powdered ferrite with glue.

Thanks, Mike. I saw all this months ago in your posts here.

you know what's made outo iron oxide powder and hot-melt resin?

photocopier toner.

Maybe 10%, if it's a *heavy* layer (thickness comparable to the coil O.D.)?

If 50% of space is occupied by high permeability material, and the rest is random gaps, the average permeability is about 2. The permeability of the material doesn't matter much, as long as it's a few times the average (i.e., >10 or so).

This goes for the entire space around the coil. The biggest sin you have, with a pancake coil, is not getting any field from the front face to the back face. That's 50% of your space. So placing the coil broadside smack against an infinite block of ferrite will still only get you about double the inductance.

So, like I said, probably a relatively thin layer of lumpy, gappy material will only do about 10%.

You can buy ferrite plates (e.g., Ferroxcube PLTxx), what's wrong with those?

A pair of plates, clamped around a winding, can offer an effective permeability of 5-10. This is pretty handy for planar PCB inductors, if you don't want to deal with the planar shape cores and holes in the PCB.

Tim

-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website:

I want to smash to powder some ferrite material, mix it hot melt glue and spread it over a sheet of rigid plastic such as polystyrene. Why? Because I want to move the sheet over a pancake coil to make a variable inductor. I understand there will be millions of air gaps between the ferrite particles and also an air gap to the coil. Before I actually go and do that I wonder how much change in inductance I will actually get? If you use a copper or aluminum sheet you can get quite a good change in inductance as the sheet basically cancels out the AC magnetic field of the wire underneath it. But then of course the AC resistance stays more or less constant while you are getting less inductance. If you put two pancake coils face to face and slide a sheet of aluminium between them you can arrange things to get an even bigger chance in inductance.

A very little. Inductance goes basically as 1/ (integral of 1/mu along the magnetic path), so even small gaps dominate the effect of the ferrite. The OP's magnetic stuff will be almost all gap, and so will be nearly indistinguishable from air. JL's estimate of a 1% change is probably within a factor of 3 either way, I'd guess.

Cheers

Phil Hobbs

I had it wrong! The Q gets better with the ferrite rod. Also the the inductance with the rod out of the core is 6.5uH. My previous statement of "Varies from 25uh to 242uh" had the core partially inserted. Should be "Varies from 6.5uh to 242uh".

Without any rod Q @ 570kHz=126, 970kHz Q @=134, 1700kHz Q @=166

Rod 1/2 in coil Q @ 510kHz=512, 1000kHz Q @=490, 1700kHz Q @=380

Rod fully in Q @ 540kHz=540, 1000kHz Q @=520, 1700kHz Q @=430

But, I repeat , if you want Q's near 1300, try these rods on ebay.

Mikek

Interesting. What's its average mu? (I talk about it in my draft third edition as follows:

"Its density is only 2.2 g/cm3 , so there isn?t actually much steel in it. Normal epoxy is around 1.25, whereas steel is about 8, so the mass fraction of steel is (2.2-1.25)/(8- 1.25)?0.14 and the volume fraction is therefore only a couple of percent."

Cheers

Phil Hobbs

Thanks Tim, That's useful information. A permeability of 2 is a bit low. Obviously it' s for a zero cost application (regenerative receivers dare I say it.) Pan cake coils are also used as coin detectors. I guess I will try to improve t he solenoid variable inductor by using as small an air gap as possible, or accept the loss of Q and use an aluminum sheet with the pancake coils. Marc oni (the company) actually used that idea in one early radio where they use d a copper spade in front pancake coil. Well, that idea never caught on.

If you want to tweak the L of a pancake coil, you could slide another coil parallel to it. Or a figure-8 coil that would add/oppose.

Hmm I was going to say that was weird. But I guess not. Since Q is ~ 2*pi*freq.*L/R, a bigger L with no change in R.

George H.

Toner is carbon black, no?

and spread it over a sheet of rigid plastic such as polystyrene. Why? Bec ause I want to move the sheet over a pancake coil to make a variable induct or. I understand there will be millions of air gaps between the ferrite par ticles and also an air gap to the coil. Before I actually go and do that I wonder how much change in inductance I will actually get?

in inductance as the sheet basically cancels out the AC magnetic field of the wire underneath it. But then of course the AC resistance stays more or less constant while you are getting less inductance. If you put two pancak e coils face to face and slide a sheet of aluminium between them you can ar range things to get an even bigger chance in inductance.

just little to make it black, afaik ~90% is polyester

-Lasse

Right, the "black" is carbon black (not iron-oxide). Sure, it's on a transport mechanism.

Not usually (but there's lots of manufacturers of toner). Oak-gall ink, the black ink on a dollar bill, and toner, usually have Fe3O4 magnetic iron oxide. I just did a magnet test on the output of an old LaserWriter; it's magnetic ink, for sure.

On Thu, 16 Jul 2015 13:11:13 -0400, krw Gave us:

If I recall correctly, MICR-compatible toner (for printing routing numbers on checks) contains a significant amount of iron oxide or some other magnetic material. There is enough present to allow magnetic sensors in high-speed check processing equipment to "read" the MICR font.

Yes, the real test would be a 242uH inductor wound with 660/46 Litz. (like my variable) And I have seen others report their air core inductors of this value have Q's 800 to 1000 in the BCB frequencies. To be nit picky, R is changing but XL is changing faster. :-) Mikek

the particle are magnetic bits in for the transfer path to the image drum or to the developer, I forget the exact details.

Is there any way you can distinguish between the magnetic and electrical properties? That is, how is the coil's stray capacitance changed vs the magnetic properties?

I have wrapped some empty plastic bottles with wire, measured the resonant frequency and then filled them with water (dielectric constant increases about 80 times) and noted a large reduction in resonant frequency. So, some materials may appear to increase the inductance while it actually increases the stray capacitance.

This is very interesting. Thanks for your post.

That's strange... the water effect. We put water in a big coil to do Earth's field NMR, I've never seen much of a change, (but I haven't looked closely either.) Well that's at 3 kHz... what's your frequency?

George H.