Ferrite U core question

If you take 2 x tv flyback transformer cores, join the square sides together without an air gap and wind the primary and seconday on the round sides then would this work as a DIY flyback transformer?

I would presume the primary and seconday coils would be more loosely coupled due to the greater distance between them.

I would presume the primary and seconday coils would be more loosely coupled due to the greater distance between them.

Click to expand...

Very loosely. Do you expect any positive effect?
What do you assume for the primary inductance? Does the device effectively work as a transformer?

FvM said:

Very loosely. Do you expect any positive effect?
Does the device effectively work as a transformer?

Click to expand...

OK you have answered my question quite emphatically then that it wont work.

Can you breifly explain why you think it wont work given that one of the cores is in two halves with a tiny air gap between them.

I was assuming the two u cores back to back, with a tiny air gap, would behave like the two halves of one core. In effect I was assuming I would end up with an E core.
And that with the windings at opposite ends the device would behave a little more like a tesla coil with its very loose coupling between primary and secondary.

Perhaps I should play around with one of these instead: **broken link removed**

FvM said:

Very loosely. Do you expect any positive effect?
What do you assume for the primary inductance?

Click to expand...

I have no clue FvM. Be patient with me matey as I have only just decided to have a ***** at making a flyback transformer and I have by no means got my head around the mathematics of it all yet. Think I need to get myself an appropriate book on this subject for a little bed time reading. Do you know any really good websites on this subject form a raw beginners point of view?


I believe I have sort of got my head around the capacitance vs resonance issue after a bit of reading. My random wound flybacks have a capacitance (as measured with my multimeter) of about 2nF or something like that. Their resonant frequency was quite low as you can imagine - it took a 690nF - 1uF timing capacitor to produce about a half centimeter arc. Not bad all the same but I would prefer higher frequency.

I have been reading about winding techniques and found one website where they wind a series of 3mm wide coils, embedded in epoxy compound, and then stack a number of such disks on a core.

I am trying to figure out a way to replicate this that does not involve epoxy which is a pain.......at least the two part glues you get at the likes of Bunnings which are not fluid enough to fill all the gaps between the wires and start hardenihng too quickly.

Have been toying with the idea of using petroleum jelly as before with disks of over head transparency to divide the coils. I would have to freeze the petroelum jelly before winding the next coil to prevent the last coil from collapsing before I finish winding the next one. Got some petroleum jelly in the freezer now actually to see if it solidifies enough. An alternative might be candle wax but how do you apply that while you are winding.

I'm not even sure if I understand your idea correctly. I presume you have an UR type 2 core or similar.
Joining the "square sides" (yokes) would give a "H"-shaped core. But where do you want to place primary and secondary windings on the 4 limbs?

FvM said:

I'm not even sure if I understand your idea correctly. I presume you have an UR type 2 core or similar.
Joining the "square sides" (yokes) would give a "H"-shaped core. But where do you want to place primary and secondary windings on the 4 limbs?

Click to expand...

I am beginning to understand why my back to back ferrites wont work....

To combat this, transformers that are subject to DC in the windings use an air gap in the core, so it is no longer a complete magnetic circuit, but is broken by the gap. This lowers the inductance, and means that a larger core must be used because of the reduced permeability of the core material due to the gap. An air gap also increases leakage inductance because of the flux "fringing" around the gap, and resistive (copper) losses are increased as well, because more turns will be needed.

Click to expand...

https://www.pocketmagic.net/?p=1913

The problem FvM is how to acheive the above with something other than standard araldite glue. When you mix this stuff it is very viscous and does not flow into the gaps between the wires particularly well and it starts setting within about 10 minutes or so that in no longer flows at all. It would be useless for the above purpose.

I have seen that there is moulding epoxy from a website, and that would possibly be ideal. But it is only available in 500ml minimum. I think I would like to try a small amount of the stuff before committing to such a quantity. So where the hell would you get a small amount of such an epoxy with respect to a retail store.

Other than that I have thought I could use vaseline but have permanent rigid plastic dividers between the windings disks. But such dividers would take up a substantial amount of space between the winding disks on those small ferrite cores from tv flybacks. So I would have to find a larger ferrite core to use this method.

But if I could get the vaseline to solidify sufficiently for long enough for me to get my bobbin onto my winding motor and wind the next disk before popping it back in the freezer. Once all the winding disks are done and the bobbin is tightly packed with windings then I could allow the vaseline to soften again. If this worked then the dividing disks could be overhead transparency sheet and they would not take up a signficant amount of space on the ferrite core.