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Re: Why does running an NST on an async gap kill it?
Original poster: "Gerry Reynolds by way of Terry Fritz <teslalist-at-qwest-dot-net>" <gerryreynolds-at-earthlink-dot-net>
If you look at the RC portion of Terry's filter (1000 ohm series R and .0006
uf effective shunt capacitance to ground, it is a one pole low pass filter
with a cut off frequency of 265 KHz. Any frequency below this will
essentually be unattenuated by the filter, especially 60 Hz resonance
effects. I believe the RC portion of the filter is intended to only filter
the high frequency harmonics of the spark gap firing from getting back to
the NST. It looks like the resonance frequency of the tank (assuming
resonance < 265 KHz) will not be attenuated much. However, the conduction
resistance (~2 ohms) of the main spark gap will attenuate much of the tank
voltage. I have scoped the voltage across a static gap and found only RF
hash during conduction (measurement is a little tricky to keep RF fields out
of the measurement loop). The magnitude of this hash was maybe a couple
I believe, resonant charging effects is the main culprit on killing the NST
when the spark gap of an ARSG fails to fire at the appropriate time
(especially if the ARSG is not turned on before the NST is turned on). The
MOV's and safety spark gap will help clamp this overvoltage resonant
I'm wondering, if the safety spark gap was moved to be directly across the
NST bushings and ground, if the NST would be better protected. Since the
charging current is coming from the NST, the sooner this can be shunted the
better. . I have to think what this would do to the 1000 ohm power
resisters since any Cp energy would then be dissapated in the resisters.
Maybe this isn't such a good idea. Comments??
Ft Collins, CO
> I can personally attest that the MOVs will fail if you don't set (forget)
> to properly set the safety gap....
> The wirewound resistors in Terry's filter design do have a fairly large
> inductance. I remember trying to measure it once (the 1kohm, 100W
> Ohmites), but the RLC meter I was using had issues with the large
> resistance, and wouldn't give me reliable results.
> Another aspect of async gaps killing NSTs is that when used in conjunction
> with near resonant sized capacitors, resonant rise can overvolt things if
> the gap fails to fire due to falling on a voltage null. I would expect a
> good "Terry" style filter and LTR capacitors to significantly reduce NST
> deaths from use with async gaps.
> Mark Broker
> Chief Engineer, The Geek Group