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DIY Powdered Iron Tranny Core

Hello Everybody,

I am new to this forum, and was going to wait until I was further along with this project before I posted anything on it, but with all the activity around tranny winding lately, I decided to go ahead and put my ideas out here.
I have been intrigued with the idea of a powdered core audio tranny for some time. The cost of producing a core suitable for audio seems to be prohibitive, since I’m not aware of any. That doesn’t mean they don’t exist somewhere, I just haven’t seen them. Either way they’re not readily available to the diy’er.

I hope to change this injustice. :D

The first step was to find a suitable material. It has to be iron, not iron ferrite (the material most “iron powder” cores are made of), it has to be readily available, and most importantly (to me anyway), it has to be cheap. The answer to this problem came from a surprising place…your local brake shop. Brakes are made from a material called “grey iron”, and every time a brake drum or rotor is turned, a pile of granular iron drops in a tray. I searched the net to find out the composition of grey iron to see what was added to the raw iron, and the main additive is…wait for it…SILICON. It seems that silicon not only helps or tranny to avoid saturation, it helps are breaks from warping from the heat. Now, I don’t know the exact composition of the iron, but it no doubt will work. And the price ? How about FREE!! Its just waste to the shop owner.

Before I go any further, a word about safety. Iron dust is not to be breathed, even our un-processed bucket of iron has dust in it. ALWAYS wear a dust mask if your playing with it.

The next thing is to process our bucket of dirty brake turnings into our iron powder. I first tried wacking the stuffing out of it with a hammer and cast iron pan. A weekend of effort netted me about pound of dust fine enough for my purpose. This will not do. The solution, I believe, is a simple device called a “ball mill”. Basically it is a small drum filled with steel balls that spins on rollers (I hope to have mine finished this weekend). As it spins around the balls smash the grains to powder. Before it goes in the drum (or frying pan if your more inclined to beat it into submission), I heat it up in my metal furnace ( I have a hobby aluminum foundry) to a cherry red heat, and while hot, pour it into a metal pot. As the iron hits the air, it cools rapidly, hardening it. This does 3 things, first it burns any residual oils off the iron, second, it makes the iron quite brittle for easy smashing, and third it puts a very thin layer of oxide on the surface as an insulator. Once pulverized I’ll re-heat it in the furnace to oxidize the broken edges, but this time I’ll let it cool slowly to anneal the metal back to its natural “soft” state.

Once processed, the powder will be mixed with an epoxy binder, and rammed under pressure into a break-away mold. These details will be worked out when I get there.

Comments and suggestions would be greatly appreciated.
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
www.diyaudio.com
> powdered core audio tranny for some time. The cost of producing a core suitable for audio seems to be prohibitive, since I’m not aware of any.

They have silver-wire transformers. I don't think the reason for no iron-powder is "cost".

It probably does not work well.

Consider. Use solid (sheet) iron, you get high inductance. Break the solid iron with an air-gap, you get lower inductance. In applications where the iron is soaked in DC, a small air-gap improves AC performance.

Your plan seems to have a ga-zillion air-gaps in it. Inductance could be rediculously low.

There are ferrite cores for audio. I remember when you could not get 1% of the inductance of iron with ferrite, so it was only used at high (radio) frequency. Apparently there has been great progress in the ferrite racket. Certanly switcher power supplies have pushed 20KHz core performance.

> a surprising place... your local brake shop.

Brake rotors are iron, yes. But the rotors are contaminated with brake pad lining. This used to be asbestos (a potent carcinogen) and Bakelite/phenolic resin. Asbestos pads are nearly gone from the market, but brake shops may have decades of old asbestos dust everywhere, and it isn't clear that the asbestos-replacement materials are entirely benign to the body.

As you found, processing iron shavings into fine iron dust is VERY hard work. You can find pre-made iron dust in several places. The inside of drum brakes accumulates a nasty mix of brake-pad dust and iron-drum dust. The rear drums on modern cars hardly wear enough to be worth the collection, but big trucks still use big hard-worked drum brakes and must generate a pound of dust between reshoeings. Magnetic separation would get the iron out of the pad-dust. There is also fine iron in engine oil, though with modern oils and low-wear engines, you might have to process a lot of oil for a little iron.

