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Re: center tapped transformers

Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>

At 07:41 AM 11/3/2005, you wrote: 
Original poster: "Qndre Qndre" <qndre_encrypt@xxxxxxxxxxx>

John,

I'm not talking about grounding one end of the secondary but about not grounding anything on the secondary side. This way I don't see why there should be a voltage potential between the transformer's secondary and the transformer's primary where insulation could break down. If the secondary is held seperate from the primary the circuits are fully galvanically seperated.

Galvanically isolated, but certainly not AC isolated. There's significant capacitance from windings to core. Let's say, for grins, its 1 nF... At 60 Hz, that's about 2.6 Meg impedance, which sounds like a lot, but at 7.5 kV, that would be about 2 mA of current flow.

At higher frequencies (like, say, 120 Hz, from the break rate of a gap), the current could be even higher.

"real" isolation transformers have a metallic shield between windings and core for just this reason.




From the glorious pupman archives:
http://www.pupman.com/listarchives/2001/October/msg00752.html Discussion of RF stress

http://www.pupman.com/listarchives/1999/June/msg00046.html Measurements of C from bushing to ground, cites 2nF for a 60mA NST.

http://hot-streamer.com/TeslaCoils/MyPapers/NSTWindingStress/NSTWindingStress.html some tests by our moderator 
Q

----Original Message Follows----
From: "Tesla list" <tesla@xxxxxxxxxx>
To: tesla@xxxxxxxxxx
Subject: Re: center tapped transformers
Date: Wed, 02 Nov 2005 21:34:12 -0700


Original poster: FutureT@xxxxxxx

In a message dated 11/2/05 4:22:09 PM Eastern Standard Time, tesla@xxxxxxxxxx writes:

Q,

The secondary insulation of NST's and OBIT's cannot withstand
the voltage stress of what you're suggesting.

John