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 Post subject: Toroids
PostPosted: Dec Wed 13, 2006 12:46 am 
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There seems to be a general unfamiliarity with toroids and what they are and how they work so I'll put forth a few words that may ease the mystery.

Basically a toroid core is simply a donut shaped piece of material. It can be made of powdered iron or a ferrite mix, not unlike the materials you find as slugs in tunable coils, if xfmrs, etc. The coil winding is wound on the core in a fashion like threading a needle...over and over.

The material that the core is made out of determines its amount of inductance for a given number of turns. It also determines the suitability for the core to be useful at various frequency ranges. A core that is good for a power supply choke for instance generally will be unsuitable for an RF coil...and vice versa. And even within the realm of RF coils there is a usable frequency range for each type of core. A coil made with the incorrect core can have the 'same' inductance but at different frequencies than intended could appear as a dead short - just as if you had used a piece of metal as the coil form.

From a nuts-n-bolts perspective toroids offer several advantages. The main ones are small size and self-shielding characteristictics. They are a real boon for compactness in modern power supplies where in the old days it may have taken a big old choke with thousands of turns to provide the same effectiveness in filtering. In RF circuits they can be small, require no shielding and are relatively stable both for temperature and physical inductance value stability.

On the downside, precision as to the value is harder to come by at rf frequencies since they are not adjustable beyond increments of a turn of wire and the consistency of the cores isn't very exact for precision work.

There's 3 or 4 main vendors...Amidon is well known amongst hobbyists thru the 70s and 80s and many older projects refer specifically to Amidon cores. CWS Bytemark is the up and comer these days. Then there's the OEMs like Fair-Rite and some others who aren't so well known at the hobbyist level. And in true corporate fashion there's all sorts of Plant A makes Company B's products, etc.

Some buying tips:
Most cores are first distinguished as either powdered-iron or ferrite. Yes Virginia, two totally different things. Next will be the core size. 50 will refer to a .500 inch core, 68 will be a .680 inch core, etc etc. The third main parameter is the type of material and this is the most important part. -61 as a suffix is the ferrite compound that is best for applications in the broadcast band range. -77 tends to find more uses in things like lower freq IF transformers. -2 and -6 are typical in shortwave applications.

These are the numbers most familiar in the hobby world and Amidon, CWSByteMark continue to use them. OEM numbers are convertible but a number like 0200-4567-1234 isn't very user friendly.

Take a look at these charts and you'll see the recommended freq range for various 'mix' types.

http://www.oselectronics.com/ose_p88.htm
http://www.oselectronics.com/ose_p89.htm

The 'magic' number for calculating turns is that AL number. Its directly related to the permeability "U" of the core material.

http://www.cwsbytemark.com/prices/toroidal.php

Amidon (and CWS) use a color coding on most of their powdered iron cores but NOT on the ferrites. The explanation has been something to do with the effects of the solvents and/or heat curing of the paint that bothers the consistency of the molded mix. These cores are NOT conductive like a metal, thats why you can break them and glue them back together just like a BCB ferrite rod antenna...which, btw, is usually -61 material for those interested :)

And the OEMs have all sorts of internal color coding schemes so when it comes to sorting out junkbox/grab bag toroids the color is of no help. In fact, for grab-bag cores you can count on 99.9% of them not being suitable for RF work. The industry usage by far is chokes for power supplies, etc. Rule-of-thumb, if you don't know where it came from then its a power supply capable core but no good for RF. Make your wife some earrings out of them :)

Oh...and a strong magnetic field will severely and permanently bollox up a ferrite core! Store them accordingly.

I've done a lot of investigating of these things via DX crystal radio work and they are wonderful little devices but I too found that there was little info available about the intricasies as to how to use them or what to expect.

Hope this helps.

-ex


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 Post subject:
PostPosted: Dec Wed 13, 2006 3:22 am 
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Outstanding Bill... Thank You...

I dun learn't sumpthin' today...

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 Post subject:
PostPosted: Dec Wed 13, 2006 3:58 am 
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its a rare case where I can do any good but I'll claim the 'walk supercedes the talk' when it comes to 'roids. Everybody wants to save a nickle because they have a 'toroid' in their junkbox and they wind up in all sorts of problems because "it didn't work".


Oh well. That was my point.


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 Post subject:
PostPosted: Dec Wed 13, 2006 6:45 am 
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I have some toriods in my Ramsey AM-25 which isn't too far away from being junk box material... Been thinking about trying to use it's digital osc in one of Norms xmitts... Last time I tried it the osc was running but had no carrier...

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 Post subject:
PostPosted: Dec Wed 13, 2006 10:50 am 
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And all this time I thought 'toroids' were hemorhoids that bull fighters got.


