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Building a simple trigatron

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gargamel

Fri Aug 14 2015, 06:42PM

Registered Member #56093 Joined: Wed Jul 29 2015, 04:57PM
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Posts: 8

Requirement:
-Switching my 4500V/1ÂµF MMC cap as quickly as possible, using a piezo igniter as trigger
-The piezo is hand operated.
-It should be possible to fire the cap without a load on the output.

After some brainstorming done I found 3 ways to do it, please note my drawing.

My fear is the operation of the piezo switch. Somehow the operators finger is near the business. Maybe the circuit is fired with open output, without the load resistance. In this case the spark might discharge through the piezo switch (which would explode?) more tempting.
I would place some resistors in series to limit the current to few mA.

I wonder if version 2 would be any safer than 1.

Also in both cases the HV could discharge prematurely through the trigger circuit, even if slowly.

Version 3 would eliminate this but the spark would only cross the main firing path, not run parallel to it.
How much less effective woulf that be?

hen918

Fri Aug 14 2015, 09:24PM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

I wouldn't use a piezo (rather weak ignition), but if I would, I would use a mouse trap and a long length of string to fire it. The piezo material itself has a high dielectric strength, however there will probably be a small gap that could arc across.
There is another method, that involves two gaps, one for your trigger spark and one for you main arc it goes like this: output of capacitor electrode, load electrode, piezo electrode. The ground of the piezo is connected to the load electrode. The gaps should be configured in a V so UV light from the piezo spark ionises the air in the main gap and causes a breakdown, but the piezo spark does not jump across the top two electrodes.

gargamel

Fri Aug 14 2015, 11:06PM

Registered Member #56093 Joined: Wed Jul 29 2015, 04:57PM
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Posts: 8

My piezo is quite strong, compared to the ones built in lighters.
It reliably shoots through about 12mm. Much more than the main circuit.

There is another method, that involves two gaps, one for your trigger spark and one for you main arc it goes like this: output of capacitor electrode, load electrode, piezo electrode. The ground of the piezo is connected to the load electrode. The gaps should be configured in a V so UV light from the piezo spark ionises the air in the main gap and causes a breakdown, but the piezo spark does not jump across the top two electrodes.

I only count 3 electrodes, isn't that what I decribed in 2. ?

The resistor in series to the piezo must be a HV type of course.

For the sake of safety I'd prefer Number 3, even if the output is accidentally open, the cap cannot unload itself through the piezo... The electrodes could be aligned like a flattened X, so that both the main axis and the trigger axis could run as parallel as possible.

gargamel

Sat Dec 12 2015, 08:27PM

Registered Member #56093 Joined: Wed Jul 29 2015, 04:57PM
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Posts: 8

I'd like to ask you guys another question:

The perfect trigatron should obviously fire reliably when triggered, but should not fire by itself.
There are always some uncertainties like the voltage to be switched or the airs humidity varying ect.

I'm lacking experience here, I'll have to find out how near to unprovoked breakdown the electrodes must be placed, or how save the can be placed until the firing gets unreliable.
Just curious about your experence, is this sweet spot rather slim or whide?

Concerning the main electrodes, would you rather use sharp tips and greater distance or ball shaped ones and shorter distance?
Which one of these setups will provide me with a broader sweet spot?

hen918

Sun Dec 13 2015, 04:52PM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

All trigatrons will fire by themselves if the voltage across them gets too high. This can actually be a benefit, as it prevents capacitors being destroyed by overcharging.

Pointy electrodes should not be used as the small surface area means that the plasma erodes them after only one or two shots.

I think large radius curves should be best, although I used flat ended electrodes and they worked OK.

The "sweet spot" depends on your triggering power. With a small spark from a piezo, the "sweet spot" will be quite small, with a higher current / power trigger, the sweet spot will be much wider.

Keep playing with the electrode distance until the setup is right.

Ash Small

Thu Dec 17 2015, 06:58PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

A third way is to superimpose the spark from the Piezzo onto the main circuit using an air core transformer. You wouldn't need many turns, and it wouldn't add a lot of inductance.

I don't think this would present any risk to your main capacitor, but I can't be certain.

radiotech

Mon Dec 21 2015, 08:55PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

How about spraying a fine mist of a salt solution between the sharp points?

The bang will somewhat add a bit of damping, and might help by not ruining
the capacitor with ringing.

And keep the fire point away from porcelain insulators. The tend to crack.

Signification

Fri Dec 25 2015, 11:20AM

Registered Member #54278 Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367

radiotech wrote ...

How about spraying a fine mist of a salt solution between the sharp points?

The bang will somewhat add a bit of damping, and might help by not ruining
the capacitor with ringing.

And keep the fire point away from porcelain insulators. The tend to crack.

EXACTLY (almost) what I was thinking of trying! The mist never occurred to me. My main thought was letting a drop of saltwater fall through the field. It occurred to me that this may be too small, so I considered a soap bubble made from salt water--maybe even blown through one of the high voltage electrodes with a small hole in the center. Pointy OR (more so) spherical electrode. This may even work with just the H2O.
ps:
Wouldn't you be scared that the high voltage might find a path through the mist near you (if sprayed by hand)?

Patrick

Fri Dec 25 2015, 08:11PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Signification wrote ...

. . . . . high voltage might find a path through the mist near you (if sprayed by hand)?

Yes.

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