Saturated ferrite

Inductance can approach its free air value if saturation is deep enough and the effect is instantaneous with current level, any current - not just DC current.

Now, imagine a horseshoe magnet is placed with its poles bridging the compass.

Of course, the compass is instantly rendered useless.

Get the idea?

.... Phil

On Tue, 14 Jul 2015 04:44:26 -0700 (PDT), Phil Allison Gave us:

Now you've done it... you went and saturated his brain.

Just let the big dipper do the poynting for you. :-)

Thanks Phil, I was hoping that was the case as it fits the symptoms.

Cheers

The gapped magnetic path impedance is intermediate between all air and ungapped ferrous stuff. The air gap, instead of being the whole world long as it would be without the core, is the much smaller volume where you put the Kapton tape.

The ferrous path has some magnetic conductivity, like a low value resistor, and you are adding a series resistance, the air gap, to decrease the conductivity. The gap is tuned until your current produces only as many gauss as the ferrite can handle without saturating, at the price of lower overall inductance.

You can get the same effect by using lower-mu core material, like powered iron or permalloy, which is microscopically gapped. The "cool-mu" gapped materials are good for switchers and such that have a lot of AC current.

The gap reduces the shielding, as it breaks the ferrite into two halves, sort of a magnetic dipole antenna. Gapping the center post of a pot core isn't as bad as gapping an EI or a c-pair.

But one has to be aware that many of those are resin-based and may not take it lightly in case one does overtax them. I had my comeuppance as a kid with a fresh ham radio license. In physics classes they made us believe inductors were ideal. Later, back home ... *KABOOM* and my 2" core in which I had invested more than $5 was gone. I should have stacked two but didn't quite have the funds.

Maybe you can fix it with a series lightbulb :)

NT

On Tue, 14 Jul 2015 08:01:37 -0700 (PDT), snipped-for-privacy@gmail.com Gave us:

And a three-way socket.

Yeah, and I have burned the paint off powdered iron cores, too. Smelly.

I had my comeuppance as a

It actually exploded? Neat.

I never got into ham radio. Buncha guys who want to talk.

The Kool-Mu cores are almost as good as powdered permalloy, low losses for switchers and similar high-field apps, but closer to iron powder pricing.

Actually that would work. The choke is there to stop a power line signal being shorted by a PSU input, and the impedance of a light bulb would work too.

There may be knock-on consequences to do with power wastage and dissipation...

Cheers

Somebody once made inductors with permanant magnets in the gap, to help overcome DC magnitization. Might still.

Can you parallel resonate it? That could reduce the required L.

What red core? It's black :-)

Yup, plus it dropped the wire antenna. Luckily it landed in the front yard and not on people or cars.

I love both the tinkering and the talking. It was very interesting to learn about other countries by talking to folks there. There was no other way for a kid to get that kind of exposure.

In the 70's there was mostly just whatever Philips and Amidon sold.

Unfortunately not, the signal is across three octaves.

The gap will work, no problem with that - it was meant to be gapped but was made ungapped. I just wanted to be sure what actually happens to a well saturated ferrite, which Phil confirmed.

Cheers

I like that. Interesting that the permeability of NdFeB and SmCo is only slightly more than air, something I discovered a few years ago but would never have guessed.

Cheers

It's for the same reason as your saturated ferrite--the domains are already aligned, so there's nothing left to respond to small external fields.

Cheers

Phil Hobbs

Ah yes, that makes sense, thanks.

Cheers

Chatting was not one of my desires, either, unless the object was to discuss technical issues. We had a local net whose primary purpose was to talk about technical problems and designs. It was in the days before the internet, email, and such. We learned a lot from each other just as we do on this forum.

Would that help to illuminate the problem?

On occasion, i have seen that. More in TVs than elsewhere.