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Forums
4hv.org :: Forums :: Electromagnetic Projectile Accelerators
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1kJ Electrothermal gun (ETG)

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MrFlatox
Fri Feb 08 2013, 06:35PM
MrFlatox Registered Member #9349 Joined: Mon Jan 07 2013, 08:50AM
Location: France
Posts: 102
From a flyback transformer. I assumed it to be around 30kv to 40kv judging by the distance where it starts to arc.

By twisting wires, you mean like that the picture posted ?

Will it the spark occurs on the chamber or all the energy transfered by this aire core transformer will go on the capacitor ? I suppose it would be better to placce this close to the chamber right ?

1360348518 9349 FT148882 Sans Titre
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Ash Small
Fri Feb 08 2013, 07:02PM
Ash Small Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
You MAY have some problems here. First, I'm not sure if the capacitor will 'damp' the HV pulse. Second, with the 'loose coupling' of an air core transformer, as I understand it, it will take several (read 'lots of') cycles to build up voltage.

It's possible I may be mistaken on both of these points, though.
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Yandersen
Sat Feb 09 2013, 01:50AM
Yandersen Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340
By twisting the wires I mean ETG cable and HV wire should go in parallel, touching-close to each other for a some distance to make sure those wires have better magnetic coupling. You may put several ferrite beads to increase the coupling and hold those two wires together (recommended), or simply twist them a little one around the other like a couple of fucking snakes. The HV wire should be a closed loop, following the ETG+ -> Gap -> ETG- path, so it may be coupled with both ETG cables. Hope it is possible for your flyback to connect secondary directly to that HV loop without diode and cap - alternating current is just what you need. Just make sure primary is protected from voltage spikes.
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Ash Small
Sun Feb 10 2013, 12:01PM
Ash Small Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Another, possibly simpler, solution may be to use diodes on the 'output' of the 450V capacitor.

I'm not sure what value you'd need though, or how expensive it would be.

I could draw a schematic if required.

(I'd consider a big stud schottky, possibly)

EDIT: I may look into this myself, as it seems an extremely simple method of firing an ETG.
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Yandersen
Sun Feb 10 2013, 08:48PM
Yandersen Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340
MrFlatox, end of the week is here, me becoming nervous, how are you doing? :)
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MrFlatox
Tue Feb 12 2013, 10:31PM
MrFlatox Registered Member #9349 Joined: Mon Jan 07 2013, 08:50AM
Location: France
Posts: 102
haha ! Sorry to disappoint you but, I tried to work on the project, but I was too lazy to work with an ambient temp below 0°C in my "workshop".

I will be able to work on the ETG during the next two weeks (holidays).

As for the "transformer" design, I think I will give a try to this design : Link2 which was proven to work. I don't have a clue about the results, but i will try what you said in the case in does not work properly with the design mentioned above.

PS: I have not forgotten your magnet thing, and I would surely try it out as I am curious about the results too.
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MrFlatox
Tue Feb 19 2013, 11:55AM
MrFlatox Registered Member #9349 Joined: Mon Jan 07 2013, 08:50AM
Location: France
Posts: 102
Hi there !

I post this to make an update of the current progress :

I just made a "quick and dirty" setup as a proof of concept that the air core transformer technique works, and indeed, it worked !
With a 8 turns secondary, 4 turns primary, 30kV on the primary side, primary capacitor of 2.2nF (homemade cap), it produce a 2mm spark across the main capacitor spar gap, and thus enabling main current to flow ! No more aluminium foil !

Here is a picture of the dirty setup :

21595920130219120949

Air core transformer :

76645320130219121008

Now that I know this thing works, I will do a cleaner setup.

I also rebuilt the chamber to accomodate to the new powering method (spark gap). The chamber main body is now a thick steel tube. The new chamber have less dead volume, and the distance between the central electrode and the chamber wall is approximately 1mm : low enough to make the triggered spark gap working.

47913020130219121128

Stay tuned, if all is going well, I might do first test shots this week.
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Yandersen
Tue Feb 19 2013, 09:22PM
Yandersen Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340
I'm so happy, that I start thinking about doing my own ETG... Good luck, mrFlatox!
BTW, why don't you try ferrite core transformer with just no more than a couple turns on primary and secondary? 30kV should penetrate 30mm of air - if it doesn't, than coupling in an air-core transformer is very bad, or pulse is too short.
Don't be afraid of higher inductance on ferrite core - as soon as main current will run, it will saturate ferrite almost instantly making an equivalent of air-core trans.
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Yanom
Wed Feb 20 2013, 02:20AM
Yanom Registered Member #4659 Joined: Sun Apr 29 2012, 06:14PM
Location:
Posts: 158
For a propellant/projectile, might I suggest (lead-free) solder? The ETG would melt and expand it (and the surrounding air) and force it out of the chamber (i think). Molten solder would have quite some penetrating power.

Would that work? or does the propellant have to be something that gassifies/ionizes?
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MrFlatox
Wed Feb 20 2013, 09:49AM
MrFlatox Registered Member #9349 Joined: Mon Jan 07 2013, 08:50AM
Location: France
Posts: 102
Actually, in this new design, propellant is just "air". That means, no solid material. Air will expand due to the current flowing through it and push the projectile. That's about all.

Given that the air core transformer I made has a 2:1 ratio, the primary is powered by pulsed 30kV discharge, and that on the secondary output, it doesn't produce sparks higher than 2mm (which is enough to fire the ETG btw) the coupling must effectively be very bad. I might try some ferrite core, as it would enable me to use less wire, and thus making the whole assembly less cumbersome and heavy.

I am actually thinking of how could I fire the ETG within a reasonnable distance ? In fact, how could I make the spark gap (HV side) to fire one time and only one time. If the spark gap distance is set within the range of the 30kV breakdown distance, once powered, it will discharge continuously (at a high break per second)... I keep thinking...
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