"Iron powder" on Google

If you manage to collect enough iron dust, be aware it can be flammable. Not easy to ignite, but very hard to extinguish. Mixed with an oxidizer, iron dust is "Thermite", which can weld railroad tracks or burn a hole in a safe. Iron has to be super-fine to sustain combustion in plain air, but if you carry the experiment far enough you could get in trouble.

> SILICON. It seems that silicon not only helps or tranny to avoid saturation, it helps are breaks from warping from the heat.

Actually, Silicon in iron makes it easier to cast.

> anneal the metal back to its natural “soft” state.

Transformer steel works best in the direction it is rolled in. I'm not sure that cast steel would have the same magnetic properties.

> mixed with an epoxy binder

I'm trying to picture this. With fine iron, and most epoxy resins, the thick epoxy would keep the iron apart, giving a mass that is say 50%+ epoxy, not iron. By simple conglomerate geometry, I think you want a super-thin binder.
 
"They have silver-wire transformers. I don't think the reason for no iron-powder is "cost".

It probably does not work well."

The cost would be the complications of casting larger cores.

"Consider. Use solid (sheet) iron, you get high inductance. Break the solid iron with an air-gap, you get lower inductance. In applications where the iron is soaked in DC, a small air-gap improves AC performance.

Your plan seems to have a ga-zillion air-gaps in it. Inductance could be rediculously low"

Actually its called a "distributed air-gap"..its an advantage. Properly done, the casting would have about 90-95 % the mass of solid metal, ergo, 90-95% of the permeability. I intend to build an SE amp with parallel 813's. The Idle will be about 200ma I need an air gap.

"Brake rotors are iron, yes. But the rotors are contaminated with brake pad lining. This used to be asbestos (a potent carcinogen) and Bakelite/phenolic resin. Asbestos pads are nearly gone from the market, but brake shops may have decades of old asbestos dust everywhere, and it isn't clear that the asbestos-replacement materials are entirely benign to the body. "

See the saftey precaution mentioned in original post "ALWAYS wear a dust mask". That said, after riddling the iron through a screen outdoors in the breeze the powder contaminants were effectively removed. As I mentioned in the first post , the rest are burned off in the kiln.

"Actually, Silicon in iron makes it easier to cast"

Actually, according to ALL the metalurgy sites I visited, the use of silicon was to increase its wear/heat resistance.

"Transformer steel works best in the direction it is rolled in. I'm not sure that cast steel would have the same magnetic properties."

It won't, it will be better. Because of the random placing of the particles it will have an amourphous mag pattern.

"I'm trying to picture this. With fine iron, and most epoxy resins, the thick epoxy would keep the iron apart, giving a mass that is say 50%+ epoxy, not iron. By simple conglomerate geometry, I think you want a super-thin binder."

My first crude attempt with household epoxy and poorly rendered powder and no real pressure yielded about 75% mass, the 25% waste was due mostly to not enough pressure, as I used 5% by weight of epoxy. True enough thinner is better, however with proper mulling thick works fine. It's more a question of proportion by weight than anything else. You want just enough binder to hold it together, no more. As I said in my original post I will work out the exact method later.

Thanks for the reply.
 
Actually its called a "distributed air-gap"..its an advantage. Properly done, the casting would have about 90-95 % the mass of solid metal, ergo, 90-95% of the permeability. I intend to build an SE amp with parallel 813's. The Idle will be about 200ma I need an air gap.

The main principles you exposed fly 100,000 feet above my head.
I just want to point that, using "standard laminations" the gap represents 1 to 5 per 1000 of the total magnetic lenght (not 5 to 10 per 100).
This reduces the permeability by a factor of about 20.

I really don't know if my remark is pertinent in this context, just a remainder.

Yves.
 
You're both wrong, silicon is added to keep it from turning brittle! :D The carbon content (mostly present as graphite flakes and pearlite) is what provides the lower melting point and great fluidity.
Silicon does increase temperature and wear resistances a little though.

Dumping the stuff hot through air is going to burn *everything*, depending on just how fine the stuff is. Magnetite is still an okay material, but it's not known for a high permeability. (But hey, you can always bond it with some clay and make your own ferrites.)

Aside from dense epoxy bonding, which evidently does work well enough? You could try sintering it, heat powder to about 2000øF in a refractory mold under hydrogen atmosphere.

Tim
 
Yvesm said

"I just want to point that, using "standard laminations" the gap represents 1 to 5 per 1000 of the total magnetic lenght (not 5 to 10 per 100).
This reduces the permeability by a factor of about 20."

