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Ferrite Rods (or work alikes)

D

default

Jan 1, 1970
0
I have made many ferrite rods by assembling long form
ferrite beads into a stack. At 35 kHz, the Steward LFB
material would work just fine, and Digikey sells many sizes
for little money. Buy beads of the appropriate diameter and
epoxy them together.

Yes.

Fine to just by a wood dowel and slide them on it - until you get the
inductance tuned in.
 
Btw,

As an alternative... I can just buy a bunch of 2cm 'rod' magnets from
the local craft store, heat them up to kill the magnetism, and then
align them end to end until I've got what I need, right?


Just a thought.

thanks!

-phaeton
 
D

default

Jan 1, 1970
0
Btw,

As an alternative... I can just buy a bunch of 2cm 'rod' magnets from
the local craft store, heat them up to kill the magnetism, and then
align them end to end until I've got what I need, right?


Just a thought.

thanks!

-phaeton

It never occurred to me to try that. I happened to have a few small
Radio Shack ferrite magnets so I put one on a piece of steel wire (has
a small hole through the center) and put the magnet and wire into a
propane torch flame. The wire glowed orange the magnet never got hot
enough to glow.

The magnet is still magnetic, but compared to one that wasn't heated,
there is no doubt the heated one is much weaker. Unheated picks up my
small pliers - heated barely moves it (they are both at room
temperature now)

I expected it might break apart when heated but seems no worse for the
heating - can't see any physical difference between the two. Heated
one feels a little different but that is probably just skin oil being
burnt off or carbonized - little more friction with the heated one.

You may be on to something. Next time I fire up the barbie, I'll wire
a magnet to the coal basket and see what happens.
 
S

Sjouke Burry

Jan 1, 1970
0
Btw,

As an alternative... I can just buy a bunch of 2cm 'rod' magnets from
the local craft store, heat them up to kill the magnetism, and then
align them end to end until I've got what I need, right?


Just a thought.

thanks!

-phaeton
NO :) ;)

Even though you killed the magnetization, you did
nothing to soften the cores (magneticaly).
Only a 'soft' core would act like ferrite.
They are very difficult to magnetize, and you
might just as well use a piece of wood.
 
J

John Popelish

Jan 1, 1970
0
Btw,

As an alternative... I can just buy a bunch of 2cm 'rod' magnets from
the local craft store, heat them up to kill the magnetism, and then
align them end to end until I've got what I need, right?

Sorry, no.

That kind of ferrite has a very square BH loop (called hard
ferrite. It has a low permeability, even when it is
demagnetized. High permeability ferrite is called
magnetically soft ferrite.
 
H

Homer J Simpson

Jan 1, 1970
0
Btw,

As an alternative... I can just buy a bunch of 2cm 'rod' magnets from
the local craft store, heat them up to kill the magnetism, and then
align them end to end until I've got what I need, right?

http://www.amidoncorp.com/aai_cost_ferriterods.htm

R33-033-450 0.33 inch 4.5 inch $8.00
R33-037-400 0.37 inch 4.00 inch $7.00
R33-050-200 0.50 inch 2.00 inch $4.00
R33-050-300 0.50 inch 3.00 inch $4.50
R33-050-400 0.50 inch 4.00 inch $8.00
R33-050-750 0.50 inch 7.50 inch $20.00
 
D

default

Jan 1, 1970
0
Sorry, no.

That kind of ferrite has a very square BH loop (called hard
ferrite. It has a low permeability, even when it is
demagnetized. High permeability ferrite is called
magnetically soft ferrite.

I think you may be wrong about that.

I'm seeing something that leads me to believe it becomes soft with
heating.

After all that's what is done to transformer laminations for 60 HZ
cores. The steel is rolled from a slab or billet and becomes hard as
it is worked. It is annealed after it is rolled to lower the
hysteresis losses.

The empirical results:

I fooled with the magnet that I partially demagnetized yesterday -
figuring that putting it between a pair of neodymium super magnets
would re-magnetize it - after all that's what should happen if it is
hard ferrite, right?

It can't be re magnetized. It is now soft.

So my first thought is that it takes some special killer magnetizer to
magnetize the stuff. Then I took one of the unheated magnets and used
the neodymium magnets to reverse its polarity. It doesn't stand a
chance against the neodymium - reverses polarity back and forth
instantly - and has the same strength as the magnets that I haven't
done anything to.

Conclusion - fire makes it soft.

Permeability? I can't really say for a certainty. My only way to
judge the various surplus ferrite cores I have is to use a magnet -

The demagnetized ferrite is strongly attracted to a magnet so I think
the permeability is relatively high.

The hole in the magnet is really too small for a toroid core, but the
next time I use the barbeque, will take one and do a proper job
softening and annealing it then see if I can't get some 32 gauge wire
around it and make a blocking oscillator.

