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

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

Original poster: "Luc by way of Terry Fritz <twftesla-at-qwest-dot-net>" <ludev-at-videotron.ca>

Hi Gary,

Tx an other interesting paper to add to my library. 

Tesla list wrote:
> 
> 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'm not sure this is making a big difference with tesla coil
operation, if you want a nano second switching of the gap, like
in UV laser, this is probably a good advice, but for tesla a
little jitters is not a big problem.

>From analyze and report of people using them R-C protection
network seem to have more benefit than drawback.

Cheers,

Luc Benard  




  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