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Triggered Spark Gaps...

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Hon1nbo

Wed Apr 29 2009, 01:50AM

Registered Member #902 Joined: Sun Jul 15 2007, 08:17PM
Location: Pacific Northwest USA
Posts: 1042

ok, so I have decided to use a triggered Spark Gap for my Capacitor Pulse Bank, economical, powerful, simple...
well, I realized how much space my current design takes in my Cap Bank's Pelican Case installation, and want to make a smaller one, while still retaining my recycled parts for as much of my project as possible...

the first Pic is my current one with an Ignition Coil trigger making most of the bulk
the second Pic is my hope for a new unit, but I am unsure of how much power it could safely take (though I would be willing to replace the bolts and dome nuts with thicker ones, I hope it would not have to change much) - a penny is used for size reference...
the third Pic is the entire "trigatron" unit I want to make work, and a CM600 brick is used for reference (more on why that's on my workbench is for another time ; - )
the last Pic is the Pulse transformer I now want to use... I have used the think for CW operation for some time, but I feel it is time to use it as it was intended for...

How much power do you think I could safely pass through this newer gap design at around 1000V? just any ballpark estimate would be fine, but this is my first project of such a magnitude to use a triggered Spark Gap... I normally use SCRs, but I hate replacing them and I am going to need bigger ones than I have ever used...

Proud Mary

Wed Apr 29 2009, 10:05PM

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

A newly patented (2007) trigger device that may interest you - that does away with a third pin - consists of a placing a momentary large over-voltage pulse on one of the electrodes by means of a transformer.

This does away with the third 'trigger' electrode, which is the element that requires the most frequent, and troublesome, replacement in trigatron circuits.

See: Trigger arrangement for a Marx generator
United States Patent 7170198

Hon1nbo

Thu Apr 30 2009, 02:26AM

Registered Member #902 Joined: Sun Jul 15 2007, 08:17PM
Location: Pacific Northwest USA
Posts: 1042

I'm assuming that my HV Diodes on the Cap bank will have to be able to withstand this to protect the Caps?

Anyways I still would like to know how much Energy I can pump through a gap of the shown size (smaller one) without issues... I really hate that my current gap setup takes up more sqaure inches that four of my Caps!

Proud Mary

Thu Apr 30 2009, 03:03AM

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

Find a copper wire table online, and go to the wire gauge that has the same thickness as the bolt shown in your lowest picture, and read off its current carrying capacity. Reduce this a little to take account of the screw thread and the different metals, and this will give you a rough 'ballpark figure'

jpsmith123

Thu Apr 30 2009, 02:05PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

Spark Gaps are inherently high energy devices.

Generally speaking, unless the associated conductors have a very small cross sectional area, the voltage dropped across the hardware will be only a small fraction of your capacitor voltage, thus the limit on how much single shot energy it can throughput depends mainly upon how often you'd like to repair/replace the electrodes (which in turn also depends on the electrode material and the geometry).

IOW, a small amount of material will be ablated from the arcing surfaces with each shot, so the gap properties will slowly change, eventually necessitating repair/replacement.

OTOH, if you're talking about high duty cycle operation, i.e., high average power, then, depending on your design, you may run into a thermal problem as well, in which case you may need convective cooling.

Somewhere I have some literature on spark gap design that gets into estimating such things as the power dissipated, efficiency, etc.

In any case, most commercial manufacturers use a copper/tungsten alloy for the arcing electrodes, and the design usually involves an insulated, pressurized case with slow gas flow, so that heat and contaminants can be carried away and the physical gap can be as small as possible to minimize the inductance and also the rise time.

Edit:

I just realized you stated your voltage is 1000 volts. Is that a mistake? Did you mean to say 10,000 volts?

Hon1nbo

Thu Apr 30 2009, 02:18PM

Registered Member #902 Joined: Sun Jul 15 2007, 08:17PM
Location: Pacific Northwest USA
Posts: 1042

[quote]
Spark Gaps are inherently high energy devices.

Generally speaking, unless the associated conductors have a very small cross sectional area, the voltage dropped across the hardware will be only a small fraction of your capacitor voltage, thus the limit on how much single shot energy it can throughput depends mainly upon how often you'd like to repair/replace the electrodes (which in turn also depends on the electrode material and the geometry).

