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

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

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

Let me add my 2 c.. The  triegger circuit is a grounded circuit. The NST is
center grounded with 10KV on each side of center.The capacitor discharges
through the primary and the spark gop with as low an impedance as you can to
produce 100 amps or so to pulse the coil. The NTS is not in this circuit. It
is a seperate circuit to charge the cap and neads protection from over
current loads. A wire wound resistor/coil acts to disconect the NTS from
harm.The trigger circuit is yet another circuit to fire the gap. Once it
dose its job it also neads to be disconnected from the ARC current. This is
why we use sparkplug wire or a high value resistor in series with the
trigger to act as a disconect to save the trigger circuit Even though the
NTS dose influance the resonant frequency, it is not in the current circuit
at the moment of gap firing and should be disconected for that microsecond
or so. Contrast my thoughts.I'm not in accord with you.
   Robert  H 

> From: "Tesla list" <tesla-at-pupman-dot-com>
> Date: Sun, 13 Jan 2002 07:41:15 -0700
> To: tesla-at-pupman-dot-com
> Subject: RE: Trigger xfmr "grounding", and R-C protection networks
> Resent-From: tesla-at-pupman-dot-com
> Resent-Date: Sun, 13 Jan 2002 08:31:59 -0700
> 
> 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