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Re: Triggered Spark Gaps - Why Not?

Hello List.

I'm sorry for not making myself a little more clear. I don't think the
threads concerning triggered spark gaps should cease. Experimentation with
TSG configurations could potentially yield insights and improvements for
Tesla coils. However, I believe it is necassary for some of us to play the
role of devil's advocate, especially when complex and inappropriate
technological solutions are suggested.  

Any Tesla coil design is a compromise between theory and constructability.
I strongly believe that any design improvements that are not "good,
practical, or cheap" are not really improvements at all.

Safe coiling,

Gavin Hubbard
 

>Original Poster: "Steve Young" <youngs-at-konnections-dot-com> 
>
>Gavin,
>
>Your observations are right on, but I hope they don't discourage the idea
>of using practical, inexpensive triggered spark gaps for TCs.  I still
>think it could yield valuable improvements.  
>
>I started this thread in hopes of promoting some serious experimentation
>with triggered spark gaps.  My mistake was mentioning laser triggered gaps.
> I had hoped technology had progressed to the point that an affordable
>solid state laser trigger could be pressed into service.  All responses
>were educational, and have convinced me that the answer is no--not yet. 
>Then the thread took off with nitrogen lasers, etc.  Let's get back to the
>practical.
>
>Mainly I wanted to encourage experimentation with spark gaps triggered by
>high voltage pulses, such as is readily available from ignition coils,
>flash tube pulse transformers, home made flyback transformer cored pulse
>transformers, miniature SS tesla coils, etc.  
>
>A year or so ago I downloaded an interesting paper "Pulse Power Switching
>Devices - An Overview" by John Pasley.  Unfortunately, I don't know the URL
>I got it from, but a search engine could find it.  It covers interesting
>stuff like Sprytrons, Krytrons, etc.  But the applicable part is the
>section on triggered spark gaps.  There are 5 different configurations
>described:  1) Field distortion:  3 electrodes; employs a point discharge
>in the creation of a conducting path.  2) Irradiated:  3 electrodes; spark
>source creates an illuminating plasma that excites electrons between the
>gap.  3) Swinging cascade:  3 electrodes; trigger electrode nearer to one
>of the main electrodes than the other.  4) Mid plane:  3 electrodes; basic
>triggered spark gap with trigger electrode centrally positioned.  5)
>Trigatron:  trigger to one electrode current forms plasma that spreads to
>encompass a path between gap.  
>
>Surely at least one of these configurations would make a good, practical,
>cheap triggered spark gap for TC use.  Seems like some experimentation is
>in order.
>
>--Steve
>----------
>> From: Tesla List <tesla-at-pupman-dot-com>
>> To: tesla-at-pupman-dot-com
>> Subject: Triggered Spark Gaps - Why?
>> Date: Saturday, November 07, 1998 7:02 AM
>> 
>> Original Poster: Gavin Hubbard <ghub005-at-xtra.co.nz> 
>> 
>> Hello list members.
>> 
>> I don't want to sound unnecassarily negative, but I believe that some of
>> the exotic triggering systems that are being suggested e.g. trigatrons,
>> laser triggers, and pseudo-spark systems, are missing the point. IMHO the
>> real beauty (and design strength) of a conventional Tesla coil is in its
>> simplicity. 
>> 
>> For those not familiar with controlled HV triggering, a conventional
>> trigatron is essentially an insulated rod that penetrates an earthed
>> sphere. The tip of the rod is positioned flush inside a hole in the
>> sphere's surface with approx 1mm annular clearance. A 10kV tripping pulse
>> is applied to the needle which produces a spark between the tip of the
>rod
>> and the earthed sphere. The space charges of the auxiliary spark distort
>> the electrostatic field in the main gap and cause it to break down at a
>> voltage much lower than that required in the absence of the trigger
>pulse.
>> The sphere containing the trigger rod must be kept at earth potential or
>> else insulation difficulties occur in providing the high voltage pulse to
>> the rod. This means the trigatron IS polarity sensitive. This makes it
>> UNSUITABLE for a conventional tesla coil tank circuit unless a polarity
>> change over switch is placed in the trigger circuit. 
>> 
>> Similarly, laser triggering is also an unneccasary burden. Disregarding
>the
>> obvious problems of optical containment and termination, the major
>problem
>> is in producing a dicharge at a given time. For example, a nitrogen laser
>> (as discussed in previous threads) is essentially a pulsed discharge
>across
>> a nitrogen filled cavity. The pulse is usually produced by charging a
>bank
>> of capacitors until the breakdown voltage is reached. I hope that the
>irony
>> of using a non-controlled discharge to initiate a regular discharge in
>the
>> TC is not lost. More importantly, most nitrogen lasers can NOT operate
>> above 30-40HZ, this makes them UNSUITABLE for a tesla coil tank circuit.
>> 
>> The near impossibility of obtaining psuedo-spark gaps, in itself makes
>them
>> UNSUITABLE for a tesla coil tank circuit.
>> 
>> 
>> I hope that I am not interpreted as being anti-innovation or
>unimaginative.
>> I too have many blue-sky ideas. However, at the end of the day I believe
>> that a well designed and constructed tesla coil is worth two unrealisable
>> projects in the spares box. 
>> 
>> Cheers,
>> 
>> Gavin Hubbard
>> 
>> 
>> P.S. If any people out there are interested in simple triggered spark
>gaps.
>> I have used the following circuit with great success in a seven stage
>1.4MV
>> Ferranti impulse generator. Because it is not designed to quench, I do
>NOT
>> recommend this design for Tesla coil use.
>> 
>> Two copper spheres are arranged close together. A pair of contacts is
>> placed flush in a small hole in the sparking surface of one sphere. The
>> contacts are connected to a low voltage battery (9V) through an
>inductance
>> (230mH) in series. When the contacts are made to open, the energy stored
>in
>> the inductance appears in the form of a spark across the contacts and the
>> breakdown of the main gap follows. For safety, the contacts are operated
>> using a photosensitive cell and a relay. 
>> 
>> The advantage of this circuit is that unlike a trigatron, the circuit can
>> be situated at any point in the high voltage circuit, whether insulated
>> from the earth or not. The operating range of voltage is about the same
>as
>> that for a trigatron, for the smae polarity and gap length, and is hardly
>> affected by the value of either the voltage or the inductance.
>> 
>> 
>> P.P.S. To all of those who requested information on transformer design, I
>> will use this weekend to start translating my notes into HTML.  
>> 
>>    
>> 
>> 
>
>
>