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Are ferrite beads affected by magnetic field? How?

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ricko_uk:
Hi,
are ferrite beads characteristics affected/changed if a DC or AC magnetic field is placed next to them?

How? Can it be detected/measured?

Many thanks :)

duak:
You bet they can, but it's not easy unless you've got fairly strong external fields.  If you have a inductor with a medium to high permeability material for a core, measure its inductance both with and without a supermagnet in contact with it.  I just tried this on a 10 uH axial inductor.  One supermagnet reduced the inductance to 1.4 uH and adding another didn't make a difference; somewhat better than I thought it would.  What's happening is that some or all of  the core is getting saturated by the supermagnet and its effective permeabily is reduced, thus reducing the inductance.

This effect is used in Flux gate magnetometers and can be insanely sensitive.

T3sl4co1l:
Note that you get the same effect, albeit magnified greatly, due to direct DC bias.  (An external magnet doesn't have a closed path, so they are relatively insensitive that way, but self-bias does of course.)  Ferrite chips aren't good for more than 10s or 100s of mA.  Beads on leads, and cable beads and rings, aren't good for, much more than say 1-10At, depending on aspect ratio and material.

Tim

ChristofferB:
IIRC, some ferrite materials will get permanently altered by contact with a magnetic field.

I've heard an old story (maybe early 1960's) about a fairly large danish radio/TV manufacturer who suddenly had a huge surge in malfunctioning products, after they had all their soldering irons replaced with the 'new' Weller ones with magnetic temperature control in the tip, which what traced back to ferrite cores damaged by close contact with soldering iron tips.

exe:

--- Quote from: T3sl4co1l on March 25, 2020, 01:54:24 pm ---Note that you get the same effect, albeit magnified greatly, due to direct DC bias.

--- End quote ---

Thanks, now it all makes sense why this is happening.

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