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Re: Triggered Spark Gaps - Why?

Hi Gavin,
    There is a new theory for the triggering mechanism in trigatron gaps
substantiated by considerable proof.  Here is the reference:  

                    Triggering in Trigatron Gaps:  A fundamental study

                    P. F. Williams and F. E. Peterkin

                    Journal of Applied Physics, No. 66 (9), 1 November 1989



    A trigatron or swinging cascade can be rigged for AC mode with a
circuit similar to a triac trigger circuit.  In this case the voltage
divider network is across the switch.  A voltage is tapped off from the
divider.  It is used to charge up a capacitor.  When the capacitor reaches
a certain charge level (position on the AC sine wave) it sparks across a
small trigger gap into the primary of an isolation coil consisting of about
ten turns of RG-8 on a 4 inch PVC form.  A couple of doorknobs should work
for the capacitor (a few nanofarads).  The output of the trigger isolation
transformer is applied to the trigger pin on the trigatron or the midplane
of the swinging cascade switch.

Barry

P. S. BTW I would be interested in your transformer design notes.


  
-----Original Message-----
From: Tesla List +ADw-tesla+AEA-pupman-dot-com+AD4-
To: tesla+AEA-pupman-dot-com +ADw-tesla+AEA-pupman-dot-com+AD4-
Date: Saturday, November 07, 1998 12:06 PM
Subject: Triggered Spark Gaps - Why?


Original Poster: Gavin Hubbard +ADw-ghub005+AEA-xtra.co.nz+AD4- 

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.