True..but a powder core has the gap distributed. Think of how the bricks lay in a wall, for every two bricks laying end to end with morter in between, there is a brick on top bridging the gap. Thats not to say there isn't a loss, its just not a linear loss like with laminations. I wont know what the total effect will be untill I actually have a core to test.

Sch3mat1c said

"Dumping the stuff hot through air is going to burn *everything*, depending on just how fine the stuff is."

How fine, and, how hot. I'm "only" heating it to 1200-1400 deg. The idea here it not to burn the metal, rather to temper it "full hard" so that its brittle enough to pulverize, and to generate a thin oxide layer to act as an electrical insulator from one particle to the next. If you have a piece of "hot rolled" steel around, the black layer on the surface is from the steel coming into contact with the air while its still red hot coming out of the rollers. This layer is a lousy conducter. Just ask a welder..he has to grind a spot down to metal to put his ground clamp on.

"Aside from dense epoxy bonding, which evidently does work well enough? You could try sintering it, heat powder to about 2000øF in a refractory mold under hydrogen atmosphere"

Aside from the fact I don't have the resources needed to sinter, sintering would provide an uninsulated path for eddy currents. One of the main advantages of a powder core, is that each particle acts as an individual lamination (ideally anyway), hence very little loss due to eddy currents.

Epoxy and oxidizing weren't my ideas. This is how industry does it when working with real iron as opposed to ferrite. The difference is, the big boys oxidize with a acid bath. I tried this with vinegar as a mild acid, it worked, but what a mess :dead:
 
Well.. Its looking like my original impulse to hold off posting on this topic until I was at the point of testing the electrical/magnetic properties of an actual core was the correct one.
I understand that you guys hear a lot of off the wall design ideas from nubes. I also realize that the people here don't know me from Adam, that I have been a component level service tech for 25+ years, that I have been a hobby machinist for the better part of 10 years, ect.,ect.. I don't profess to be a MIT research scientist, but I have had an idea or two that has panned out (along with several that didn't). Will this one be worth the effort ? I don't know yet, but clearly I believe its worth a shot.
What I didn't expect was the chorus of negativity, especially when you consider the potential it has. From reading the responces you would think ...A)There can't be a break man alive over the age of thirty, what with all the dire consequences of being around break shavings 40 hours a week B)If a powdered iron core for the 5hz-20khz range was such a good idea Radio Shack would have 'em already.
I didn't come up with my methods out of thin air. I spent a great deal of time reading up on powdered cores and how they are manufactured. Basically I took industries approach, and figured out how to replicate it on a much smaller scale..what I'm trying to say is the basic approach is sound. Will I pull it off ? Time will tell. Will it be worth it if I do? Boy Howdy!!
I was hoping to get suggestions on how to improve my approach, to generate just a little excitement about the possibility of making a core to match the tranny, instead of winding a tranny to match the core. To be able to make any style of core you want, not having to pull apart a tranny to get the lams, or meet the minimum order requirements for an industry that wants to sell a ton of lams not the few pounds you need.

I'm still hopeful that there is some imaginative soul(s) out there with a inventive spirit that would like to chip in with an idea or two.
 
Powdered iron cores of various shapes and materials are readily available and cheap:
http://www.amidoncorp.com/aai_cost_ironpowderpower.htm
or www.micrometals.com (they even give out samples)

I'm sure you can find some real cheap cores on Ebay too if you look for a while. Problem is, powdered iron material is typically very low permeability, ranges from Mu relative of 3 to 100. Also, any magnetic material formed from powder will be high loss. This is because the magnetic field is highly concentrated where the particles touch, but low in the bulk of the material. This causes saturation losses to set in early. Somewhat better are 20% iron/ 80% nickel powdered cores (powdered permalloy) for Mu up to 200.

Ferrite material has much better magnetic properties (Mu 2000, 5000 even 10,000, TDK, and very low loss and good initial permiability at low fields) and you can definitely find E cores for switch mode power supplies on Ebay cheap. (E cores are easy to gap for SE use and easy to wind too) Only drawback is lower (about 1/4) maximum saturation field. Fair Rite Products sells a 6 inch OD ferrite core. Stack 2 or 3 of them up and you can handle 100 watt audio. Toroids are a major pain to wind however without automatic equipment.

