RE: Protection Circuit

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Lau, Gary by way of Terry Fritz <twftesla-at-qwest-dot-net>" <Gary.Lau-at-compaq-dot-com>

The high inductance of the NST's secondary makes it a virtual open-circuit
as far as high frequencies go, so that can be ignored.  But, if you really
want to explain all of the considerations that go into such filter
circuits, it may end up over the heads of high school teachers.  And in
fact, there is still not total agreement on just how necessary such filters
are and just what it is that the filters are protecting against.

Since most of our coils operate at a higher frequency than 88 KHz, it's not
clear that such a filter would be very worthwhile.  I believe the merit of
such filters is in attenuating the much higher voltage and higher frequency
transients generated during the zero-crossings of the spark gap.  This is a
seldom mentioned phenomenon that I describe on my NST filter page,
http://www.laushaus-dot-com/tesla/protection.htm.  But if I was attempting to
describe it to anyone not insanely interested in Tesla Coils, I would dumb
it down to a one-liner, just saying that it attenuates high frequency
transients excited by the spark gap.

Chokes used to be favored for such protection circuits because a much
sharper low pass filter can be built using chokes.  But this was before
circuit simulators were widely used.  L-C filters work well when
_continuous_  signals are applied.  But it was discovered that when used in
a spark gap circuit, inductors tend to oscillate at voltages no less
hazardous and possibly more-so than an unprotected circuit.

Regards, Gary Lau
MA, USA

 -----Original Message-----
From: 	Tesla list [mailto:tesla-at-pupman-dot-com] 
Sent:	Monday, April 22, 2002 7:16 PM
To:	tesla-at-pupman-dot-com
Subject:	Protection Circuit

Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<dlynch-at-reyercorp-dot-com>

Hello,

I am studying various circuits from several people and it seems the
protection circuit is 
a low pas filter, like Terry's small coil specs, where he uses 360pF and 5K
resistor 
which gives a 3dB point of 88.4kHz. Is it that straight forward or do you
really need to 
factor in the secondary inductance of the NST?

My daughter is doing a High School project and I am trying to explain each
section of 
the circuit based on what I use to know and have long since forgotten.

I have also seen RF chokes used in the protection circuit. Is it really
necessary?

I have a OBT 10Kv 23mA and so we are on the low end of the power spectrum.
I've 
ordered some 400pf 30KV doorknobs and so I guess a 5K resistor seems just fine.

The more we include, the more has to be explained and I want to keep it as
simple as 
possible so it doesn't tax my brain too much.

Thanks.

Don