[Prev][Next][Index][Thread]
Triggered Spark Gaps - Why Not?
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.
>
>
>
>