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While trying to disassemble some SMPS transformers for rewinding I have managed to break the ferrite core. I broke the 'arms' of the 'E' half.

Also, I found broken core transformers in apparently working SMPS. After I removed the surrounding adhesive tape, I saw the core broken in two.

If I glue the pieces together will it have the same performance? The question applies to 'E' and 'ETD' (cylindrical) cores with air gap of 0.5 - 1.5 mm.

And what is a good method for ungluing a SMPS transformer's core?

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    \$\begingroup\$ Does it have an air gap? Heat should destroy the adhesive. Assuming you've stripped to bobbin, wire, tape etc. off, reasonable temperatures won't affect the ferrite, but there still might be some toxic fumes emitted. \$\endgroup\$ – Spehro Pefhany Apr 21 '14 at 18:52
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If the transformer already has an air gap of ~1.5 mm, an additional glue gap of 0.05 mm will likely make no difference.

If the transformer was glued before you disassemble it, it should be broken right where it was glued, cleaned to bare ferrite and re-glued in the same way. It's likely to keep its performance close to what it was.

If you break the core in some random place and re-glue it, you introduce an additional gap. It depends on the transformer's and the SMPS's design how much it will affect performance.

I have unglued a few transformers putting a knife at some of the glue joints and hitting it with something about 100 g heavy. It succeeded in about half of the cases.

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    \$\begingroup\$ I'd also like to add that just regular super glue (cyanoacrylate), preferably the very runny kind, is the glue of choice for ferrites and ceramics in general. Most other glues, especially contact-type, will not be able to bear the heat and stress. \$\endgroup\$ – user36129 May 5 '14 at 13:38
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This blog entry explains it better than i could in this short section. http://jimlaurwilliams.org/wordpress/?p=3337

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    \$\begingroup\$ Welcome to EE.SE! While it's fine to post external links in an answer, it's recommended to add an excerpt that helps the reader and remains if the link is broken. How to answer. \$\endgroup\$ – clabacchio Dec 3 '14 at 9:48

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