RE: Trigger xfmr "grounding", and R-C protection networks

["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>

If I may respond to my own post, or maybe just think out loud...

Regarding the possible problem of having the trigger transformer current
passing through the 1-2Kohm NST protection network resistors, I found a
triggered spark gap application note from R. E. Beverly III & Associates at
http://www.reb3-dot-com/pdf/sg_appl.pdf, which specifically states:
"Under no circumstances should a resistor be placed in series with the
trigger plug."  I assume the same goes for anywhere in the xfmr/trigger gap
current loop.

This made me look hard at the possible trigger xfmr configurations.  Since
trigger xfmr's have a limited pri-sec holdoff voltage rating, it means that
the low end of the secondary wants to be connected to RF ground.  So with
an NST where neither side is grounded, the widely used NST
protection-network resistors get in the way.

This made me begin to question the necessity of the R's in the R-C network.
 The question of just exactly what the R-C protection network protects the
NST against has not received a great deal of attention.  My personal theory
is that the offending thing is the brief but high voltage spikes generated
during the gap's zero-current crossings.  If true, bypass caps alone across
the NST secondary will shunt these spikes to ground.  When the gap is
conducting, its impedance is low enough that there are no hazardous
voltages across it to protect against.  And when the gap is not conducting,
there is no tank circuit and no HF oscillations, so again, nothing to
protect against.

So, am I overlooking something, or are the protection network resistors
less than essential?

One further observation of a resistor-less protection network - the 2 NST
bypass caps would be in series and across the main spark gap.  Each time
the gap fires, the bypass caps would be discharged directly into the gap
with no current limiting, so perhaps some very low impedance resistor would
be advisable.  I don't know if this would have any other repercussions
relating to gap operation.

Gary Lau
MA, USA

 -----Original Message-----
From: 	Tesla list [mailto:tesla-at-pupman-dot-com] 
Sent:	Thursday, January 10, 2002 10:04 AM
To:	tesla-at-pupman-dot-com
Subject:	Trigger xfmr "grounding"

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

Another detail of triggered spark gap hookup that is stalling me is, what
to connect the "low" side of the trigger transformer secondary to.  Most
folks using an automotive ignition xfmr connect the low side to RF ground.
This makes sense in that it is the most appropriate place to reference the
trigger electrode voltage to for a field distortion gap configuration.  My
problems with this are twofold.  First, the trigger spark current will have
to travel through the ~1-2Kohm resistor in the R-C NST protection network.
I'm not sure how significant this is to a 30kV pulse.  The other problem is
that the RF ground is typically riddled with HV spikes, relative to the
mains and 3rd wire ground.  It's unclear how long the ignition xfmr pri-sec
insulation will survive this, as I would imagine that in a car, the low end
is chassis-grounded, while the primary may have a couple hundred volts max
on it if there is a C-D ignition.  I'm actually using an EG&G trigger
transformer, with a!
 pri-sec holdoff voltage of 1500V max.

Assuming a trigatron configuration with an NST (neither side grounded), the
best approach appears to be to have a 1Meg HV bias resistor between the
adjacent and trigger electrodes, and a pair of HV doorknob caps from each
side of the xfmr secondary to the adjacent and trigger electrodes.  This
leaves both sides of the xfmr secondary at an AC float.  So, how to bias
the low side of the secondary to a safe voltage /wrt the primary without
coupling too much HF to the primary side?  

Gary Lau
MA, USA