Thane Heins Perepiteia.

[Steven Dufresne]

Or create an airgap in the center?   

Edit: input (primary) on the center, output (secondary/ies) on the outer paths. The airgap will destroy the efficiency, but the principle should still stand

Where can you buy large ferrite cores?

M@rcel,
If you're saying to leave the existing center core material and just introduce airgap(s) that wouldn't be working towards the principle. The idea is to prevent the flux due to the secondaries' back EMF from going back into the center coil. Airgaps would allow the flux from the primary to enter the secondary core but also allow the flux from the secondaries' core back EMF back into the primary core.

So a low permeability (i.e. high reluctance or high resistance to magnetic flux) core for the primary and the opposite for the secondary core will allow the primary's flux to enter the secondary but prevent or at least reduce the secondary's flux from going back into the primary. It's basically a diode for magnetic flux. That's why the permeability 200 for the primary and 100,000 for the secondary in Thane's diagram.

What you can see on the link that Larry gave before:
  http://info.ee.surrey.ac.uk/Workshop/advice/coils/mu/#mur
is that the permeability (more properly the relative permeabity, also ur) for ferrites is around 10,000 but for grain oriented silicon steel it is higher, 40,000 in the table. I'm no expert though, so I don't know if 10,000 is good enough or if new problems are introduced if you go too high. I know there's a frequency issue, lower permeability for higher frequency and vice versa.
-Steve
http://rimstar.org
PS. Stefan, I'm having problems downloading Larry's attachment. Can you have a look at my post re this at: http://www.overunity.com/index.php/topic,3893.15.html, #28 in the "Re: Forum access problems" topic?

[M@rcel]

yes, I understand that, but I thought an airgap in the centre would do the same. The primary flux has no alternative than going through the gap. The secondary flux takes the easy way, all around the outer core.

[LarryC]

Or create an airgap in the center?   

Edit: input (primary) on the center, output (secondary/ies) on the outer paths. The airgap will destroy the efficiency, but the principle should still stand

M@rcel,
If you're saying to leave the existing center core material and just introduce airgap(s) that wouldn't be working towards the principle. The idea is to prevent the flux due to the secondaries' back EMF from going back into the center coil. Airgaps would allow the flux from the primary to enter the secondary core but also allow the flux from the secondaries' core back EMF back into the primary core.

Hi All,

When I first seen the Bi-Toroid Transformer drawing I wondered about the air gap between the primary member and the toroid. Didn't know wheather it was intentional or a drawing problem. I ended up assuming that it was intentional due to the statement about the primary being magnetically isolated from the secondary.

I pictured it like this: If the primary bar was an electromagnet without any other metal around, all the magnetic flux would just expand around the bar to complete the N S connection. If it was placed inside a toroid all the flux would complete the N S connection by jumping into the toroid because the small air gap at each end is a much easier path then the longer air path around the primary bar. In a normal toroid all flux will contain itself completely in the core. If back emf is created in the toroid, the flux would not want to jump the air gap to the primary bar because the toroid is still the easiest path. That's why I thought the design was pure simple genius, but I still wonder about the need for the big permeability difference.

Guys, please feel free to correct me if any of this is illogical as you know what assuming causes.

@Luc,

If you haven't heard from Thane yet, maybe you could ask about the air gap in the drawing.

Thanks, Larry

[M@rcel]

I think that the use of the airgap could be sufficient to prove that the bemf flux doesn't go through the primary. For practical purposes this may however cause too much loss. Therefore a toroid with a higher perm. than the center (which now has no gap) would be better.

my 2c

[gotoluc]

@Luc,

If you haven't heard from Thane yet, maybe you could ask about the air gap in the drawing.

Thanks, Larry

Larry, that was one of my questions also plus the permeability numbers. I will be talking to Thane tonight!...our time is E.S.T.

Luc