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Forums
4hv.org :: Forums :: Electromagnetic Projectile Accelerators
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ETG spark gap firing

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Wizzup
Tue Jan 22 2013, 05:19PM Print
Wizzup Registered Member #3302 Joined: Sun Oct 10 2010, 02:21PM
Location: Finland
Posts: 42
I got very interested from MrFlatox's ETG and i want to build a very similar one myself, but instead of using aluminum foil or salt water or something like that, i got an idea that I think should work well, but as I am not very experienced in electronics i would like to hear your opinions before possibly destroying my caps or some other part of the circuit.

I made this very high quality picture with ms paint to explain what i have in mind:
3462mx5

So it would work kind of like a trigatron, but with two spark gaps - the primary gap is inside the ETG chamber and the second one is between the capacitor bank positive and HV spike positive, the meaning of that is so that the HV capacitor isnt directly connected to the main cap bank. So when the cap bank is charged to 450V, the HV pulse will ionize the air between the main electrodes and make it conductive so that the main cap bank can discharge through it making the ETG fire. The main spark gap would be 1-2mm and the secondary about 1mm.

My worry is in that I dont know if the HV spike is able to damage the main cap bank, my logic would say that the HV cap will discharge through the gaps until its voltage reaches 450V that is the voltage of the main cap bank and then the secondary spark gap would "shut down", but I would like to hear the opinions of other more experienced members about the design in case it has a major flaw, that is not impossible as I am still pretty n00b in this stuff ;)

The reason i would like to do it this way is to remove the need of aluminum foil making it easier to operate and also for it to be technically a bit more advanced as it practically wouldn't need any extra fuels in the chamber.
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MrFlatox
Tue Jan 22 2013, 05:38PM
MrFlatox Registered Member #9349 Joined: Mon Jan 07 2013, 08:50AM
Location: France
Posts: 102
I am also very interested in modifying my ETG to a triggered spark gap firing one. I thought about the same design as yours, but my main concern is that the spark from your pulse will not jump onto the "primary gap" because the 450V bus is close to ground compared tto the 5kV, I don't know if it make sense, but I think that the spark from your hv capacitor will juste discharge in your main capacitor bank positive terminal. The only way to know is to give a try :)
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DerAlbi
Tue Jan 22 2013, 06:30PM
DerAlbi Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727
..You need some Inductnace n between your main cap-bank and the sparkgap... i think this design is not verry good. It would be much better if the 5kV-spark fired nearby the main sparkgap so you get more ionized gas initially.

If a pure air plasma will be usefull is an other storry. I think use a denser start material provides much more gas expansion as every air can provide. But thats nothing i now anything about^^
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Wizzup
Tue Jan 22 2013, 06:47PM
Wizzup Registered Member #3302 Joined: Sun Oct 10 2010, 02:21PM
Location: Finland
Posts: 42
I thought about that too, but I think that as my main capacitor banks total ESR is relatively very large compared to a 5kV 2nF ceramic capacitor, the HV caps energy would move to the main cap bank slow enough that the voltage would remain high enough for long enough time to find a new way to discharge through the primary gap.

So what i mean is that the main cap bank couldnt probably accept the incoming pulse energy as fast as its coming so it would still trigger the primary gap. I am not in any way sure this is the case but i believe it could work like that.

I dont have a 5kV supply or cap yet as i just got the idea for this today, otherwise i would have tested it already with my old 200J cap bank that i dont care about that much.
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DerAlbi
Tue Jan 22 2013, 06:57PM
DerAlbi Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727
Well.. depending on the size on your cap, you need to change the way you look at the caps ESR. The ESR is not seperateble from the cap, as every equivalent circuit diagram suggests. Thats why its called _E_quivalent_ series resistance. Its not a real series resistance!

Imagine your Cap as a verry lare (long and small width) aluminium foil, connected on one side at just one point (for illustration).
So part of the foil (close to the connection) has very low resistance and areas farer away have higher resistances due to the long way the electrones must travel through aluminium. Average that behavior, you estimate an ESR.
However: if the Low-ESR-region of the Foil (close to the connection) already has enougth capacitance to absorb your pulse, your cap may be destroyed there, and in any case the 5kV pulse will vanish.

If you have already a high ESR an additional ESR of an inductance will not matter much - you dont need much inductance anyway, and since this is only low current shielding you can use a big screw as core and thats it. Sould be verry low ESR..
This is the only -engeneering wise- reasonable way to decouble a voltage pulse from a capacitor.
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Ash Small
Tue Jan 22 2013, 11:21PM
Ash Small Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Would a ~5kV diode (or diode string) between the 450+ terminal and 5kV spark gap help?

Would this prevent the 5kV pulse from just charging the cap. and force it to 'jump the gap'?

I'm also of the opinion that some 'fuel' in the chamber would help, but what do I know? smile
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MrFlatox
Wed Jan 23 2013, 10:19AM
MrFlatox Registered Member #9349 Joined: Mon Jan 07 2013, 08:50AM
Location: France
Posts: 102
Your diode string would have to withstand the main current when firing (couple kA) so I don't think it worth it because the diodes would have to be beefy as hell.
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Ash Small
Wed Jan 23 2013, 10:43AM
Ash Small Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Yes, I realise that.

An inductor could work. You could test this without the capacitor in the circuit, and see whether the 5kV pulse jumps the gap, or manages to short through the inductor.

You could also try two gaps, at right angles to each other, in a 'cross' formation, so that the 5kV pulse jumps from north to south, and the 450V pulse might jump from east to west, if the air is sufficiently ionised, although the 5kVpulse may jump from north to east, destroy the capacitor, then jump from west to south, unless the 5kV gap is smaller than the 450V gap, in which case the air possibly won't be sufficiently ionised for the 450V spark to jump. (hope that makes sense)
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MrFlatox
Wed Jan 23 2013, 11:15AM
MrFlatox Registered Member #9349 Joined: Mon Jan 07 2013, 08:50AM
Location: France
Posts: 102
It makes sense, but I was wondering that the main capacitor bank would not be destroyed, the energy in the 5kV capacitor, is very low compared to the energy in the main bank. So I am guessing that if the 5kV discharge on the 450V line, it will charge ip the main capacitor so that the energy of the 5kV cap is just added to it own energy (few volts more). I don't know if this makes sens, but I don't know how to put the idea in words...
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Yandersen
Wed Jan 23 2013, 11:41AM
Yandersen Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340
May work if wires for the main discharger are few meters long while 5kV is applied directly to discharge terminals.
There is no way to ionise 1mm of air between two electrodes being hold at 450V by the cap and without inductance - the curent will take the shortest path: it is easier to go through the 450V cap circuit rather than penetrate 1kV of air gap. So some inductance is anavoidable anyway. So why not to add transformer to make sure HV spike goes nowhere else except the air between main electrodes?
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