External screen heats up tranformer rapidly

Bas Horneman · 13th July 2005, 10:39 AM

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?

Nisbeth · 13th July 2005, 11:02 AM

If you connected the two screens together, it sounds to me like you're making a shorted turn

/U.

heater · 13th July 2005, 11:03 AM

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.

Bas Horneman · 13th July 2005, 11:26 AM

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..

Sch3mat1c · 13th July 2005, 12:58 PM

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

Bas Horneman · 13th July 2005, 01:08 PM

I had it on the windings

like in the pic

Sch3mat1c · 13th July 2005, 06:02 PM

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.

Tim

mzzj · 13th July 2005, 08:06 PM

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

13th July 2005, 10:39 AM	#1
Bas Horneman diyAudio Moderator Emeritus Join Date: Apr 2002 Location: The Netherlands Blog Entries: 18	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? __________________ Boatanchor amps forum My "adventures" in audio

13th July 2005, 11:03 AM	#3
heater diyAudio Member Join Date: Jul 2004 Location: Helsinki	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. __________________ For me the past is not over yet.

13th July 2005, 12:58 PM	#5
Sch3mat1c diyAudio Member Join Date: Jan 2003 Location: Milwaukee, WI	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 __________________ Seven Transistor Labs Projects and Resources

13th July 2005, 06:02 PM	#7
Sch3mat1c diyAudio Member Join Date: Jan 2003 Location: Milwaukee, WI	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. Tim __________________ Seven Transistor Labs Projects and Resources

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13th July 2005, 11:02 AM	#2
Nisbeth diyAudio Member Join Date: Jan 2001 Location: Denmark	If you connected the two screens together, it sounds to me like you're making a shorted turn /U.

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