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Ferrite cored inductors

 
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Alex
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Joined: 27 Jul 2006
Posts: 5

PostPosted: Thu Aug 31, 2006 10:26 am    Post subject: Ferrite cored inductors Reply with quote

Vis a vis recent dialogue re Codas/Crossovers and still mulling over the options. Could someone perhaps throw a rope on the following?
Is there any perceived problem with ferrite cores.? Descriptive data about them sometimes refers to saturation and I can understand that if this condition occurs any resultant change to network values would be a setback. But to date I have not found any meaningful explanation of why saturation occurs nor how it can be avoided. When does the system regain it's normal status etc ?
Alex
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speakerguru
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Joined: 18 Nov 2005
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Location: Green Hut, Tovil

PostPosted: Thu Aug 31, 2006 12:17 pm    Post subject: Cored Inductors Reply with quote

The main limitations of magnetic core materials are non-linearity and saturation. Both are due to the magnetisation hysterisis loop that they following during magnetic field reversals caused by ac current.

They are used because they reduce the size and therefore the resistive losses in inductors. They make inductor designs more practical.

It should be noted that most crossover ferrite core inductors are only partly cored. The return path between the two ends of the cylinder is through the air! This is why crossover boards with many inductors should be built with all inductors at right angles and/or far apart to prevent unwanted coupling or transformer action. This air gap considerably reduces both non linearity and saturation that a closed core would have had. i.e don't use toroids for XO inductors.

Some so called "ferrites" are in fact iron dust. KEF inductors were at one stage all iron dust and had the best saturation performance available at that time.

Saturation occurs when the magnetisation causes the magnetic flux in the core to rise above the maximum specified level. The magnetic resistance of the core increases and the flux then has no reason to travel in the core rather than through the air. The inductance at this point falls (the core is doing nothing) and the current increases thereby causing distortion. This happens at peaks in the audio waveform. The point of saturation is reached sooner with small diameter cores as this increases the flux density in the core and with large number of turns which increases the magnetisation for a given current.

If you monitor the current through an inductor on a 'scope you can see when it saturated by the blips on top of the peaks of the waveform. You can crudely determine the saturation point by putting a steel croc clip against the open end of an inductor core and winding up the signal until the clip starts to bounce. Watch out for things getting hot though.

Want any more? You'll have to buy a book on magnetism Wink


Last edited by speakerguru on Fri Nov 14, 2014 9:24 am; edited 2 times in total
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Alex
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Joined: 27 Jul 2006
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PostPosted: Sun Sep 10, 2006 12:23 pm    Post subject: Ferrite Cores Reply with quote

Many thanks, Speakerguru, for reply. All's well with the new speakers and I don't want to buy a book on magnetism!

Alex
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