SpudFiles

I want to build a completely portable electrothermal cannon...

So, I'm looking at purchasing:

2x 4400uf 700vdc banks of 6 caps each. Each cap is 2200uf and 350vdc.

I don't know exactly how to charge these things... I'm guessing I'll need some sort of transformer to step up my 120vac from my wall outlet... And then some sort of rectifier to turn it into DC... Then probably some resistors that will lower the voltage to the right level... I don't know the appropriate charge current for these things....

Can I just charge them from a 12vdc lead-acid car battery if I find some way to step up the voltage?

Also, I really don't want to get vaporized err.. shocked.. Do I need some insulating rubber gloves and what's the safe way to handle and work with these capacitors?

Lastly, I can turn some things on a lathe, but can I make the barrel and chamber out of steel piping?

Thanks,
-Matt

you need to do research.
For charging, a voltage multiplier of of mains will give you rectified power. A resistor will not drop the voltage, the capacitors provide a voltage drop so the charging can be cut when it is the correct voltage.
As for safety, Rubber gloves are good, but I never use them. the best thing is have a discharge method, and not go prodding around while the capacitors are charged.
As I said, you need to do more research.
I have some pages with a wee bit of info:
http://rp181.110mb.com/index.php?p=1_5_ETG
http://rp181.110mb.com/index.php?p=1_10_Charging
http://rp181.110mb.com/index.php?p=1_12_Switching
DYI will probably respond, but here is his project thread:
http://4hv.org/e107_plugins/forum/forum ... .php?63596

EDIT: how could i forget larda's?
http://www.spudfiles.com/forums/28-8kj- ... 13449.html

Yea... You should probably do ALOT more research. These things are really complex and dangerous.

Also i dunno how okay the mods will be with this as it does use some solid propellant, which are banned from discussion on this forum.

They are not complex at all, but still dangerous.
ETG's are fine with the forum rules, thats why larda's thread wasn't locked. Just don't start discussion about ETC's.

Have you read this ?

It's a good place to start, and a much shorter (not to mention more useful) read than my 4hv topic.

2.2kJ is a lot to be messing around with to start off - I've been using 450J for about seven months now, and I still haven't been able to build a launcher that will operate without fairly severe mechanical stress issues (i.e., everything expands under the pressure spikes, to the point of being useless). It's possible, but get ready to start machining stainless steel.
Also, you really do need to do more reading on the capacitors. Those things can kill ya, you know

Pipe fittings are of no use for ETGs. They operate on too small a scale, generate too much pressure, and require parts too complex to be jury-rigged out of pipe fittings.

ET launchers should not be confused with ETC (electrothermal - chemical) launchers, as seems to happen frequently here. ET launchers do not derive any significant portion of their propulsive force from chemical energy, and it completely within the site rules to discuss them here.

Also, I must disagree with rp181 and say that ETGs are complex - I have what seems to be the most efficient amateur ETG (at around 17% efficiency), and I still know next to nothing about the topic. The behaviour of plasmas and their interactions with other matter are still major topics of professional research.

Yes, they are complex, but not at this level. I highly doubt kiwi is interested in plasma physics and material science, mearly trying something new to shoot things (I do not mean that in a bad way, everyone starts off like that )


May I ask where you are getting the capacitors? whats your plan to switch the power?

You could go straight into making a heavy duty chamber and making all sorts of capillary tubes and such, but I would start with just vaporizing some metal powder in open air, try some in a metal tube with one end blocked, and just build your way up to get a feel for this kind of power. Start with only one bank.

Also, regarding charging, resistors do not lower voltage, but capacitors have a voltage drop. This of it like taking a 120PSI air line and connecting it to a large tank. When you turn on the air, the chamber is not instantly at 120 PSI, but climbs, so you can cut the air at 60PSI. The higher your air flow (eg. current), the faster the pressure builds. that is why DYI was able to use a MOT (around 3,000 volts) to charge a much lower voltage capacitor, by cutting the power when the voltage is right. Despite the MOT's high voltage, my 500V tripler would charge the same bank faster, as it provides around 2A of current, while a MOT supplies around 250ma. I would still go with a supply closely matched. Capacitors, when used in the wrong polarity, charged too high, or used with AC power is very nasty, especially with capacitors this size. A capacitor around 1" diameter and 1.25" length was able to fill my room with dense white smoke (that smells like bad corn....)


Im thinking of starting in ETG's in a more serious manner soon, the railgun has gotten boring (never thought i would say that before making it!)

kiwi of nitro wrote:
2x 4400uf 700vdc banks of 6 caps each. Each cap is 2200uf and 350vdc.