In bygone days, a lot of audio xfmrs for telephone equipment was made using magnetic wire (oxidized coating) to form the cores. Old ignition coils too. Hard to find decent material now though. Could look for the old magnetic recording wire spools maybe.

Don
 
valveitude said:
Well.. Its looking like my original impulse to hold off posting on this topic until I was at the point of testing the electrical/magnetic properties of an actual core was the correct one.

Ah, now don't let a few skeptics keep you from experimenting. And I hope you didn't take my chuckle at Schematic's post as any critique of your plans. I don't know enough to criticize. I just found humor the image of cooking up stuff with hydrogen in the back yard. It's the kind of crazy thing I would be tempted to try.

Sheldon
 
If you want to make an output transformer fast, easy and cheap, disassemble a military surplus modulation transformer (can be had for $10-$20). The laminations are of excellent audio quality.

I don't want to dampen your enthusiasm, but making your own powdered iron cores for audio transformers is a complete waste of time and will only result in frustration. Purity and exact formulation are absolutely critical in Fe-Si magnetic materials. Any sintered powdered core, even permalloy, is unsuitable for an OPT. I had a couple of good-sized permalloy powder cores (free) I wanted to use in the worst way and, after doing all the research and calculations, I gave it up.

Why try to re-invent the wheel?

John
 
Ok..heres the challenge. Find a powdered core for a 80-100 watt tranny (40 watt SE running hard class A) with a useful freq. range of 5hz-100khz with a permeability of 250-450. No Toroids please. Oh yea, it has to be affordable,you know $50-$100

Or..a source of 4% silicon iron powder under $2.00 a pound in smallish (100lb or smaller) quantitys.

For bonus points find a source at any price that can provide a "cup" core that meets the above specs. At any price.

Here's a hint, Micrometals nor Ebay have 'em.

If anybody can source either of these, I will stop my folly post haste, and speak highly of you at all future social gatherings.
 
My last post was originally up a little ways in the thread. Being a new member I got stuck in moderator pergatory for a while. Ergo, I'm a little out of sequence.



If you want to make an output transformer fast, easy and cheap, disassemble a military surplus modulation transformer (can be had for $10-$20). The laminations are of excellent audio quality.


Yep...fast and easy...Thats what diy Ultra-Fi means to me.


I don't want to dampen your enthusiasm, but making your own powdered iron cores for audio transformers is a complete waste of time and will only result in frustration.


:eek: Wow..thanks. Clearly I would have saved a lot of undue stress if I had only asked first.


Purity and exact formulation are absolutely critical in Fe-Si magnetic materials


Well... If your in the biz of producing cores, and you want the cores you sold, in say, 2004 to perform exactly the same as the cores you sold back in 1997 this is very true. On the other hand, if you want to make a "one off" pair of cores for a project...not so much.


Any sintered powdered core, even permalloy, is unsuitable for an OPT


Were to start..a)as mentioned earlier, I am not sintering b)of all the materials, Permalloy would be one of my last choices for an OPT. It was designed for low level signals and saturates quickly. I made some good coin back in the early eighty's relpacing my customers permalloy heads in their tape decks with sendust heads..the increase in dynamic range was stunning.


I had a couple of good-sized permalloy powder cores (free) I wanted to use in the worst way and, after doing all the research and calculations, I gave it up.


There you have it...it simply CAN'T...BE...DONE!!!.


Why try to re-invent the wheel?


We haven't actually re-invented the wheel since Ork was whooping it up around the cave with his round rock, but dare I say, we have made an improvement or two on the concept since then.

I admit this post got a little ..umm..snarky, but at this point in the thread the defeatism is starting to wear on me. Maybe by page 4 or 5 somebody will have somthing positive to contribute. At this point a simple "welcome to our forum" would help my attitude.
 
I admit this post got a little ..umm..snarky, but at this point in the thread the defeatism is starting to wear on me. Maybe by page 4 or 5 somebody will have somthing positive to contribute. At this point a simple "welcome to our forum" would help my attitude.

I apologize for sounding defeatist, but I was only trying to save you a lot of heartache.

Yep...fast and easy...Thats what diy Ultra-Fi means to me.

