temperature stable inductor design

[Edwin G. Pettis]

Check out this article from 1920 on General Radio inductors, description on page 1 and photo on page 2:

https://books.google.com/books?id=IjEiAQAAMAAJ&pg=PA542&lpg=PA542&dq=General+Radio+Inductance+Standards&source=bl&ots=hTKaESMzKq&sig=1oX30xOl53e3S2ycxQqnEuuF2UM&hl=en&sa=X&ved=0ahUKEwi97q3KuZnZAhVH7mMKHa3kAVoQ6AEIVjAJ

[Alex Nikitin]

Thats fascinating. Did you see hysterisis when it cooled down? And how big is your ferrite?

Do you know the rated permeability of it?

Interesting that a common core has the same stability as a standard inductor

I wonder how much stability could be gained by going through the whole song and dance that the standard inductors use. I mean their pretty much air coiled, since their on ceramic with no magnetic properties.. So it looks like the core material adds very little. Guess we need to know its design permeability

All the manganese zinc ferrites i can find have a mu between 400 to 4000.. I wonder if that book is filled with a bunch of baloney

are you sure you dont have a nickle zinc core? They have mu that would agree with the book

1) There is no hysteresis that I can measure.

2) The core is a "pot" type, about 1'' in diameter, material is EPCOS M33 , with a small air gap and an adjuster, so the inductance can be tuned between 80uH and 150uH. Litz wire was used for the winding.

3) The initial mu for that ferrite is about 750.

Cheers

Alex

[CopperCone]

That means the book is wrong or is talking about a very specific ferrite material. Good to know. Or its just the pot core structure is stabilizing it.


You don't happen to have a cylinderical core of the same material do you to compare? I want to know so I have some kind of rational for potentially finding better options. It looks like going to a low mu material to get better stability may not be the case.

[Kleinstein]

The air gap can help a lot to stabilize the inductance. It makes the the details of the ferrite part less important, as much of the magnetic resistance is from the small air gap and not the core itself. In this case the core material should actually have a high Mu, but the effective Mu of the core with gap should be small due to a significant gap.

I think that scan from the old book was talking about a special class of cores, likely with an gap to lower the effective Mu.

The pot core type helps to keep the mechanical stress on the core low.

[CopperCone]

Whats the benefit of doing so, rather then just using a material with a different mu?

[T3sl4co1l]

Air doesn't have a tempco...

[CopperCone]

Yea but don't you need more windings if you stabilize a core (which seems to be lowering its mu) to get the same effective inductance.. so why not just use a different material?

does its behavior change in any other way then just for tuning purposes?

i.e. for my stable inductor, is there a difference in using a nickle zinc core vs a gap stabilized zinc manganese core? (Does the air gap core's high frequency behavior change? like it seems to be a intrinsic material property, so if the effective mu of the two inductors are the same due to design difference, the nickle zinc core would still work better at high frequencies? )

seems like there would be no point to airgap a core with a lower permeability because with the enhanced winding requirement you would need more size, so for a filter it seems like it would be better to go with a lower permiability material rather then going for an air gapped core with trimmed permeability, because it would filter higher frequencies better..... any benefits on the other side?

[T3sl4co1l]

How much energy do you think you'll store in a core with mu_eff = 1000?

Energy density is B^2 / (2*mu).

The air gap stores energy, the core merely concentrates flux into it.

NiZn and MnZn are very similar materials.  They are both soft ferrites with Bsat ~ 0.3T.  The primary difference is NiZn has higher frequency roll-off and lower mu.  I doubt there is any substantial difference in terms of tempco.

I don't know much about tempco, but there is a MnZn ferrite (Fair-Rite #33) with low tempco near room temperature, intended for AM radio (loopstick) antennas.

Tim

[Alex Nikitin]

Here is a good reference on soft ferrites from Ferroxcube.

Cheers

Alex