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4hv.org :: Forums :: Electromagnetic Projectile Accelerators
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Low Voltage Capacitor ETG

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spectre
Fri Oct 31 2014, 07:47PM Print
spectre Registered Member #42773 Joined: Sun Jan 12 2014, 03:38PM
Location:
Posts: 7
Is it possible to use low voltage capacitors to power a ETG/Railgun? i have i 16 Volt 50 Farad Car audio Cap bank that is collecting dust. Total stored energy i 6.4 KJ at 800 Coulombs.

If this is to low a voltage to drive the circuit properly, can i discharge it through a transformer core?

I am in the process of designing a ETG, and i am sourcing materials needed. thinking of a long barrel, large chamber, multiple electrodes and low melting-point material connecting the electrodes. lead, tin, potassium.
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hen918
Fri Oct 31 2014, 10:32PM
hen918 Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556
Well the thing with ETGs is, the Thermal bit is caused by a thin piece of wire of other metal, vaporising when it has a multimegawatt pulse of electricity through it. More voltage = more current, with a low resistance. Basically you are trying to get the thing you want to propel the projectile to explode as violently as possible. A low voltage just isn't going to do that.

Discharging those capacitors though a transformer will cause the transformer to get hot, but nothing else: transformers work with AC, capacitors are DC.

As for the type of propellant metal needed, you want a low boiling point and low resistance (with the resistance being more important)
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spectre
Sat Nov 01 2014, 03:54PM
spectre Registered Member #42773 Joined: Sun Jan 12 2014, 03:38PM
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Posts: 7
oops, i see my mistake, was tired when i wrote that. i meant a dc-dc transformer, cant remember the name, switch-mode something something.

and i see your point about low voltage, will try to get my hands on something larger, series parallel 400 volt caps or something in that area, get up to about 800 volts

thinking of trying tin as metal, in a chamber filled with pure oxygen, or possible other gases. in the process of machining a valve that can contain the blast, in theory it should hold pressure from vacuum to 100K psi.

using tin, it has i melting point of 239 degree celsius and almost no resistance, 115 nano-ohm per meter.
easier to read: using a 1 centimeter piece of tin, it has a internal resistance of 0.000001 milliohm at 0 degree celsius

i am not sure how to calculate potential energy from capacitors. does anyone have a formula i could use??
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hen918
Sat Nov 01 2014, 04:25PM
hen918 Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556
You would have to use a switch-mode power supply (my favorite is the Mazzilli flyback driver with a homemade 3kv flyback transformer) to charge up the large 400/800v capacitors.
Aluminium works very well, because it is actually flammable as well as having the third lowest resistance of any element. (about 5 times less than tin)
The boiling point of tin is 2602 degrees Centigrade, meaning that it may not even vaporise: all of the energy would be dissipated in the connecting wires.
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spectre
Sat Nov 01 2014, 04:35PM
spectre Registered Member #42773 Joined: Sun Jan 12 2014, 03:38PM
Location:
Posts: 7
well don't i feel like an idiot. should probably have read or checked, not trust my on brain. all the numbers were drawn form memory, which apparently is not that good...

back to the practical, let other people handle the theory and the science

as for charging the capacitor, i have built a variable transformer with ac and dc output. 0 to 4000 volts, 0 - 400 hertz. filtered through a custom sinus filter. use it for heat testing/drying and current/voltage supply on large generators and transformers.
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Houdini0118
Sat Nov 01 2014, 05:13PM
Houdini0118 Registered Member #8558 Joined: Thu Dec 06 2012, 11:38PM
Location: usa
Posts: 57
Why do you want a large chamber? From my experience with these they are detrimental unless you need the space to hold the electrodes the chamber should be the size of the projectile.
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spectre
Sat Nov 01 2014, 07:24PM
spectre Registered Member #42773 Joined: Sun Jan 12 2014, 03:38PM
Location:
Posts: 7
the large chamber idea was to have enough gas/material to plasmatize(?). this went out the window since a cant use my low voltage capacitors. they have a combined power of 64 kJ. (16 Volt and 500 Farad)

the chamber size will now be reduced, will be using tungsten-alloy as electrodes, from high temp welding equipment.

what gases can i experiment with? have access to argon, oxygen, acetylen, argon, nitrogen, xenon, helium, hydrogen, carbon disulfide, propane and butane. any more suggestions??
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hen918
Sat Nov 01 2014, 08:32PM
hen918 Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556
well if you want a big bang, why don't you just fill the chamber with hydrogen and oxygen? :)
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Houdini0118
Sat Nov 01 2014, 09:02PM
Houdini0118 Registered Member #8558 Joined: Thu Dec 06 2012, 11:38PM
Location: usa
Posts: 57
Any amount of material will turn into plasma and as i understand it the less material the hotter what is there gets and the more it expands. With plasmas negative temperature coefficient you are better off with less material. I am not an expert on this however so i could be completely wrong.
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spectre
Sat Nov 01 2014, 11:20PM
spectre Registered Member #42773 Joined: Sun Jan 12 2014, 03:38PM
Location:
Posts: 7
Houdini0118 wrote ...

Any amount of material will turn into plasma and as i understand it the less material the hotter what is there gets and the more it expands. With plasmas negative temperature coefficient you are better off with less material. I am not an expert on this however so i could be completely wrong.

so what you are saying, i should apply vacuum to the chamber and fill it with whatever gas to somewhere between 0.1 and 0.9 bar (assuming normal pressure is 1 bar.)

and yes, a hydrogen/oxygen mix could be fun. maybe a chamber filled to 0.5 bar with the mixture.

will design the chamber to be excessively strong, made from chrome-moly steel or titanium...held together with bolt for easy dis-assembly for replacing electrodes and inspecting blast chamber. 6 or 8 M6 bolts, 8.8 grade, each bolt have a tear strength of around 2 metric tonnes.
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