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RE: Blown tranformer

Original poster: "Lau, Gary" <gary.lau@xxxxxx>

I don't agree with the posted advice on how to construct an RF filter.

The secondary winding inductance of an NST measures in the THOUSANDS of
Henries.  Adding a few microhenries in series with each winding will
accomplish nothing useful.

I was unfamiliar with ignition ballast resistors so I had to Google
that.  They would appear to range in value from 1.3 to 1.6 Ohms.  Given
that the resistance of an NST's secondary is several KOhms, adding such
a low series resistance will accomplish nothing useful.

Even if an L-C low pass filter were used, this would only be useful for
attenuating a steady-state signal.  When L-C filters are used with spark
gap coils, the series inductors will ring at hazardous voltages each
time the gap fires, and the NST will experience worse RF than if no L-C
filter were used.  Adding a resistor (typically several KOhms) in series
with the inductors will reduce their Q and reduce their ring-duration,
but the initial oscillation is just as "loud".

The current wisdom for constructing an RF filter is to use an R-C
configuration.  About 500pF from each NST HV bushing to RF ground, and
about 1K from each HV bushing to the main gap terminals.

I whole-heartedly agree that a PROPERLY SET safety gap should be used in
addition to any filter, as each protect against different hazards.  My
complete thoughts on this topic are on my web site:

Regards, Gary Lau

> Original poster: jdwarshui@xxxxxxxxx
> You can make an  r.f. filter by winding two inches of 28 gauge on a
> toilet paper core. Place this in series with a generic ignition
> ballast resistors ( used on older cars ) Place on each 15k output.
> A safety gap can be a simple L bracket from each 15k output to the
> case or to a ground. Don't open this gap any more then you absolutely
> need to (less then 1/2 inch) If your coil is out of tune they will go
> off like mad (which is good!).