Interested in hearing your comments on my experiments and conclusions.

I realize heating magnets may not be the best ferrite core material,
but this guy is only working at 35 KHZ and there are steel laminations
that are good at those frequencies.
 
D

default

Jan 1, 1970
0
Btw,

As an alternative... I can just buy a bunch of 2cm 'rod' magnets from
the local craft store, heat them up to kill the magnetism, and then
align them end to end until I've got what I need, right?


Just a thought.

thanks!

-phaeton

To follow where this is going regarding hysteresis and the magnetic
properties of hard and soft iron

http://hyperphysics.phy-astr.gsu.edu/hbase/solids/hyst.html#c4

Explains it really well

The fact that the heated magnet can't be re magnetized supports the
idea that the hysteresis curve is a lot narrower than it was before
heating.
 
J

John Popelish

Jan 1, 1970
0
default said:
I think you may be wrong about that.

Could be. Ive been wrong about lots of things.
I'm seeing something that leads me to believe it becomes soft with
heating.

After all that's what is done to transformer laminations for 60 HZ
cores. The steel is rolled from a slab or billet and becomes hard as
it is worked. It is annealed after it is rolled to lower the
hysteresis losses.

Steel (and metals, in general) have a grain structure that
changes with cold working and heat history. Spinel
(ferrite) is a ceramic material that changes grain structure
only with sustained heating well above red heat. Just
heating ferrite through its Curie temperature (where it
demagnetizes, completely) doesn't change its grain structure.
The empirical results:

I fooled with the magnet that I partially demagnetized yesterday -
figuring that putting it between a pair of neodymium super magnets
would re-magnetize it - after all that's what should happen if it is
hard ferrite, right?

Only if the magnetic field strength was high enough to push
it all the way to saturation. I think that the external
field from a neodymium iron magnet is not quite high enough.
It can't be re magnetized. It is now soft.

You haven't re magnetized it, but you haven't proved that it
can't be re magnetized with a high enough field.
So my first thought is that it takes some special killer magnetizer to
magnetize the stuff.
Bingo.

Then I took one of the unheated magnets and used
the neodymium magnets to reverse its polarity. It doesn't stand a
chance against the neodymium - reverses polarity back and forth
instantly - and has the same strength as the magnets that I haven't
done anything to.

That is a pretty good test. You originally said that the
heat did not eliminate all the magnetism, but only weakened
it. Does that weakened magnetism reverse when you apply the
external magnet?
Conclusion - fire makes it soft.

Perhaps. But how did they make the ferrite in a kiln,
originally, without making it soft? Perhaps it was fired in
a low oxygen atmosphere (or some other gas than air), and
heating it in air has changed its chemical makeup. The
oxygen content of ferrite has a big effect on its properties.
Permeability? I can't really say for a certainty. My only way to
judge the various surplus ferrite cores I have is to use a magnet -

You need to do an inductance test of a coil with either the
unheated ferrite or the heated ferrite in it, as a core.
The demagnetized ferrite is strongly attracted to a magnet so I think
the permeability is relatively high.

You may be right.
The hole in the magnet is really too small for a toroid core, but the
next time I use the barbeque, will take one and do a proper job
softening and annealing it then see if I can't get some 32 gauge wire
around it and make a blocking oscillator.

I would try heating it with a propane torch on a piece of
fire brick.
 
J

John Popelish

Jan 1, 1970
0
I have a collection of ferrite pieces. If you live in the
U.S., send me your address and I will send you some to
experiment with.
 
D

default

Jan 1, 1970
0
pruning away


Steel (and metals, in general) have a grain structure that
changes with cold working and heat history. Spinel
(ferrite) is a ceramic material that changes grain structure
only with sustained heating well above red heat. Just
heating ferrite through its Curie temperature (where it
demagnetizes, completely) doesn't change its grain structure.

Agreed, that's what I've been taught and believe.

Then I wonder how important grain structure really is? In small
transformers (less than 5+ KVA) they talk about the importance of
grain orientation for maximum efficiency. But with larger
transformers and motors the physical strength or hardness begins to be
a consideration. I guess (never worked with big transformers) if you
have a lamination that is a few feet long, you'd worry about it
bending before you got it secure in the core - so perhaps (I really
don't know, just conjecture) the relatively high efficiency intrinsic
to big, versus little, is more important.

I did work with a small - wind the transformer to spec - company some
time ago. My boss (only 25 people) kept mentioning his claim to fame
was not taxing the iron too much - we used some grain oriented steel
with moderate cost for most stuff, and stacked the laminations at 1X1,
and our clients would rave about how cool the transformers ran. I
assumed, back then, that not everybody was using grain oriented steel.
Only if the magnetic field strength was high enough to push
it all the way to saturation. I think that the external
field from a neodymium iron magnet is not quite high enough.


