How do I make my HV chokes?

To: "'Tesla List'" <tesla-at-pupman-dot-com>
Subject: How do I make my HV chokes?
From: Tesla List <tesla-at-stic-dot-net>
Date: Fri, 20 Feb 1998 23:33:48 -0600
Approved: tesla-at-stic-dot-net

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From:  Bill Lemieux [SMTP:gomez-at-netherworld-dot-com]
Sent:  Friday, February 20, 1998 12:02 PM
To:  Tesla List
Subject:  Re: How do I make my HV chokes?

> An air core inductor as described above would yield only about 0.14 mH

Admittedly two 140uH inductors is only 350ohms at 200+ kHz.  But keep
reading...

> and there's no need to use wire as large as 18 AWG.

Agree.
 
> More importantly, adding this meager inductor in series with the
> HV xfmr, whose secondary is hundreds of HENRIES just by itself, can do
> nothing to keep the high frequencies on the outside of the xfmr. 

Consider this: the inductance of the NST secondary (which, by the way,
is almost certainly less than 1H- have you measured it?), will
effectively block the RF _current_ from travelling through the NST
secondary.  This means that, barring a protection network, the outer few
turns of the secondary will see a very high
voltage at a frequency the insulation was never meant to live with.

An external blocking inductor (of rather higher inductance than Mr. Cox
suggests)
will help block RF currents, and thus, prevent the NST secondary from
seeing high
RF voltages.

> What's
> needed is a bypass capacitor to form a low pass filter, IN ADDITION to
> the series damping R and a MUCH larger inductor.

I agree.

> To have a low pass
> filter, there must be some circuit element routing the HF energy to
> ground, and a series-only L-R circuit has none.

Doesn't mean an inductor can't block RF currents, it just means that
it's not
as good as a pi network or full LC low pass filter.

> I have seen advice advocating L-and/or-R-only protection networks posted
> to this list many times but have not seen any rationale for using these
> configurations beyond anecdotal "I've been using this and had no failures
> yet".  Can anyone defend series-only protection networks in a more
> analytical manor?

I suspect the only reason would be cost- it's much easier to make an
inductor
than a HV RF cap.  Also, one has to be careful of the bypass cap
values.  I
saw nearly continuous arcing on the lightning arrestor of a pole pig
when I
added a 300pF vacuum transmitting cap as a bypass on the bushing.  (on
the 
bushing side of a sizeable blocking inductor)

On the whole, I agree with you, but I don't think you should discount
the value
of inductors alone, if they are sufficiently large.  I use largish
toroidal 
ferrite cores wound with HV wire, and no bypass caps with good luck.

And just between you and me, so does DC Cox on his smaller coils- I had 
occasion to take one apart recently.  He also uses PVC pipe for his
secondary,
something which he once tried to tell me on the phone was completely 
unacceptable.  I have learned to take what he says with a grain of salt.

-Gomez

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