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Triggered Spark Gaps...

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Bert

Sun May 03 2009, 04:49AM

Registered Member #118 Joined: Fri Feb 10 2006, 05:35AM
Location: Woodridge, Illinois, USA
Posts: 72

Wavetuner wrote ...

Hi,
I have a 5KV/240uf Maxwell that I want to fire at 5kv. I machined 2 very flat and parallel 2" brass dowels. One is to be screwed into the cap the other is to be suspended from an insultaed, robust steel structure for the business end work.

I considered a triggered gap but then thought a very precise gap set to 5KV would surfice. Once I roll the cap around back I will be ready to test it. Fisrt discharge will be into a large copper sulphate water resistor. Is there a down side to this approach?

If you aren't already doing so, I'd recommend enclosing the gap inside a Lexan box to help muffle the incredible noise from the discharge. You'll also find that, with a fixed gap, your actual breakdown voltage will vary significantly from shot to shot as the electrode surfaces become eroded and roughened. I personally like to control when the gap will fire - I don't enjoy the suspense of waiting for a 5 kJ surprise "sometime" during the charging cycle. Having a method to reliably trigger the switch (electrically or mechanically) also provides a wider working range of stable/controllable energy levels for greater flexibility.

Also, running your cap to its faceplate voltage may overvolt it if the discharge has any significant ringing, since many of Maxwell's Series C metal-cased energy discharge caps are rated at only 10-20% voltage reversal. YMMV - check with GAEP to get the specifics for your particular capacitor. Although you can exceed the voltage reversal spec at the expense of reduced shot life, failure is usually catastrophic (at least for Series C caps). Even if you have a newer CMF (self healing) capacitor, lifetime will be reduced. If the cap was purchased via the surplus market, it may already have significantly reduced shot life from its previous application.

Wavetuner

Sun May 03 2009, 06:04AM

Registered Member #1500 Joined: Sat May 24 2008, 04:38PM
Location: Ojai, Ca.
Posts: 44

Hmm, Yes I see that a big surprise would be disconcerting and the stress on the cap is something I had not considered. I wonder if a modified stun gun would work? It is isolated with a 9volt battery and the trigger could be modified to drop by pulling a string from a distance. It should trigger the avalanche, yes?

Conundrum

Sun May 03 2009, 09:36AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062

there was an article somewhere about triggering spark gaps of the atmospheric variety (i.e. lightning) using a pulsed UV laser.

might work, in this case using a bluray diode over an optical fibre to ionise the gap.

Also see this

seems to work using neon and relatively little power, worth a look..
-A

Wavetuner

Mon May 04 2009, 12:15AM

Registered Member #1500 Joined: Sat May 24 2008, 04:38PM
Location: Ojai, Ca.
Posts: 44

Hello, Yes, I had considered the clean solution of a 250nm led (deep into UV) but they were hard to come by. I have sent an email to a company that specializes in these diodes and sterilizing UV lights. A fiber link would be way cool and a much more safe firing mechanism for us big HV joule nuts:-^)

I wonder if a cheap Eprom eraser lamp would work?

My real work horses are dual 50KV 8uf. But lets walk first! I have grown accustomed to these limbs... I want a robust and trustworthy gap.
Jim Mora

Proud Mary

Mon May 04 2009, 12:21AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

I understand that laser pulses of 50kW peak power from a Q-switched laser focused by a microscope objective will trigger a 0.33mm gap with 2kV across it.

See:

A simple laser-triggered spark gap for kilovolt pulses of accurately variable timing

Journal Optical and Quantum Electronics
Publisher Springer Netherlands
ISSN 0306-8919 (Print) 1572-817X (Online)
Issue Volume 1, Number 1 / February, 1969
Category Letters
DOI 10.1007/ BF01476795
Pages 62-64

...

Mon May 04 2009, 12:24AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

I bout that a led (uv or otherwise) will work to trigger a spark gap, a uv tube might work, but really to have any appreciable effect you need to have super high peak powers (ie, q-switched yag lasers). In the mean time, a circular electrode around the spark gap should work fine, and doesn't have the issue of the trigger electrode getting vaporised.

