ETG spark gap firing
Wizzup, Tue Jan 22 2013, 05:19PMI 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:
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.
Re: ETG spark gap firing
MrFlatox, Tue Jan 22 2013, 05:38PM
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 :)
MrFlatox, Tue Jan 22 2013, 05:38PM
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 :)
Re: ETG spark gap firing
DerAlbi, Tue Jan 22 2013, 06:30PM
..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^^
DerAlbi, Tue Jan 22 2013, 06:30PM
..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^^
Re: ETG spark gap firing
Wizzup, Tue Jan 22 2013, 06:47PM
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.
Wizzup, Tue Jan 22 2013, 06:47PM
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.
Re: ETG spark gap firing
DerAlbi, Tue Jan 22 2013, 06:57PM
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.
DerAlbi, Tue Jan 22 2013, 06:57PM
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.
Re: ETG spark gap firing
Ash Small, Tue Jan 22 2013, 11:21PM
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?
Ash Small, Tue Jan 22 2013, 11:21PM
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?
Re: ETG spark gap firing
MrFlatox, Wed Jan 23 2013, 10:19AM
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.
MrFlatox, Wed Jan 23 2013, 10:19AM
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.
Re: ETG spark gap firing
Ash Small, Wed Jan 23 2013, 10:43AM
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)
Ash Small, Wed Jan 23 2013, 10:43AM
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)
Re: ETG spark gap firing
MrFlatox, Wed Jan 23 2013, 11:15AM
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...
MrFlatox, Wed Jan 23 2013, 11:15AM
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...
Re: ETG spark gap firing
Yandersen, Wed Jan 23 2013, 11:41AM
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?
Yandersen, Wed Jan 23 2013, 11:41AM
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?
Re: ETG spark gap firing
Wizzup, Wed Jan 23 2013, 04:14PM
I found a circuit made by Lomy here:
His design uses a simple homemade air-core HV pulse transformer in series with the ETG and cap bank and by watching his videos it seems to work very well with just 200J of energy using just air as "fuel". I checked with an inductor simulator that if i make my secondary 12 turns from 20mm2 cable i have it will have just 1mOhm resistance and about 250nH inductance.
I will also use few kV higher voltage than he is, so it should work well with some fewer secondary turns, same time lowering the resistance a bit.
Do you think i will need a big kickback diode for the secondary or will i be good without one? Regarding his schematic he doesnt use one.
Wizzup, Wed Jan 23 2013, 04:14PM
I found a circuit made by Lomy here:
His design uses a simple homemade air-core HV pulse transformer in series with the ETG and cap bank and by watching his videos it seems to work very well with just 200J of energy using just air as "fuel". I checked with an inductor simulator that if i make my secondary 12 turns from 20mm2 cable i have it will have just 1mOhm resistance and about 250nH inductance.
I will also use few kV higher voltage than he is, so it should work well with some fewer secondary turns, same time lowering the resistance a bit.
Do you think i will need a big kickback diode for the secondary or will i be good without one? Regarding his schematic he doesnt use one.
Re: ETG spark gap firing
MrFlatox, Wed Jan 23 2013, 04:42PM
Very nice find !
What do you mean by air core transformer ? just wind the primay and secondary around a plastic tube for exemple ?
MrFlatox, Wed Jan 23 2013, 04:42PM
Very nice find !
What do you mean by air core transformer ? just wind the primay and secondary around a plastic tube for exemple ?
Re: ETG spark gap firing
Wizzup, Wed Jan 23 2013, 05:06PM
With air-core i meant that there is no iron rod or anything in the core, so its made on top of plastic pipe or something just as you said.
Wizzup, Wed Jan 23 2013, 05:06PM
With air-core i meant that there is no iron rod or anything in the core, so its made on top of plastic pipe or something just as you said.
Print this page