Toroidal Transformer Failure

[CBP]

I was tearing apart a cheap disco light unit the other day which still worked but I no longer needed. As I was tearing it down I noticed that the toroidal transformer that powers the lamp and control circuit seems to have suffered some sort of issue and I am wondering out of interest what people think is the most likely cause. AFAIK it still worked when I tried it recently so I still think that the winding is intact however the outer insulation seems to have broken down and burned through leaving the copper exposed.

I was thinking overload but why (unless there are impurities/weaknesses) this particular spot on the windings and not at the cable joints or elsewhere if it has burnt enough to damage the copper and melt the plastic? The unit is cheap as DJ lights go so I would say it is hardly any great quality.

Bit alarming really because this was bolted into a metal case with very little distance between the exposed copper and the case however I guess the outer winding is the biggest and as such probably the secondary?

Thanks...here is the photo

[grumpydoc]

Given that it looks localised I'd guess that a turn became shorted, overheated and then managed to cleared the short as the insulation melted

How, exactly, the turn became shorted I haven't a clue.

[peter.mitchell]

Given that it looks localised I'd guess that a turn became shorted, overheated and then managed to cleared the short as the insulation melted

How, exactly, the turn became shorted I haven't a clue.

possibly vibration or incorrect mounting, but to me it is more of a worry the fuse didn't blow!

[Richard Crowley]

It seems to me that a flaw in the very thin enamel insulation, weakened by the traumatic way torroid transformers are wound (first wound onto a transfer loop, and then back around the core) caused a single turn to short and the heat melted that turn, but "welded" the winding back together to "bypass" the toasted turn.

The fuse didn't blow because the transformer continued to operate nominally. The loss of one turn likely resulted in the output voltage going up a fraction of 1%, not something that would blow the fuse, and not something that would even cause anyone to notice, even in unregulated circuits.

[nihilism]

It seems to me that a flaw in the very thin enamel insulation, weakened by the traumatic way torroid transformers are wound (first wound onto a transfer loop, and then back around the core) caused a single turn to short and the heat melted that turn, but "welded" the winding back together to "bypass" the toasted turn.

The fuse didn't blow because the transformer continued to operate nominally. The loss of one turn likely resulted in the output voltage going up a fraction of 1%, not something that would blow the fuse, and not something that would even cause anyone to notice, even in unregulated circuits.

I agree. Shorted turn on the secondary causing a heap of current flow in just that turn burning up the insulation. Then either the short has gone open (or high resistance) again or just that turn has gone open with the shorted section now bypassing it, allowing the transformer to still operate at a fraction lower output voltage.

[CBP]

Thanks for all the replies, very interesting...all of the transformer failure stuff on Google seems to be geared towards power line stuff. The wiring in the whole thing leaves a lot to be desired and a valuable lesson in buying electrical stuff on eBay, the fixing of the (high temperature) lamp holder cable to the transformer outputs with sellotape I think was the best.

[Zero999]

I agree with all of the above and you're right, it probably is the secondary. Even if it still works,  it's probably a good idea to scrap it, as there's a risk it could catch fire.