IOW, a small amount of material will be ablated from the arcing surfaces with each shot, so the gap properties will slowly change, eventually necessitating repair/replacement.

OTOH, if you're talking about high duty cycle operation, i.e., high average power, then, depending on your design, you may run into a thermal problem as well, in which case you may need convective cooling.

Somewhere I have some literature on spark gap design that gets into estimating such things as the power dissipated, efficiency, etc.

In any case, most commercial manufacturers use a copper/tungsten alloy for the arcing electrodes, and the design usually involves an insulated, pressurized case with slow gas flow, so that heat and contaminants can be carried away and the physical gap can be as small as possible to minimize the inductance and also the rise time.
[/quote1http://4hv.org/e107_plugins/forum/ forum_post.php?quote.68736241100581]

I would certainly not mind replacing the bolts every now and then, as they are cheap (and I can just get a contractor pack next time I go to the hardware store)... If I ever can, I will attempt to get tungsten electrodes, but I have actually thought about using something like a tungsten/Thorium alloy welding rod - since it has a doping of thorium the gap will form easier, but there are two things I am worried about: one is the fact it is a welding rod, and this is a spark gap unit; two is that the Thorium dopant might also cause the gap to not quench when it should... but being a capacitor bank I do not expect that to be too much of an issue.)

anyways... for my size bolts, which are 1/4" and around gauge 2, current charts state they can handle around 94 amps... I don't know if that will be enough... I can upgrade them if I have to... being my first High Energy Triggered Spark Gap I am still uncertain anbout a lot of things in it...

jpsmith123

Thu Apr 30 2009, 02:36PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

Current carrying capacities listed in wire tables are based on acceptable temperature rise for a given physical arrangement (single wire, bundle of wires, multilayer transformer coil, etc.) usually with continuous current flow and no forced air cooling or anything.

Since yours will presumably be a low duty cycle, low average power application where you don't have to worry about things like a time and temperature based degradation of an organic insulator over a wire, you can put a lot more current through your conductors...you will not melt the conductors.

Plasma Lover

Thu Apr 30 2009, 03:18PM

Registered Member #1911 Joined: Mon Jan 05 2009, 06:30PM
Location: Salem, Oregon, USA
Posts: 165

The amperage rating for wire is for continuous operation. If it's a single pulse, sure, it'll heat 'em up a bit, but it's not going to make them explode at any reasonable power. If your bank is using less than 2KJ per pulse and it's not a rapid pulse, I would assume that you may need to change the bolts out somewhat frequently, but they won't melt your enclosure.

Here's a good idea - use an aluminum enclosure to hold the elctrodes - that way your box will be a heatsink! (And your box will make a short-circuit!)

My suggestion is to put it in a make-shift blast chamber and try it out.

;)

Proud Mary

Thu Apr 30 2009, 03:51PM

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

I should have made myself more clear, and said that any significant heating of a triggered gap will cause thermal expansion of the bolts, which may narrow the gap sufficiently to alter firing conditions for subsequent shots. They should thus be sized conservatively.

Unnecessary thermal stress is in any case to be viewed as objectionable from an engineering point of view.

Hon1nbo

Thu Apr 30 2009, 07:19PM

Registered Member #902 Joined: Sun Jul 15 2007, 08:17PM
Location: Pacific Northwest USA
Posts: 1042

Harry wrote ...

I should have made myself more clear, and said that any significant heating of a triggered gap will cause thermal expansion of the bolts, which may narrow the gap sufficiently to alter firing conditions for subsequent shots. They should thus be sized conservatively.

Unnecessary thermal stress is in any case to be viewed as objectionable from an engineering point of view.

ok then... I don't mind having to adjust the gaps a little... my trigger pulse is large enough to support a large range on gaps for the larger portion, and the other half of the assembly can't be that hard to do... at least by my standards.

as someone mentioned above, by the time I am done I will have well more than 2 KJ... I have a third Cap in the mail giving me 2.1 KJ for my larger caps, and the smaller caps add slightly more, but I can only get the larger caps one at a time... so I will have time to tune the setup before going full power.

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