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Old 5th January 2012, 12:56 AM   #1
tvrgeek is offline tvrgeek  United States
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Default effect of shield shorted winding

If I snake a copper foil around the winding's of a standard e-core transformer, solder carefully, does it have any effect on the output voltage? Should this shorted winding be isolated or connected to ground? Any guideline on how heavy the foil should be with respect to the transformer voltage/current?
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Old 5th January 2012, 01:01 AM   #2
dhaen is offline dhaen  Europe
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Why would you want to? It'll destroy the transformer..
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Old 5th January 2012, 01:18 AM   #3
dhaen is offline dhaen  Europe
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Lightbulb It just occurred to me

I think you mean around the whole transformer (all 3 legs of the "E"), in which case it will not act as a shorted turn and there will be no change in voltage. It should be grounded to chassis.
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Old 5th January 2012, 06:57 AM   #4
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Quote:
Originally Posted by tvrgeek View Post
If I snake a copper foil around the winding's of a standard e-core transformer, solder carefully, does it have any effect on the output voltage? Should this shorted winding be isolated or connected to ground? Any guideline on how heavy the foil should be with respect to the transformer voltage/current?
That sounds like shield for the transformer, however if one turn of cooper foil and you solder it, it will act like a winding, and you have just shorted that winding, and will cause a dead short circuit at transformer.

So, you should make it 80% filling without shorting. this way its shield and goes to ground
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Old 5th January 2012, 09:52 AM   #5
DF96 is offline DF96  England
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Immediately around the windings, or outside the core?

The first is called a shorted turn, and will stop the transformer working. If left connected it could destroy the transformer through overheating in the primary.

The second is called (in the UK) a belly band and can reduce stray magnetic fields.
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Old 5th January 2012, 09:59 AM   #6
AndrewT is offline AndrewT  Scotland
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DF96 is correct.
Be very careful that you recognise the difference between belly band/gauss band and shorted turn.
Inter-winding screen also has a physical geometry that prevents it becoming a shorted turn.
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Old 5th January 2012, 07:16 PM   #7
tvrgeek is offline tvrgeek  United States
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The goal is to reduce the stray fields within the chassis when you have a cheap existing transformer, not a high dollar torrid inside a mu-metal can. I have a couple of instrument problems where I need my test set to be far cleaner than what I am testing and batteries are not an option.

Not an inner winding screen as I am not taking about trying to wind my own transformers. Literature has shown it both around the windings and and completely around the legs. The latter makes more sense. It is my understanding from the various WIKI pieces that a shorted turn does create a current with subsequent field, that opposes the leakage field. It is only left unsoldered because they are cheap. Even the pooge articles talk about soldering the band in the Haflers. ( I need to crack one of mine open to see how the band is situated. It makes sense that it would have a negative effect on the transformer. If current flows, that energy came from somewhere. The question is how much? Guess I just have to try it. I had not heard the term belly band. Something to search . Thanks.

I can't see how an 80% wrap would do a thing as copper has about the same permeability as air. Yes, I recognize transformers are actually a horribly complicated device.
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Old 5th January 2012, 07:38 PM   #8
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by tvrgeek View Post
The goal is to reduce the stray fields within the chassis when you have a cheap existing transformer, not a high dollar torrid horribly complicated device.
That's called a woman.
Back to the subject: the belly band is mainly useful to reduce dispersion fields, caused by the leakage inductance.
But the leakage inductance is only exercised when there is a large secondary current demand (=heavy load).
If the stray field is caused by an insufficiently dimensioned magnetic circuit that saturates, the most common instance in cheap transformers, a belly band will do nothing (or thereabout) to contain the stray field.
A complete, closed conductive container could, but then it's probably simpler and cheaper to opt for a decently designed transformer.
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Old 5th January 2012, 07:49 PM   #9
DF96 is offline DF96  England
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It's not the permeability of copper which does the trick, but conductivity. The copper band is a shorted turn, but outside the main magnetic circuit. It only affects stray flux. In my experience it tends to be seen on mains transformers in professional items such as test equipment, rarely on domestic stuff.
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Old 5th January 2012, 08:17 PM   #10
tvrgeek is offline tvrgeek  United States
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DF, Exactly what I am trying to do. The test equipment just happens to be an interface box between my probes, A to D, and my bench amp, a DH 120. I am trying to get the residual 60 and harmonics, some quite a bit higher than the 60, down to the system noise floor of about -130dB @ 1V RMS. The interface box I run on batteries right now, but they don't last long and that is totally out of the question for the amp. I hope in the process to learn a bunch on how to build a really decent power supply. Note I am also dealing with the rectifiers, caps, physical construction, and everything else. This is no quick band aid effort, but it is my usual really cheap-skate effort. It is just one of the many pieces in the chain.

So, the suggestion is the external band should not effect the transformer output? I guess I'll know when I find some copper flashing remnant I can afford. 50 feet of 14 inch wide is not cheap experiment range. That was all I could find today.
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