Homemade Ferrite - what updates do you seek most?

First a massive thank you for the interest in my project - I certainly was expecting this great response!

Some might already know my first tutorial on how to make your own ferrite, here:
As the muber of followers grow and so the hits on the linked video I was wondering what updates you would most interested in!
I want to make some custom pot cores for small HF transformers in the 2-20kHz region.
So the question is what new recipe you would like to see...
There are several options, all would include some mould making first:
Clay as a binder
Ceramic as a binder
Using some non-hygroscopic salts to melt into the ferrit powder as a binder - all three would require to finnish the parts in a good oven
2K glue or fibreglass resin as a simple dry mix
2K glue or fibreglass resin with vacuum curing to remove all air - this might require special glue as in my experiments resin and glue start boiling long before total vacuum is reached, was thinking of casting resin istead as this would be rated for vacuum preparation

From past experiments these are the most likely candidates to result in a good product, if you have other ideas and suggestions post them here.
Being a guy that loves to keep things simple and prefering the use of scrap / easy to obtain parts I would obviously love to know where you struggled with your creation of ferrite.
I still did not find a good source for rare earth materials in fine powder form to add to the mix but don't think it is an issue as for high performance there are always commercial cores available.
Some people asked my what the best was is to make a big toroid core for Rodin coils and other unuasally big cores for transformers.
If you are one of them, leave your feedback here as well as the size is only an issue in terms of avoiding cracks during the curing.

Let's get the party started! :)

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gmoon2 years ago

One of these days I'd like to make a variable inductor, and one with a large amount of inductance--something in the range of 2 - 3H to 1 - 0.5H. Similar inductors exist, but they are pretty hard to find.

I've got some vintage variable inductors in the 800-200mH range. They have a coil wrapped in a ferrite shell, with a movable ferrite slug inserted to change the inductance. They almost meet my criterion, but the DC resistance is a little too high, approx 200 ohms, vs the 40-50 ohms that would be perfect... A larger value inductor would be more forgiving of the higher DC resistance in my application.

Anyways--I'll go back and reread your instructable. I'm sure it will be useful.

Downunder35m (author)  gmoon2 years ago

For something like that I would make a silicone form for both parts.

The slug a bit bigger in diameter than required.

As a base mix I suggest 2k glue or resin a very thin castin resin might be best as it mixes best.

Should be slow curing to give enough time for a proper mixing.

Once cured use a lathe to add a thread on the slug.

If the coil is would on a thin but soft plastic the slug can be used like a screw to go in and adjust the inductivity.

Just thinking: the outer shell could be a solic piece that you drill a hole into according to the specs of the coil, after all it is just resin and not ceramic material.

Cool. Good ideas.

In my case the slug needs to move freely and quickly. The vintage inductors had a threaded brass rod embedded in the ferrite slug. It could move freely up and down, once the shell and small collar with the threads was removed from the top.

I'll probably need to experiment quite a bit to get what I want. A whole lot of variables here, and they all interact...

The overall construction of the vintage one: the coil is wrapped around a thin hollow cardboard core (where the slug moves), and everything is inside a ferrite cylinder and sealed with glue at the ends. No real way to check the internals without "deconstruction." Hey--just remembered there's already an image in my library...

Downunder35m (author)  gmoon2 years ago

Juding by the pic it seems to be just a shell with the core attached to a threaded rod.

If size is no problem you could do the same, make the coil, insert into the outer core and secure with resin glue or similar.

Mount it on a base where the screw for the centre goes through and test it.

I have done some small scale tests trying to find a way to sinter the iron oxide in a normal oven.

Sadly all I treid ended in very brittle and fragile materials.

Another problem with it is to keep a high pressure on the material during the sinter process.

I tend to say it is no option for home use unless you have access to a kiln reaching around 1500° Celsius.

With a spare oven outside I would think a mix of tar and oxide should work too if the mixing works good.

With a thick clay consistency and a bit of pressure the tar should should burn to carbon and "glue" the iron oxide particles together with a bit of shrinkage.

For my early model rocket experiments I used bentonite clay for the smaller nozzles.

If you have a spare hydraulic or manual car jack you can make a frame similar to what is used on a small press or log splitter.

A bentonite iron oxide mix should at least work fine for the centre rod.

Of course you would have to mill the bentonite to a very fine poweder first and mix it with the iron, preferable in a ball mill.

Use a steel tube of suitable diameter and with thick walls as the form for the centre rod- it needs to be polished on the inside otherwise it is next to impossible to press the finnshed rod out without cracking it.

The outer core could be done the same way but you need two suitable tube pieces and a press ring to in between the two.

Have you considered using a square instead?

I mean it makes no massive difference if the outer core is round or not.

4 flat plates are easier to make and can be glued together with only minimum losses.

Center part can still be round.

Thanks- again, good comments.

I do have a hydraulic log splitter that needs fixing. If the 15 year-old rebuild kit doesn't work (father-in-law never got to it), I'll probably replace the piston & engine with an air hydraulic jack. That could be pressed into use (pun intended).

During my college years I worked for a time in a factory that made shock absorber parts. We used a sintered metal process to make the parts, so I'm familiar with it.

I expect it would be premature to use this process until I had a working prototype. Too many variables: shape, size, ferrite composition, wire gauge, turns, size & shape of the ferrite plug, etc, etc...