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1.4kJ electrothermal gun

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Thomas W

Sun Mar 03 2013, 01:29PM

Registered Member #3324 Joined: Sun Oct 17 2010, 06:57PM
Location:
Posts: 1276

Very nice, when i get a very big SCR i may try this.

klugesmith

Sun Mar 03 2013, 04:39PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714

Ash Small wrote ...
Sorry about the mix up. As I said, according to international convention, F stands for capacitance (Farads) and C stands for charge (Coulombs).
C=F*V

That's like stating Ohm's law as Î©=V/A instead of R=V/I (or R=U/I; see below)

Everyone: This is a good example of confusion caused when one
casually replaces the names/symbols of constants or variables,
with the names/symbols of their units of measurement.
Discussed at 4hv.org not long ago:

"Are 250 and 6-8 LeBron James's pounds and feet and inches, or his weight and height?
Are 98.6 and 37 our normal body degrees F and degrees C?
We say "current = 10 kA", not "kA = 10". One appears naturally when working out a formula; the other does not. I have tried to make a good example in this spreadsheet:

Unfortunately, there is one near-exception that can lead newbies astray.
Today it's popular, in fact practically universal, to use the word voltage (and symbol v or V) for the thing that is measured in volts (V)."

At least in English, since E (for electromotive force) fell out of favor in the last century.
European documents often symbolize voltage (Spannung) as U (Unterschied), which avoids the confusion. Wouldn't've worked in the Roman Empire era.

Everyone agrees that V is the SI symbol for the SI unit volt.

Someone else brought up the prefix letters in circuit diagram component reference designators.
For basic passive elements R, C, and L, they match the symbol of the variable provided by that component. Two out of three happen to match the name of the component type.
I think it stops there. Q1 is a transistor, not a charge element. U1 is an I.C.

Wizzup

Sun Mar 03 2013, 07:24PM

Registered Member #3302 Joined: Sun Oct 10 2010, 02:21PM
Location: Finland
Posts: 42

New chamber ready! I worked today the whole day on it and it looks pretty good in my opinion:

overview:

back:

top:

front:

I didnt take a video of the first test shot but I made a 0.25g airsoft round penetrate 3.5cm into wood.

Still needs some work but I guess I can get a video of it in action tomorrow.

Ash Small

Sun Mar 03 2013, 07:53PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Wizzup wrote ...

I didnt take a video of the first test shot but I made a 0.25g airsoft round penetrate 3.5cm into wood.

Still needs some work but I guess I can get a video of it in action tomorrow.

That's pretty amazing. How many coil guns of equivalent power are that destructive?

Can't wait to see the videos.

klugesmith wrote ...

Everyone: This is a good example of confusion caused when one
casually replaces the names/symbols of constants or variables,
with the names/symbols of their units of measurement.

It is certainly partly that. I think it's also me not fully understanding 'charge' and 'capacitance', charge being capacitance multiplied by volts, and capacitance being charge divided by volts. The symbol for capacitance being 'C', and the symbol for units of charge being 'C'.

I don't recall ever using 'Q' for charge. I've always thought of charge as coulombs, 'C', and capacitance, 'C', as 'the charge stored in a capacitor'.

I thought 'Q' was a property of resonant circuits.

I know one coulomb is one amp for one second, and energy is a function of coulombs multiplied by volts (kW hour or joule, etc.).

Energy is also a function of capacitance multiplied by volts squared.

This is where I thought I'd found some 'extra' energy. (I didn't think it was 'free', I just thought it must 'store' more energy.)

My mistake. I've learned something today.

Yandersen

Sun Mar 03 2013, 08:17PM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

Ash, we don't care, really. Just relax.

Adrenaline

Mon Mar 04 2013, 02:31PM

Registered Member #235 Joined: Wed Feb 22 2006, 04:59PM
Location:
Posts: 80

Impressive results for your ETG. You definitely need a chrono, I'm curious as to the efficiency. What about making a ballistic pendulum?

For the OT conversation:
Its seems there is a confusion of units and variables...
C is the units of coulombs
F is the units Farads
What is L? Liters or inductance?
When used as a variable (in an equation), C is capacitance, q is charge, f is frequency.
f=1/2*pi*sqrt(L*C)

What about e=1/2 m*v^2
is m meters or mass?
You can see where this is going...

Wizzup

Mon Mar 04 2013, 04:05PM

Registered Member #3302 Joined: Sun Oct 10 2010, 02:21PM
Location: Finland
Posts: 42

First post updated with a new video :)

Ash Small

Mon Mar 04 2013, 04:09PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Adrenaline wrote ...

For the OT conversation:
Its seems there is a confusion of units and variables...

As I pointed out, it wasn't just that, it was also me not understanding the charge in a capacitor, which is capacitance multiplied by voltage, and energy being capacitance multiplied by voltage squared. The mistake I made was thinking that energy was charge multiplied by voltage squared, thanks to me not understanding/mis-reading an equation.

Probably a pretty basic mistake, but I now have a much better understanding of capacitors, thanks to this forum.

I think we can probably drop this subject now, and leave this thread to it's intended purpose.

Wizzup

Mon Mar 04 2013, 06:23PM

Registered Member #3302 Joined: Sun Oct 10 2010, 02:21PM
Location: Finland
Posts: 42

Seems like I have to design my chamber all over again... The nylon parts inside the chamber that work as insulators cant take the pressure created when firing heavier projectiles. I will spend more time designing this next chamber and I will probably start constructing it next weekend.

Yandersen

Mon Mar 04 2013, 08:34PM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

Wizzup, how about making the device safer and more durable?
My considerations here: Aluminum foil around both of electrodes, but with 1mm air gap between those (or a paper insulator to ensure the right distance); triggering transformer on a ferrite ring with 1:20 turn ratio and non-polar 0.1uF x 450V cap to connect to primary to trigger the spark between aluminum pieces? Here is the schematic I stick to in my air gap discharges (so it is proved to work well):

1362429259 6944 FT151239 Etg Switch 450v

Diode is not necessary unless you have an inductor in circuit and you have positive voltage left after shoot, but still preferable, as IF, due to some magic circumstances, cap bank get repolarized, your charger will blow.
And here is an example of an ignition material for your chamber:

Still I would recommend two copper rods going along the barrel, but distance between those should be less than distance between any of them and a barrel (if it is made of conductive material). MrFlatox was right about using an epoxy fill to fasten electrodes at the end of a barrel - it is hard, non-conductive and can kinda sustain high temperatures.
You can try the following chamber construction:
copper pipe as a barrel and external electrode of the discharge chamber;
copper rode as another electrode going axially in center of the copper pipe - second electrode;
the roll of two aluminum foil tapes insulated by a paper tape as an ignition material: make two thin tapes of aluminum, put a paper between those and roll it in a aluminum cap way; ensure one "electrode" contacts with barrel and another "electrode" contacts wit central copper electrode.

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