What ferrite cores are good for winding inductors? - diyAudio
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Old 13th June 2003, 11:43 PM   #1
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Default What ferrite cores are good for winding inductors?

I was winding inductors for my speakers. I got done with one of the two 12 mH inductors I had left, when I realised that I didn't have enough wire to wind the other one (judging by the weight of wire left). I had 30 lbs. of 12 gauge magnet wire and according to calculations I only needed 24 lbs., but I guess I didn't wind them very neatly and therefere thighly enough and I ran out of wie.

So I will make the 12 mH inductors ferrite core instead. The designer of my cross-over (alvaius) recommended ferrite core anyway.

I've heard that only some ferrite is suitable for winding inductors. I only read two things on this forum about choosing cores: You should look for ferrite to use for inductors, look for ones with high "H" and B" values. Also Marcus wrote that "When choosing your core you should look at the data sheet and make sure that the BH-graph is linear and that it has as small as possible hysteresis."

My question is, is this all I need to know about choosing ferrite cores? And where can I buy ferrite cores on the internet or in what stores in general?
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Old 15th June 2003, 01:21 AM   #2
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I meant ferrite rods, I think ferrite cores are circles. After seaching the web I found this website that sells ferrite rods http://www.cwsbytemark.com/prices/rods.php. Are these rods any good? I think I need the type 33 rods, but I'm not sure. Any imput, anyone?
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Old 15th June 2003, 10:17 AM   #3
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Ferrite cores come in various form factors. Typical are rods, toroids, Pot Cores (in various forms), E cores etc. The field is huge and the biggest manufacturer is www.mag-inc.com. What is also interesting is high-flux materials and KoolMu where the latter has an integrated air-gap. Magnetics also has software for designing inductors on the site I gave.

For various reasons, Ferrite's do not seem to be used much in speakers, and I assume this is mostly a cost issue + unfamiliarity with selection process (there is a huge choice). What you can do with the rods that you have already made is to add something to them to reduce the magnetic reluctance (shorten the path).

Petter
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Old 15th June 2003, 03:35 PM   #4
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You can probably get decent results using a low carbon steel rod(say type 1006) of about 1" diameter as a core. The issue you get with a solid material is the volume resistivity issue and some energy is dissipated as eddy current losses. A better way to make a core is to use sheets of low carbon steel with thin insulating material like varnish or paint between them. This reduces eddy currents.

Low carbon steel has a flat B-H curve (low hysteresis) and is much more easily available than ferrite cores.

What you want in a core is high permeability and low volume resistivity. Basically the permeability reduces the number of turns of wire you need to make the inductor (air has a permeability of 1.0). Core materials are made for specific frequency ranges because the permeability of the core materials varies with frequency. You wouldn't want an inductor that changes value with frequency, would you?

You also have to worry about saturation, making the core big enough to stay on the linear part of the B-H curve. The permeability changes with signal level as well....

All of these reasons are why it is cheaper and easier to buy an inductor - or make an air core one (as you attempted). I'm having trouble not picturing the scramble-wound 20Lb pile of wire you must have

The problem with winding your own cored inductor is that it is likely to be a trial and error process unless you can get good information on your core material's properties....
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Old 15th June 2003, 04:41 PM   #5
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Thanks for your imput, I think I will go find those sheets of low carbon steel and paint them and roll them into a core.

Do you know where I can buy some of these low carbon steel sheets? I know where I can get steel sheets from (Home Depot), but I don't know if they're low carbon steel sheets. And when I ask them if they're low carbon they won't know what I talking about, I'm willing to bet.
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Old 15th June 2003, 07:36 PM   #6
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Also do the ferrite rods have to be 1" in diameter cause I could only find .5" diameter ones?

I tried looking for those sheets, but couldn't find ones thin enough to bend. And I would have to order them just like the ferrite rods. But like I said I might order them anyway cause the only rods I could find are .5" in diameter.
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Old 15th June 2003, 11:24 PM   #7
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BTW, the core can be square - you would not have to roll the sheets.

Say you bought 18 gauge sheet - this is .048" thick, so you would need about 20 pieces that are 1" wide and just a bit longer than your coil to make 1" thick core.

A 1/2" core may work just fine - you might just have to try it.

Do a search on the web for "inductor design"
Some of the search terms could be "amp-turns", "magnetization curves", "magnetic properties", "saturation", "flux density", "reluctance", "permeability"

A fairly good page:
http://www.fnrf.science.cmu.ac.th/th...omponents.html

A fairly technical page - subject is magnetics, not just coils:
http://www.ee.surrey.ac.uk/Workshop/...ils/terms.html
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Old 16th June 2003, 02:35 AM   #8
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...and the best link of all:
http://www.epanorama.net/links/componentinfo.html#coils
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Old 16th June 2003, 08:56 PM   #9
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Default Thanks for the tips RON E

I got a whole bunch of 1x1.5" pieces 16 gauge low carbon steel. Some guy cut them and gave them to me for free when I offered to pay.

I painted them with automotive primer and painted then with rustolem enamel. I will soon glue them together and begin winding. Only I don't know how I will wind all the way to the edge. I can't use a wooden dowel and some wooden circles with holes in the middle and nails though the dowel to hold the circles in place; like with the air-core ones. I will have to think of something else.
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Old 18th June 2003, 10:24 PM   #10
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What the difference between iron powder rods or ferrite rods in terms of winding inductors?

I just got some ferrite rods.
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