I see now. I assumed you were trying a project that had little cost and made from found materials - you know, resistors from old pencils, capacitors from Reynolds Wrap and brown paper sacks, tubes made from old projection lamps, etc.

core for a 80-100 watt tranny (40 watt SE running hard class A)

For an output transformer that large, you really only have one choice for material - silicon steel, either amorphous strip or grain-oriented, preferably Hypersil c-core.

John
 
"Ok..heres the challenge"

O....ooKay......y, here's my answer. Visit the town dump and sequester a quantity of worn out steel belted tires. Burn or strip the rubber off to retrieve the steel belts. Use a torch to slightly oxidize the steel surfaces. Can either try using as-is for a giant toroid or can strip out the steel wire to use as a more compact wound core.

Now, back to reality. If you want to make a low cost one-of xfmr., just find a good scrap metal yard in a big city. Locally, I can get nice used xfmrs of almost any size or shape for $.50 a pound. My problem is finding two alike.

If you are more interested in bizarre xfmr construction, then try this one on for size. Wind your xfmr. windings on a large plastic spool (maybe 12 inch ID). Then tape the windings over well to seal them, and drop the whole assembly into a bucket of ferro-fluid. Not cheap though unless you make your own fluid.

Welcome to DiyAudio by the way!

Don
 
A glimmer of light

You don't know untill you try it. So in the spirit of be optimistic I offer my 2 cents to this idea.

To make your C core or donut you need a form. I understand that you will probably be using a slow set epoxy and a muller to mix your dough. You will need a form which you already alluded to. To get the pressure you could use a vacuun pump and a form and a bag. If you don't have a vacuum pump a compressor will also draw a vacuum on the intake side. A bag needs to be thick plastic. Like the stuff old ladies covered their sofas with. A bag can be constructed quite cheaply. Fold it over and seal the edges with silicone. Use duct tape to hold it together along the edges untill it dries. Keep it flat for at least 24 hours. A method to get the air out is up to you. You will need some sort of coupling. I personally use air hose fittings and an adapter to seal to the bag that I made up. Standard air hose works well to draw the vacuum with.

As for pressure figure it this way. Atmospheric pressure is roughly 15 psi. Or about 2102 pounds/ ft squared. Figure out the surface area of your mould and you have the toal pressure all nice and uniform. If you have a slow enough setting epoxy you will also draw out any air bubbles in the mix. But don't go to slow as you have to keep a vacuum the whole setup time for maximum effect.

Food for thought.

Mark
 
Don't listen to all the armchair experts that have not done what you are doing. There is also a quieter group here that read many posts and only rarely speak up. You will gradually get to know them.

I have played with normal ungapped ferrite cores as interstage coupling transformers and gotten quite acceptable results from them but you do need rather more turns than you would for iron. What I would like to try for a *gapped* transformer is a number of EE type ferrite cores stacked together side by side (sort of like thick laminations) so you can get any desired cross sectional area you want. EE type cores have flat sides and square limbs so they will stack together perfectly without any waste space.

[IMGDEAD]http://www.mag-inc.com/ferrites/Ferrites_EIU_Shapes_1.jpg[/IMGDEAD]
 
Why bother with epoxy. Just wind the xfmr. windings on a very large plastic spool. Then suspend it halfway in a 55 gallon drum (with castor wheels on the bottom so it can be moved). Pour in the powdered iron to the top. (suggest letting the powder rust some by heating it in air first to oxidize the surface, maybe can feed it thru an old oil burner assembly.) With such a large volume of material, the magnetic field strength can be kept low. Losses scale as the square of field strength. Low Mu will be compensated for by the large cross sectional area. (You do need to get sufficient inductance for low frequency bandpass) Roll this rust bucket onto the train or subway and ride around with it for a few hours to get the powder to settle optimally. Maybe can just ride around with it in the back of a pickup truck over a dirt road.

An old utility xfmr. would be a lot simpler though. 10KV to 120V is just about right for SE 100W. Divide voltage ratings by 3 for 20 Hz rating.

Don
 
Off the shelf xfmr for $9.99, 2000V to 120V/240V @ 150 mA, these often have a CT on the HV winding for the lower voltage ranges.

A source of HV power supplies and components (including xfmrs and capacitors) that is largely unknown in the tube audio realm are electrophoresis power supplies.

Here's one on Ebay for $9.99
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=26237&item=7508954815&rd=1

I often see these go by without a single bid on them. Just have to find a cheap one near you, so you can pick it up without shipping charges.