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Old 27th June 2004, 04:36 PM   #1
lgreen is offline lgreen  United States
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Location: San Diego, USA
Default Help with Shorted Xformer

I believe that I accidently punctured a rather large toroid, about 400VA with a teeeny tiny puncture. The primary now seems shorted to the secondary, since with no load (meaning nothing connected to any of the secondaries) the transformer hums loudly and draws enough current to blow a 6A slow blow fuse immediately. The puncture was probably caused by a screwdriver, so its near the surface under the wrapping and very small, I think one primary winding contacts one secondary winding, shorting out the primary.

Is this something that can be fixed, if so how would you recommend going about the fix?
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Old 27th June 2004, 04:58 PM   #2
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Location: South Sweden
First, measuring with a multimeter tells if you really have a short circuit between primary and secondary. I have never had this type of accident, and I have aswell run screwdrivers through the insulation ( but then the secondary was only damaged)

Ususally it is difficult to cause shorts this way, in my experience, since there is always insulation tape stuck in between, and the wire itself is insulated with a thin layer of laquer, that prevents contact. It is sort of self healing when the insulation tape is damaged (not in the real sense). Once I wanted to measure on the primary, and used a sawing needle to puncure through insulation in between secondary turns, and boy , I had to punch hard to get a contact at all.

You could also have casued a short between two secondary turns. This is more likely, and is almost impossible to measure with a multimeter, since the secondaries are in itself low ohmic.

If you remove the outermost layer of tape from the transformer at the point where the screwdriver hit, try to pry the secondary windings apart and se if you can find any damage to the insulation on the primary. A visual inspection always helps.
This could also lead to a possible short on the secondary to break up.

And for gods sake, remove it from mains voltage when doing this!

If it is a short circuit on the primary, the only way I can think of fixing this if it is a is by unwinding the secondary, reinsulate the primary, when making sure that the turns of the primary dont touch each other. Then rewind the secondary back on. Considering the labour for this( it is a lot!) , and also the fact that the transformer insulation can no longer be guaranteed, I would recommend to scrap it. it is not that expensive to risk working with mains voltage on a transformer .

If it is on the secondary it is fixable. If it is on the primary, use it as a boat anchor.

good luck

listen! Can you hear that ...
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