Wavetuner

Mon May 04 2009, 12:51AM

Registered Member #1500 Joined: Sat May 24 2008, 04:38PM
Location: Ojai, Ca.
Posts: 44

***,
Ok, so I know the diode won't have enough energy by ^'s to begin the avalache. I am interested in your circular gap encloseur trigger -I never though of that!

What needs what connection to what inrespect to the other gap? I built a 555 countdown with a ignition coil capacitive dischage cuircut that this may work with this. I then would know the precise time of fire. The one gap is at ~5KV and attached to the Cap. The working system is insulated above ground until the gap breaks down and drives an attached archimedes spiral or blows a wire to capacitor ground.

I suspect one end of this could be machined and attached to one of the dowels thereby solving some of the explosive noise or do I need to float boath ends to get the gap to fire?

Wavetuner

Tue May 05 2009, 12:30AM

Registered Member #1500 Joined: Sat May 24 2008, 04:38PM
Location: Ojai, Ca.
Posts: 44

Hmm, What about FW rectifining a 12 kv Neon transformer with the polarities attached to the gap approiately? A series bank of 1N5408 diodes with 2xV 12Kv should withstand the the breakdown. Would this work?

I suspect the diodes and transformer would be very good isolation from the target gap or will the cap suck up all the energy?

Proud Mary

Tue May 05 2009, 09:06AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Wavetuner wrote ...
I then would know the precise time of fire.

Would ye, indeed, me hearty?

You'll think I'm splitting hairs here, or splitting microseconds, but half of the science of spark gaps is about trying to control "the precise time of fire."

It won't make much difference to your experiments, its true, but you should know something at least about it, so Google "spark gap jitter" to get a little taste of what 'jitter' and other firing erratics are!

Bert

Tue May 05 2009, 03:58PM

Registered Member #118 Joined: Fri Feb 10 2006, 05:35AM
Location: Woodridge, Illinois, USA
Posts: 72

Wavetuner wrote ...

...
My real work horses are dual 50KV 8uf. But lets walk first! I have grown accustomed to these limbs... I want a robust and trustworthy gap.
Jim Mora

Since you don't need precise triggering AND you want reliable switching at 5 kV, a simple mechanical gap is simply the way to go. You can use a solenoid, an air cylinder, or even a pull string(!) to reliably fire it.
The advantages:

1. It's simple, robust, and the trigger circuit/method can be completely isolated from the High energy circuit
2. Triggering is insensitive to minor changes in electrode surface condition from electrode erosion/melting
3. It's impervious to shorting along insulating surfaces from evaporated and redeposited electrode material
4. Standoff voltage is easily controlled by initial electrode separation.
5. Minimum "firing" voltage is controlled by closed gap spacing. It can even be set for hard closure with little risk of welding for multi-kV, multi-kJ discharges if you use massive brass or copper-tungsten electrodes.
6. Because the minimum firing voltage is INDEPENDENT of initial standoff voltage, the switch will reliably fire over a MUCH broader voltage range than virtually any other type of triggered gap.

The disadvantages are:

1. It's "low tech". It's definately not as cool as a laser triggered gap...

2. It's not capable of precision triggering
3. It's physically larger than a comparably rated 3-terminal triggered gap.

Since you'll be switching 10's of kA, I really recommend keeping it simple. When I used a trigatron to switch 25 - 100 kA currents, the switch required periodic maintenance after only 20 - 30 shots or it would premature trigger (SURPRISE!!) or became difficult/impossible to trigger (

). Premature firing can take out your HV rectifiers, and failing to fire leaves you with an "armed" bomb that requires an alternative method for safe discharge.

Since I converted to the electromechanical replacement, it has not required ANY maintenance after >3,000 shots. Furthermore, it has NEVER failed to fire, nor has it ever prematurely fired. It's the Timex or EverReady Bunny of high voltage switches. The only problem is that some zinc has vaporized out of the brass to form a light zinc oxide coating on the interior of the Lexan box housing the switch. Based upon real world experience with high energy trigatron and mechanical switching over a 10 year span, unless you require precise triggering, I would NOT recommend using a triggered gap for reliably handling multi-kiloJoule discharges.

Bert

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