RE: center tapped transformers

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>

At 02:39 PM 11/4/2005, you wrote:
> Original poster: "Lau, Gary" <Gary.Lau@xxxxxx>
>
> There is not a consensus on this question, and there are valid reasons
> for both configurations.
>
> RF ground is what the secondary base is tied to, and should go to a
> ground rod separate from the mains-ground.
>
> If the NST mid-tap is wired to mains-ground, the NST core (connected
> internally to the mid-tap) is at the same potential as the
> mains-neutral.  The NST case is safe to touch (the main reason cited for
> connecting it to the mains-ground).  But if there should be a streamer
> strike to the primary coil, the safety gaps (you have these, right?)
> will fire, channeling the streamer current to the NST case and into your
> mains-ground.  Not healthy. And I don't think the safety angle carries
> any weight as you shouldn't ever be touching the NST when the coil is
> powered.

Except that if you don't connect the greenwire ground, when that same 
safety gap firing will put the HV RF into the 110V line, which is 
even worse than the greenwire ground.



> If the NST mid-tap is wired to the RF ground, there may be a very high
> RF potential between RF ground and mains-ground (neutral wire), and it's
> been suggested that the NST primary insulation may at risk.  But should
> there be a streamer strike to the primary coil, it would be safely
> channeled to RF ground.  While the potential across the NST
> primary-to-core insulation is worrisome, I've never heard of an NST
> failing for a primary side failure.  But the devil's advocate must ask -
> if there was primary-to-core leakage, would that even be detected?  It
> could elevate the RF ground to a mains fault condition.
>
> I personally use the RF ground connection, but I acknowledge that
> neither is perfect under all scenarios.


Why not connect both?  NST case to RF ground (with low impedance 
connection) and to greenwire ground.

Then, the strike/safety gap scenario will return the RF power to the 
bottom of the secondary (where it belongs). If you're worried about 
RF propagating back up the greenwire ground, then put an RF choke on 
the greenwire.


> Regards, Gary Lau
> MA, USA
>
>
>
>
> > Original poster: "Langer Giv'r" <transworldsnowboarding19@xxxxxxxxxxx>
> >
> > when you ground the mid tap, do you earth ground it (rod into the
> > ground) or do you ground it with your house?
> > Thanks
> >
> >
> > >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
> > >
> > >
> > >>Original poster: "Qndre Qndre" <qndre_encrypt@xxxxxxxxxxx>
> > >>
> > >>There is a question I asked myself over and over again since the
> > >>first time I've seen a schematic for a Tesla coil on the net: Why do
> > >>coilers ground the center tap of the secondary of the high voltage
> > >>transformer (NST or OBIT) instead of just doing the opposite
> > >>(insulating the case from ground very carefully or cut the
> connection
> > >>of the secondary to the case)? As far as I know this would be safer
> > >>since touching one of the output wires (or being accidently
> connected
> > >>to one by a streamer) wouldn't electrocute you in this case since
> the
> > >>output terminals weren't hot in respect to ground.
> > >>
> > >>Regards, Q.
> > >>
> >