Are ferrite beads affected by magnetic field? How? - Page 1
Forum section: electronics / projects

author: ricko_uk
author_rank: Super Contributor
timestamp: March 24, 2020, 09:59:38 pm
content: 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

author: duak
author_rank: Super Contributor
timestamp: March 25, 2020, 03:21:43 am
content: 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.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 25, 2020, 01:54:24 pm
content: 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

author: ChristofferB
author_rank: Frequent Contributor
timestamp: March 25, 2020, 06:03:02 pm
content: 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.

author: exe
author_rank: Unknown
timestamp: March 25, 2020, 06:36:31 pm
content: Quote from: T3sl4co1l on March 25, 2020, 01:54:24 pmThanks, now it all makes sense why this is happening.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: March 25, 2020, 10:45:47 pm
content: Fun anecdote:In Pre-electronic times they used to use transformers as AC switches.When open, the self inductance of the transformer winding passes very little current.To close the "switch" a (relatively small) DC current is put through an auxilary winding. The DC current saturates the magnetic field and the inductance of the AC winding is reduced to a small value, and therefore it conducts AC current.

author: g0fvt
author_rank: Newbie
timestamp: March 27, 2020, 12:26:57 pm
content: Hi, a related fun fact is that a magazine here in the UK had a challenge to build a receiver that did not use thermionic devices or semiconductors. (This was for VLF reception). The winning solution used basically an alternator driving a winding in a ferrite core which then behaved as a mixer. I believe it was published in RadCom and the challenge was to receive a CW signal from an RF alternator transmitter. After a search I found some infohttp://www.wireless.org.uk/mechrx.htm?LMCL=yqb9Uq

author: ricko_uk
author_rank: Super Contributor
timestamp: March 27, 2020, 12:46:58 pm
content: g0fvt that's very interesting! Thank youAnd thank you all very much for all replies

author: duak
author_rank: Super Contributor
timestamp: March 27, 2020, 06:01:00 pm
content: Thanx kindly, g0fvt. An excellent example of thinking outside of the box. I really liked the idea of using a stepper motor as a quadrature generator.