Re: NST Filter and Circuit Configuration?
Subject: Re: NST Filter and Circuit Configuration?
From: Terry Fritz <twf-at-verinet-dot-com>
Date: Mon, 14 Dec 1998 20:41:26 -0700
A number of comments:
Beware that some of the circuits I show in those papers are test circuits
and are very poor for filtering. Be sure to read the text so you don't try
one of the "bad" examples! Since those circuit tests were performed,
better circuits have been developed and I would no longer recommend the
circuits described in those papers. A more modern schematic diagram is at:
Here are some "rules" I would follow:
Always use safety gaps! I use electronic clamps at the transformer output
and high power gaps between the filter and the primary circuits. The
resonances and variable gap firing times may produce very high voltages in
the primary circuit. Only good properly set safety gaps will protect the
transformer. With 1/2 inch diameter electrodes, you want about a 3/16 inch
spacing to ground on each leg of the transformer. If you get arcing, DO
NOT open the space wider. That would be bad :-)
Avoid wire wound resistors. It has been found that the powerful pulses
that the resistors see will slowly damage the wirewound types of resistors.
Over time, the wire will form balls of metal as the element is constantly
overheated by spikes. Eventually, enough metal will be displaced and the
resistor will open. I have been using the metal film on ceramic core types
like the Ohmite L25J, L50J, L100J, and L225J series (available from
Digi-Key) These have very good high frequency response and they can take
powerful pulses without degrading the resistive elements.
Always place the spark gap across the transformer's output. When the gap
fires, it will short the output of the transformer and shield it from the
powerful signals created in the primary circuit. Many diagrams will show
the primary cap across the transformer. However, we now know that is NOT
the way you want to do it.
Avoid inductors in the filter. Simply placing an inductor in series with
the transformer will do no good. The inductance of the filter is no match
for the giant inductance of the transformer's secondary winding. Any
spikes will go right through an external inductor and create voltage spikes
on the outer secondary windings or the transformer. There are LC circuits
that are effective but they get complex...
You are correct in noting that I do not have ground referenced elements in
my filters (aside from safety gaps that I don't show). There is no real
reason for this other than the fact that these circuits used that parts I
had on hand and they just evolved that way over time. Since the greatest
"common mode" threat is a secondary to primary arc, I rely on safety gaps
to handle that event. The voltages involved there will easily overwhelm
any filter circuit elements (400kV) so I simply try to save the transformer
and the filter's parts from this event with high power safety gaps. Other
parts of the primary circuit are also designed to survive direct hits. The
gap motor and fan have grounded cases and the shaft of the rotor is
grounded to prevent bearing damage if something arcs. The caps also have
gaps that will arc if they go over voltage.
Be sure to have very good grounds. Use heavy wire with very good
connections and ground the system to something substantial like a water
pipe close to where it enters the house (don't use your house's AC wiring
ground). Ground rods are very effective too. The worst thing that could
happen is that the high voltages find a way to discharge into the AC
wiring. The primary caps 20kV pulse of thousands of amps can do incredible
damage. The secondary arcs are very high voltage but do not have the very
high currents so grounding is pretty effective if done well.
I can't explain every last detail here and there are significant
details... Resonant charging is very complex and can easily fry the
transformer and caps. However, safety gaps will protect against this.
I posted a diagram at:
This should protect against just about anything on a coil of 200kHz and
above. Lower frequency coils may need higher value filter caps.
At 02:42 PM 12/14/98 -0700, you wrote:
>I have found a source for the resistors and caps I need for a NST filter.
>It appears that one side of the resistors are connected to the cap and the
>other side is connected to the primary circuit. How is it connected? I am
>guessing it's parallel to the transformer output. Is no path to ground
>required for this filter? It's from Terry Fritz's paper on RC filters so
>he may be best suited to answer here.
>As for the rest, how should the primary circuit be set up? I have the caps
>built, and the transformer. Is there a schematic of the recomeded setup
>for an NST powered coil on the net somewhere? I'll be running a 15Kv 30ma
>NST for the power source. If there is no schematic a description of where
>the components are in the circuit would be helpfull.