Ferrite Rods to ferrite core?

[rentner]

Inductors are expensive. It is even more expensive, if you need an inductor of a high value and capable of many many amps. Why not make your own with more winding space? Is it possible, to use ferrite rods and make simple ferrite cores with them? Practical: Yes.
Possible forms are like U+l or U+U or even E+l or E+E

That is not the problem. The problem is, what AL values are expectable? And to shape a cylindrical ferrite rod, to fit to others, you need a flat and smooth surface. Or don't you? Will the AL values be much more horrible, if you just have a few % of the material having contact to the other surfaces? For example I wanted to make a very good core like U+l, but the core broke and what I was left with was nothing but anger. After giving up, to shape the remaining parts any more, it wondered, if it would work still quite good, if you don't have perfect surfaces and punctual contact of the ferrite material only. Will the AL losses be very high, or like non-existing?

I'm doing quite many projects, where I need inductors, but buying cores, that support that given inductance at that amperage are unaffordable.

Ferrite rods are much cheaper and looking at their size, a single rod of a lenght of 16 cm can give a core with a winding space of over 6cm^2, which is plenty. Any core, that big, will cost over 20€ or 25$. For my requirements and the number it is not possible for me.

That's, why I asked, if and how it is possible, to use ferrite rods for ferrite cores.

[JackOfVA]

The effective permeability of a rod is much less than the bulk material permeability. This is because much of the flux is not confined to the rod, but rather travels in the space outside the rod.

A closed form core, such as a toroid, has an effective permeability nearly identical with the bulk material permeability since almost all the flux is confined within the core.

Spend some time looking at Fair-Rite catalog - http://www.fair-rite.com/cgibin/catalog.pgm?THEAPPL=Inductive+Components&THEWHERE=Open+Magnetic+Circuit&THEPART=Rods#select:freq1 figure 1 as an example.

The advantage of an open core, such as a rod, is that it is much more difficult to saturate for the obvious reason. In fact, you can find (look hard enough) toroid cores with an air gap for that reason - it reduces the composite permeability but makes it more difficult to saturate.

So if you need maximum inductance in a given space, a closed form core, such as toroid yields the most efficient design but also one that will be easiest to saturate.

[qno]

In what application do you use the inductor?

In SMPS the stray field is difficult to control.

[JackOfVA]

In what application do you use the inductor?

In SMPS the stray field is difficult to control.

Indeed - that's why ferrite rods are used as antennas.

[rentner]

And that was, why I asked, if it is possible, to put 4 pieces together to a closed Ferrite loop. Of course, that WILL work. The question was, how good?

[JackOfVA]

And that was, why I asked, if it is possible, to put 4 pieces together to a closed Ferrite loop. Of course, that WILL work. The question was, how good?

Not possible to answer this question in the abstract. It's a bit like "I have a piece of string.  Will it reach from point A to B?"

Yes, one can make a closed core from rods. Will it work? For some purposes yes, for other purposes no and for some purposes it will work poorly and for some it will work better.

[rentner]

Well, since I have nothing, to measure that all, i cannot ask for that. What I can ask for is, if it makes a big difference in a closed core, if the Ferrite material is connected over the full 100% available area, or just a small area like 10%. Does something like that completely kill the magnetic flux, or just a bit? Not searching for mathematical solutions here, just for theoretical stuff.

You'll see, what I mean in the Attachment.

[JackOfVA]

You can model the composite permeability with a resistance analog.

Consider the cores as resistors with low resistance value (high permeability) and the gaps are also resistors, but of higher value (permeability of air is much lower).

Hence if you draw the equivalent resistance circuit, the overall resistance is dominated by the high value resistors (in the case of the cores, the air gaps).

You treat the magnetic reluctance as resistance, in effect. You can see some detailed equations on this at http://www.consult-g2.com/course/chapter3/chapter.html

If the ferrite rods are very high permeability, the gaps will significantly lower the composite permeability. If the rods are low permeability, the composite permeability will not be proportionally reduced as much.

If you look at a split ferrite core you will see that the connecting surfaces are polished and lapped flat, almost to a mirror finish. This should give you an idea of the importance of gaps in series magnetic circuits.