-121
Wanted 13" LCD for Mitac 6133 Samsung HT13X13-201
Bunch of stuff For Sale and Wanted at the link below.
MAKE THE OBVIOUS CHANGES TO THE LINK
http://www.geocities.com/SiliconValley/Monitor/4710/

xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

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?

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.

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.

--
Roger J.