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Old 13th July 2005, 10:39 AM   #1
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Default External screen heats up tranformer rapidly

Last night I added an electrostatic screen (3M copper tape) on the outside of my filament transformers. And soldered the outside screen to the internal electrostatic screen that runs between primary and secondary. To my suprise the transformer got warm real fast????

I don't understand why that would happen. So I removed the extrernal screen and voila..did not get warm anymore. I realize now after typing this that I should have just disconnected the external from the internal screen first...

Anyway..anyone have an explanation?
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Old 13th July 2005, 11:02 AM   #2
Nisbeth is offline Nisbeth  Denmark
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If you connected the two screens together, it sounds to me like you're making a shorted turn


/U.
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Old 13th July 2005, 11:03 AM   #3
heater is offline heater  Finland
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Seems to me that if you wrap copper tape around a transformer you have effectively added another scondary winding of one turn.
So now your primary is pushing current around this new short circuited secondary thus getting loaded and hot.
Wrap one turn of tape but keep the ends insulated from each other where they overlap so you don't have a current loop and you would be OK.
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Old 13th July 2005, 11:26 AM   #4
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Quote:
Wrap one turn of tape but keep the ends insulated from each other where they overlap so you don't have a current loop and you would be OK.
Yes..I think that would be the solution. You are right ofcourse..it's another secondary but shorted now..
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Old 13th July 2005, 12:58 PM   #5
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Did you put it around the WINDINGS, or around the whole TRANSFORMER? Big difference

Flux goes one way up through the center limb (which the windings are wrapped around), then goes back down the two outside pieces (or just one in a C core). 2 - 1 - 1 = 0 so a "shorted turn" around the outside of it all will only squash leakage, while the majority of the field happily circulates inside the core where it belongs.

Tim
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Old 13th July 2005, 01:08 PM   #6
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I had it on the windings

like in the pic
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File Type: jpg ext_screen.jpg (34.2 KB, 88 views)
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Old 13th July 2005, 06:02 PM   #7
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Yeah, that'll pull a few tens of amperes! Wrap it in the same position but around the outside of the whole transformer and it'll be good.

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Old 13th July 2005, 08:06 PM   #8
mzzj is offline mzzj  Finland
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Quote:
Originally posted by Bas Horneman
I had it on the windings

like in the pic

Classical way to get couple of hudreds amps from toroid is to fix it to chassis with trough-bolt that is in contact with metal chassis from both ends
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