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Registered Member #1321
Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843
I was thinking of the situation where the OD of the round bobbin would be equal to the length of a side of the square bobbin...in which case the square bobbin would would require more wire per turn. In the case where the diagonal of the square bobbin = the diameter of the round bobbin then yes it would be the other way around.
I don't see it as much of an issue especially if you're using magnet wire.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
jpsmith123 wrote ...
I was thinking of the situation where the OD of the round bobbin would be equal to the length of a side of the square bobbin...in which case the square bobbin would would require more wire per turn. In the case where the diagonal of the square bobbin = the diameter of the round bobbin then yes it would be the other way around.
I don't see it as much of an issue especially if you're using magnet wire.
There seems to have been some misunderstanding here.
I was discussing the ferroxcube cores you mentioned in the first post above, and whether it was better to use a square section or round former with them.
As I said, I've been working on some segmented square section bobbins for my 30mm square cores, for three reasons, it uses less wire, you get more turns for the same window, and, I assumed, that it would be more efficient electro-magnetically.
However, if both yourself and Patrick are of the opinion that performance will be exactly the same with round bobbins I 'may' change my design.
I appreciate that you are striving to achieve a high quality design, so I felt that my comments were relevant here.
I was under the impression that the closer the windings are to the core, the better the coupling, and therefore a square section bobbin would perform better, however, it seems that you both disagree.
Registered Member #1321
Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843
Having the windings a little closer to the core or a little farther away doesn't matter very much as far as coupling (ultimately to the primary) is concerned. This is because the permeability of the core is so much higher than that of free space...IOW, the flux outside the core is negligible compared to the flux inside the core for any practical situation.
If the winding is going to be something like this:
Then, IMO, you're better off (theoretically) to put as much space as you practically can between the core and the windings, since the HV across the winding can couple capacatively to the core. (This is one down side of that type of winding, IMO). Since ferrite has a small but finite conductivity and a fairly high permitivity (IIRC), it's possible to have significant "dielectric" power loss in the core and even corona due to field enhancement at the sharp edges of the ferrite.
Ash Small wrote ...
jpsmith123 wrote ...
I was thinking of the situation where the OD of the round bobbin would be equal to the length of a side of the square bobbin...in which case the square bobbin would would require more wire per turn. In the case where the diagonal of the square bobbin = the diameter of the round bobbin then yes it would be the other way around.
I don't see it as much of an issue especially if you're using magnet wire.
There seems to have been some misunderstanding here.
I was discussing the ferroxcube cores you mentioned in the first post above, and whether it was better to use a square section or round former with them.
As I said, I've been working on some segmented square section bobbins for my 30mm square cores, for three reasons, it uses less wire, you get more turns for the same window, and, I assumed, that it would be more efficient electro-magnetically.
However, if both yourself and Patrick are of the opinion that performance will be exactly the same with round bobbins I 'may' change my design.
I appreciate that you are striving to achieve a high quality design, so I felt that my comments were relevant here.
I was under the impression that the closer the windings are to the core, the better the coupling, and therefore a square section bobbin would perform better, however, it seems that you both disagree.
Registered Member #3215
Joined: Sun Sept 19 2010, 08:42PM
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Posts: 780
the last transformer I wound arced from the wire to the core after the gap had a massive purple breakdown of fierce arcs
gap arced like twenty seconds, then an arc jumped from the core to the windings (on a segmented former indeed) and blew the secondary
that was an EE core of 4*4cm, 1cm cross-section ferrite, 2000 turns secondary and 2*10 turns primary fed with 12V 5A half bridged, so nothing ultra powerful and still massive nasty effects
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Shrad wrote ...
the last transformer I wound arced from the wire to the core after the gap had a massive purple breakdown of fierce arcs
gap arced like twenty seconds, then an arc jumped from the core to the windings (on a segmented former indeed) and blew the secondary
that was an EE core of 4*4cm, 1cm cross-section ferrite, 2000 turns secondary and 2*10 turns primary fed with 12V 5A half bridged, so nothing ultra powerful and still massive nasty effects
If this was a gapped core, EE, then you probably had fringing magnetic fields causing the copper to heat by current, when it got red hot the enamel was burned off the causing the further visible arcing between windings and then the core. This is why I go to great lengths to avoid putting turns near the gap, or avoid a gapped core if at all possible. This is a common failure mode with foil windings.
Registered Member #1321
Joined: Sat Feb 16 2008, 03:22AM
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Posts: 843
As I recall I simulated it a while ago (unfortunately I can't find the files), but I couldn't find much info on ferrite conductivity and permittivity or how they vary with frequency, so the results I got at that time were based on guessing at the values and may be way off, accordingly
I think I can correctly say that, electrically speaking, the core looks like a resistive load, capacitively coupled to the HV secondary, but exactly how bad the losses and the field stress will be, I don't know for sure.
Ash Small wrote ...
That's an interesting point about HV capacitive coupling to the core, jp.
Registered Member #1321
Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843
I just did a quick simulation again and here are the results showing the high efield at the edges of the ferrite near the ends of the windings:
This is the result using values of 0.5 S/m for the conductivity and 10 for the permittivity. I don't know if these values are very representative or not. Anyway, it's showing a low power dissipation of only a few mw, but a field well over the corona threshold in air.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
jpsmith123 wrote ...
I just did a quick simulation again and here are the results showing the high efield at the edges of the ferrite near the ends of the windings:
This is the result using values of 0.5 S/m for the conductivity and 10 for the permittivity. I don't know if these values are very representative or not. Anyway, it's showing a low power dissipation of only a few mw, but a field well over the corona threshold in air.
JP what program did you use to generate those pics?
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A New Idea For Ferrite HVHF Transformer Construction
