Ferrite or iron powder? How do I tell what an unknown core is made of?

Question

Most times when I need a small inductor, I try to find the necessary core in my junk box. Depending on what I'm trying to build (RF, power, ...), a ferrite core or an iron powder core might be the better choice.

For completeness, ...

• Magnetically soft ferrites, as used for today's RF or power applications, are sintered particles of, mostly, MnZn or NiZn compositions. (Wikipedia section about how they're made)

• Iron powder cores are produced by gluing small iron particles together using epoxy resin or another polymeric glue. I've seen them pretty much exclusively as toroids. The main applications appear to be mains frequency input filters and PFC (step-up) stages at low to moderate frequencies, not so much SMPS transformers. One big advantage appears to be the fact that you can build toroids with an air gap distributed along the entire ring.

I am aware that many different types of both ferrite and iron powder exist (and it may be interesting to test them, too), and the differences do matter, but let's say I'm just hacking some proof-of-concept circuit and don't care about exact losses or saturation properties.

Still, I want to avoid really bad mistakes like using iron powder when I'm building an antenna balun. Some other time, an iron powder core may be just right and a ferrite core may be a bad idea.

Let's focus on simple toroids of different sizes, as a start, because this is what most iron powder cores look like.

Is there an easy and somewhat reliable test that tells you if your core is made of ferrite or iron powder?

Like winding ten or twenty turns of wire onto the core, carefully applying a rectangular voltage (low duty cycle, via a power MOSFET, using a freewheeling diode) to this inductor and looking at the point of saturation in the inductor current?

Or testing the inductor with a sine sweep up to some 10s of MHz in an appropriate circuit?

Also, can you sometimes tell just by visual inspection? For instance, are these color codes used by other manufacturers, too?

Answer 1

You can't tell by visual inspection, that's for sure because some of them are lacquered/painted and even those that aren't all tend to look dark-grey. What you are asking is really tricky to fathom because there are so many characteristics that look the same between two ferrites at one frequency but are vastly different at another. If you are still interested I'll try and say what I'd do (what I'd really do is throw all my unboxed/unmarked ferrites in the trash and buy some more).

I'd consider winding (say) 5 equally spaced turns and putting the coil in a circuit to see what its inductance was - maybe a colpitts oscillator with a few caps that can be switched in and out. Maybe even make a band-pass filter from it and see where it resonates if you have a signal generator.

First type of result this will tell you is the inductance of the wound core. Then using the squared relationship between turns and inductance you can deduce its "effective permeability". This should enable you to narrow down the type of core to a range of possibilities.

You need to be be avoiding "test frequencies" significantly above 100kHz and preferably more like 10kHz - this is to reduce parasitic capacitance giving you errors.

OK so far, you might have determined the approximate "effective permeability" of the core BUT there are plenty of suppliers toting vastly different materials that you'd have to read through to try and identify the part so I'd next consider seeing how the indctance varied with temperature.

You don't need to test over a vast range, maybe just 25ºC to 50ºC would give you a decent shot at trying to uncover the ferrite. Use the oscillator/filter idea mentioned earlier and a controlled temperature - almost certainly the inductance will rise with temperature although there are a small percentage that will stay stable or fall but this will give you another tell-tale characteristic of the ferrite.

So now you have effective permeability and some idea what its temperature characteristic looks like. Scanning through various supplier's websites might narrow down the ferrite to maybe five or ten types.

It's going to be a long process this way and you may never uncover what it is that is sitting in your junk box. I suppose if your effective permeability is low it's likely to be either very temperature stable (i.e. good for filters up to (say) 1MHz) or it could have very low losses up to over 50MHz. The temperature test that indicated hardly any change in inductance across 25ºC might tell you its a material like Ferroxcube's 3D3: -

Also shown is 3C90 for comparison. 3D3 has a flat curve of inductance/permeability against temperature; probably changing something like 5% in a 25ºC change around ambient. 3C90 probably changes about 20%. It also has a much higher permeabilty. I'd recognize these two ferrites from their characteristics!

I think I've definitely convinced myself to throw all unrecognizable ferrites in the bin.

Bottom line - if you have a target circuit try it.

EDIT Also, here's is a question/answer on EE stack exchange that might also be useful or provoke some other ideas.