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 Post subject:
PostPosted: Dec Wed 13, 2006 2:04 pm 
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Just trying to nail the theory down here with a question:
'roids work if the ring is a dielectric but HAS ample magnetic permeability (say enough to develope an H field)? P.S. provided the turns match the "u"!


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 Post subject:
PostPosted: Dec Wed 13, 2006 4:57 pm 
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Since I just upgraded my ham license to Extra Class and this is fresh in my mind - the formula for calculating inductance for a given toroid AL is:

L = inductance in mH
AL = mH per 100 turns (from the tables)
N = number of full turns

L = AL * N **2 / 10000

or, getting number of turns from a given inductance for a particular toroid:

N = 100 * SQRT( L / AL )

I think these are for 'iron powder' toroids.

Mike Yancey
KE5EMA
Dallas, Texas


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 Post subject:
PostPosted: Dec Wed 13, 2006 5:26 pm 
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So if I grab a likely torrid and wind a coil on it, and I get a usable inductance; will it be ok for BCB? Should I shoot for low C and high L as we do with other coils?


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 Post subject:
PostPosted: Dec Wed 13, 2006 6:19 pm 
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Scot Armstrong wrote:
So if I grab a likely torrid and wind a coil on it, and I get a usable inductance; will it be ok for BCB? Should I shoot for low C and high L as we do with other coils?


Toroid, not torrid :)

The same LC relationships are there. If you're using a broadcast band 'range' tuning cap the the coil will be in the same typical range like loopsticks, antenna coils, etc. Around 220-240 uH gives full BCB coverage with the standard 15-365pf cap.

If you don't know the heritage of the core then there's no guarantee that it will be any good at BCB. In fact, the odds are heavily against it. -61 cores aren't all that common as surplus.

-Bill


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 Post subject:
PostPosted: Dec Wed 13, 2006 6:54 pm 
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Toroid, not torrid
by electricbill
-----------------------------------------
I started misspelling it on earlier threads and Scot just copied that. Bill Hamre


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 Post subject:
PostPosted: Dec Wed 13, 2006 7:16 pm 
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:lol: :lol: :lol: Me too, JohnnySan, I saw the 'roids part and got a twinge. this forum truly cracks me up, sometimes. :lol: :lol: :lol:
But seriously folks, historically (no, not histerically) circa when did toroids make their first appearance in electronic gear?


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 Post subject:
PostPosted: Dec Wed 13, 2006 7:22 pm 
Silent Key

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Some early 1920's sets had air core toroids.
Curt

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 Post subject:
PostPosted: Dec Wed 13, 2006 7:53 pm 
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Air core? Sorry, I don't understand...


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 Post subject:
PostPosted: Dec Wed 13, 2006 7:59 pm 
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OZ132HOME wrote:
Air core? Sorry, I don't understand...


I believe they would be similar to the hole in a doughnut :lol:

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 Post subject:
PostPosted: Dec Wed 13, 2006 8:07 pm 
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Curt, are you serious? Or did you just zing me??? :oops: :oops: :oops: :oops:


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 Post subject:
PostPosted: Dec Wed 13, 2006 8:10 pm 
Silent Key

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Yes, imagine getting the toroid wound and then removing the core. What you have left is a coil of wire that looks like a coil spring doubled back on itself. Or maybe a donut.
Curt

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 Post subject:
PostPosted: Dec Wed 13, 2006 8:32 pm 
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O.K. I'll sure keep my eyes peeled for one of those... :wink: :wink: :wink:


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 Post subject:
PostPosted: Dec Thu 14, 2006 12:00 am 
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Here's an old timer using air-type toroid (donut) coils.

Image


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 Post subject:
PostPosted: Dec Thu 14, 2006 12:37 am 
Silent Key

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Location: Sandpoint, IDAHO 83864
There you go, OZ! I also had a set with them, but the turns weren't rounded, but squared off. Like a donut that was circular in shape, but a square cross section. I can't, for the life of me, think of what it was. I keep wanting to think it was a Freshman, but I also have my doubts. It was beyond restoration and all the coils were smashed flat on the upper half where something must have dropped on them at one time. But I recall the wire had an insulation on it that was reddish, almost purple color.
Curt

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 Post subject:
PostPosted: Dec Thu 14, 2006 1:00 am 
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OZ132HOME wrote:
But seriously folks, historically (no, not histerically) circa when did toroids make their first appearance in electronic gear?


Interesting question. They used them extensively in CATV distribution equipment for broadband VHF couplers, etc at least as far back as the early 60s. And I think Heathkit was using them in various kits in the 60s as well.

-Bill


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