I don't know exactly how to charge these things... I'm guessing I'll need some sort of transformer to step up my 120vac from my wall outlet... And then some sort of rectifier to turn it into DC... Then probably some resistors that will lower the voltage to the right level... I don't know the appropriate charge current for these things....

Also, I really don't want to get vaporized err.. shocked.. Do I need some insulating rubber gloves and what's the safe way to handle and work with these capacitors?

Thanks,
-Matt

Like any high stored energy item whether it be a heavy suspended load, compressed air, or elecric charge, the sudden release can be deadly. I would advise against a high voltage high energy project until after you work on power limited projects. Start with a strobe in the 10-50 watt/sec power range. Study tesla coils and coin shrinkers. Learn how they work with the high energy and safety required to remain alive. If you fail this test, you don't get a second chance.

I've worked with enough power to provide a serious arc flash. Understanding the danger and taking all the proper percautions is a given. This is not a place for beginners. Make the mistakes and blow stuff up with power in less than lethal energy levels.

What I am saying is play with popping balloons before working up do mine blasting.

agreed. Play around with a camera flash circuit first, if you get shocked by one of those, you will never come close to getting shocked by anything bigger. A camera flash capacitor was the worst shock I have ever had.

Ha try a homemade capacitor bank from the primary of a SGTC (spark gap tesla coil)

Similar to this one:
[youtube][/youtube]
To say the least, it hurt like a b*tch

Thanks for you help!

I've been googling and looking on the forum for most of the day, I've been really interested in this concept, but I didn't know where to start.

I'm having second thoughts about starting this project:

I don't know very much about electrothermal guns... and I don't understand most of what I am reading about them... I don't have any experience dealing with high voltage capacitors, all I really know is that these can explode tissue and bone...


This capacitor bank I am looking at, http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... K:MEWAX:IT is used and untested...

I was considering buying it, for a coilgun, then I started to do some research and found the electrothermal gun.

It looked simple at first but, it's not.

I may be getting in way over my head if I do this.

I still really want to try it..

Any idea how this was made?



It's "only" 200j (will only kill a person 12.5 times) and I want to copy it...

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now 200J is a much better starting point, and that system will be easy to replicate, it may take you some time as you learn though.
Those capacitors are a good deal, I would buy them, even if you dont use them right away.
The main thing with a portable version is the charger, everything else is much easier. It is probably a boost converter powered by a Li-ion or NiHM battery pack:
http://www.anothercoilgunsite.com/nf-po ... uction.htm

Crap...

I placed an offer of 35.00 +13.00 or something shipping on that capacitor bank I WON IT... Now my fate of electrocution is sealed...

I want to construct that 200j ETG I linked... I don't know enough about this to do it or to do it safely...

I'll start a thread when I begin construction, if I stop posting suddenly, you'll know I died.

I'm going to find someone I can speak with in person who can help me along. I think my physics teacher should be able to help me.

What kind of switch do you think that guy in the video I posted used to trigger his etg?

With any kind of high voltage device, you do not want to use a regular old switch. I have only worked with fairly low power stuff (camera flash circuits, single computer grade caps), but even with those you need something besides a switch to trigger. For low power stuff like camera flash circuits, you can just use a 30 amp relay, but for big cap banks like the ones you have, your only option are power transistors. Any kind of traditional switch will just weld itself together upon closing.

kjjohn wrote:With any kind of high voltage device, you do not want to use a regular old switch. I have only worked with fairly low power stuff (camera flash circuits, single computer grade caps), but even with those you need something besides a switch to trigger. For low power stuff like camera flash circuits, you can just use a 30 amp relay, but for big cap banks like the ones you have, your only option are power transistors. Any kind of traditional switch will just weld itself together upon closing.

I hate to disagree. Often high energy switches will use a LARGE contact relay with fast operation so the contact area closes rapidly to expand the hot spot to keep it below welding temperatures. Another option is a trigger arc initiated air spark gap (extremely noisy) and other options. Look on coin shrink sites for switch options.


See this page under the theory of operation.
http://capturedlightning.com/frames/shrinker.html

for big cap banks like the ones you have, your only option are power transistors

That would be why all the coin shrinkers and can crushers you've seen use mechanical switches, right?

Larda's ETG is a great example - he uses an electrolytic cap with about 500J stored energy to pulse a solenoid which closes a VERY large relay. Some high energy switches are even simpler - an arm "falling" in between the two switch terminals or a wedge being driven in between two appropriately shaped electrodes both work fine, and with zero bounce. Triggered spark gaps are also an option, even at the relatively low voltages the OP will be using.

The only time you NEED more complex switching methods is when you're trying to co-ordinate or precisely time your pulses, as is the case in a multi-stage coilgun.