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In Reply to: Re: In a grounded metal chassis, no shielding is needed for TWISTED pair. posted by kurt s on April 30, 2007 at 22:58:09:
Kurt, I do remember you making this point before. I apologize for not responding then. I got distracted by other things, overworked etc. It slipped away.Anyway, I see an ideal transformer (without center tap) as being an ideal differential device. It is sensitive only to the difference in voltage between the two ends of the primary. It is that voltage difference that creates current through the winding, thus creating magnetic flux in the core which couples to the secondary. Any voltage that is common to both ends of the primary (referenced to earth, or the core, or anything you choose) will not result in current through the winding. That is the definition of a differential device.
I agree that a ct might be the best practical solution to real-world problems, but I do not agree that it is a necessity for the transformer to be a differential device.
Follow Ups:
> I agree that a ct might be the best practical solution to real-world problems, but I do not agree that it is a necessity for the transformer to be a differential device.It's worse than that. The CT provides a third current path on the winding, which means equal and opposite currents at the two ends of the transformer are no longer ensured. Consider that when the two ends are equal and opposite, there is no signal voltage at the CT. It is an electrical null at the midpoint of the two ends of the winding. On that basis, any signal current whatsoever flowing through the CT indicates that the winding has becomed unbalanced.
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Minor imperfections (asymmetry) in the transformer create imbalance with respect to the CT. Whereas the CT would normally exhibit no signal voltage because it is halfway between the two ends, any unintentional electrical assymmetry will produce signal voltage at the CT. If the CT is connected to ground, the imbalance will cause signal current to flow to ground from the CT, and currents at the ends of the transformer will no longer be equal and opposite. The same unbalanced condition can result from imperfections in the differential source when the CT is grounded. Without the CT, the transformer offers only one current path, therefore current is forced to be equal and opposite at the ends, perfectly balanced. With the CT, all bets are off.
"It's worse than that. The CT provides a third current path on the winding, which means equal and opposite currents at the two ends of the transformer are no longer ensured. Consider that when the two ends are equal and opposite, there is no signal voltage at the CT. It is an electrical null at the midpoint of the two ends of the winding. On that basis, any signal current whatsoever flowing through the CT indicates that the winding has becomed unbalanced."Now you're advocating no center tap? Okay, how about a center tap with a 100 Megohm resistor to ground. Or will this throw the world out of balance too much ? Just wondering.
Kurt
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