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Old 6th January 2005, 04:11 AM   #1
Stocker is offline Stocker  United States
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Question unknown permeability... wind or no?

I have a few toroidal cores of unknown permeability and I was wondering what is the worst case scenario if I wound some chokes on them to put in a power supply. I am thinking the worst that could happen is that they wouldn't be very inductive and that's still not that bad. I am thinking max 2A at maybe 26V rails...

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Old 6th January 2005, 06:16 AM   #2
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Default Chokes.......on toroid cores.......

With DC flowing through them, and no air gap, right?

No.

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Old 6th January 2005, 06:33 AM   #3
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Default Ok, I'll accept that...

...but why?

My knowledge of inductors, aside from transformers' secondary windings, is nil. A friend described it as black magic, and that's pretty close to my knowledge level.

So, they get hot. what else? These are about 1" or so diameter, 1cm thick... wouldn't there be time to realise that they were getting hot and shut down, if required?
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Old 6th January 2005, 07:03 AM   #4
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hm I have no idea as well..

If they overheat (eddy currents right?), couldnt you simply reduce the number of windings on the core, reducing inductance, until it becomes stable?

Isnt there some other issues such as saturation, or is that the same problem.
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Old 6th January 2005, 01:18 PM   #5
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Try some info here

http://geek.scorpiorising.ca/windingchokes.html
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Old 6th January 2005, 01:29 PM   #6
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Yes, saturation is the primary concern in ungapped cores.

It may have thousands of microscopic gaps already, if it is ferrite. If it was used in HF applications (SMPS) it probably will be.

As a result, mu will be too low to bother using in a 120Hz power supply - only a hundred uH or so at best.

Tim
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Old 6th January 2005, 05:00 PM   #7
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If they are ferrite, they won't be of any use at 120 Hz. You could use them to make a common mode choke to keep out some RF. If the core doesn't saturate.

Wound steel tape.....the stuff that they make toroidal power transformers out of..........must have an air gap for it to work.

(They do exist...just ask Charlie Hansen.)

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Old 7th January 2005, 01:32 AM   #8
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alright, assuming there was any useable permeability to the base material...

...could I chop the toroid in one spot with an hacksaw to become an almost-circle, giving myself an air-gap?
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Old 7th January 2005, 03:56 AM   #9
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Stocker - show us some photos of the items in question, and I might be able to tell you what kind of material you are dealing with. Show the sides of the toroids if there is any numbering on them. Echoing the sentiments of others here. If it's ferrite or a tape core, the toroids will make decent common mode chokes but nothing else. If the cores are powdered iron, molypermalloy popwder, or "cool-mu", you may have a stab at making inductors that can handle some current. Ferrites and metal tape cores will saturate quickly in a DC bias situation as they are essentially ungapped. Powder cores are made of a powdered magnetic material with an inert binder, and have an inherent distributed gap. Thus they can stand off a fair amount of DC bias without saturating.
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Old 7th January 2005, 04:21 AM   #10
Stocker is offline Stocker  United States
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IIRC, these are painted or coated with something that looks like paint. No numbers but I can certainly look again. Will it be constructive if I tell you how it breaks and how it looks inside if I smash one? Photos will be tough, without a digicam.... gotta get me one....

The application is for inductive coupling of some very low level signals in a $$$ capacitance diaphragm vacuum guage, which seems like a step in the wrong direction with me feeling in the dark here...

/edit: oh yes, they are dark greyish under the paint.
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