You haven't re magnetized it, but you haven't proved that it
can't be re magnetized with a high enough field.

It does reverse polarity - even faster or with less effort than the
regular ceramic magnets do. It is the regular ceramic change polarity
about 1.25" from the neodymium and get sucked in, the wimpy heated
magnet changes polarity at ~2" and is still in my fingers.

Maybe partial magnetization is misleading - the heat treated one is
weakly magnetic.
That is a pretty good test. You originally said that the
heat did not eliminate all the magnetism, but only weakened
it. Does that weakened magnetism reverse when you apply the
external magnet?
Yes, it does indeed reverse, and some distance out from the strong
magnet, compared to the regular magnets.
Perhaps. But how did they make the ferrite in a kiln,
originally, without making it soft? Perhaps it was fired in
a low oxygen atmosphere (or some other gas than air), and
heating it in air has changed its chemical makeup. The
oxygen content of ferrite has a big effect on its properties.

Well, I don't know. One of the mysteries reserved for ferrite makers?
Working iron makes it hard. Perhaps there's some process that
converts iron billet or ferric oxide to powder without hammering on
it? Like spraying molten metal into an inert or hydrogen atmosphere?

Then how does one make hard versus soft ferrite? Maybe hard ferrite
is harder to make, and it has to be processed in a different way to
make ceramic magnets.

I had been assuming they took metal particles and heat and fuse them
like other sintered materials, now I'm not too sure. Why call it
ceramic? does it contain clay? An "off the wall question" is leading
to more questions.

So far Googling hasn't worked - too much commerce or the wrong search
words.
You need to do an inductance test of a coil with either the
unheated ferrite or the heated ferrite in it, as a core.
I haven't gotten any results I trusted with high frequency cores. The
place I worked had a "National Radio" (I think) impedance bridge that
worked very well - my home attempts haven't been trustworthy. I do
trust the 60 hertz power line tests, but not any from my audio signal
generator.
You may be right.


I would try heating it with a propane torch on a piece of
fire brick.

Not sure the propane torch is up to the task. I may have a spider
living in the works. Left it outside one night . . . now the flame
looks weak. The barbeque, on the other hand, will definitely exceed
1,000 degrees in the coal basket for more than enough time. Anyhow
the torch is probably OK but low on fuel so I'll use it till it dies.

I'm living very frugally (both a necessity and environmental awareness
predisposition) so I work the experiments in when they don't interfere
with life.

I don't believe in God - man makes the mistakes and man should be
fixing them, not praying.
 
J

John Popelish

Jan 1, 1970
0
default said:
(snip)
Yes, it does indeed reverse, and some distance out from the strong
magnet, compared to the regular magnets.


Well, I don't know. One of the mysteries reserved for ferrite makers?
Working iron makes it hard. Perhaps there's some process that
converts iron billet or ferric oxide to powder without hammering on
it? Like spraying molten metal into an inert or hydrogen atmosphere?

Then how does one make hard versus soft ferrite?

The composition is different and the atmosphere during
firing may be different. Ferrites are generally made by
mixing (and milling with a ball mill) iron oxide and other
metal oxides. I am more familiar with soft ferrites. These
usually contain nickel and zinc oxide (if they are the high
resistivity, high frequency stuff) and spiced with cobalt,
copper or other metals. This mixture is pressed and fired
to react the oxides into a spinel crystal structure.
Sometimes the mixture is prefired as a loose powder, to
start the reaction and form granules, then milled to powder
again, pressed and fired again to a higher temperature.
This reduces the shrinkage that happens during the reaction.

I think this material is usually fired at least part of the
heat cycle in a reduced oxygen atmosphere.

The low frequency soft ferrite is based mostly on iron oxide
and manganese oxide. The process is the same, but I think
this stuff is usually fired in air.

If I remember right, hard ferrite is made of iron oxide and
magnesium oxide, and I haven't heard what atmosphere is
needed during firing to achieve the best coercive force and
remnance.
Maybe hard ferrite
is harder to make, and it has to be processed in a different way to
make ceramic magnets.

I think some info may be available through Google if you
want to dig a little.
I had been assuming they took metal particles and heat and fuse them
like other sintered materials, now I'm not too sure. Why call it
ceramic? does it contain clay?

A ceramic is made almost entirely of metal oxides.

Finding the right key words helps, because I have come
across some discussion of ferrite production.

This one is pretty general:
http://www.ryston.cz/pdf/avx/ferrites.pdf
(snip)

I haven't gotten any results I trusted with high frequency cores. The
place I worked had a "National Radio" (I think) impedance bridge that
worked very well - my home attempts haven't been trustworthy. I do
trust the 60 hertz power line tests, but not any from my audio signal
generator.

I have one of these and get a lot of use out of it.
http://www.aade.com/lcmeter.htm
(snip)
 
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