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1.4kJ electrothermal gun

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Wizzup

Wed Jun 12 2013, 09:42PM

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

-- doubled post --

radhoo

Thu Jun 13 2013, 10:00AM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 699

Where did you find those ball bearings?

And would be nice to know more on the amount of water you've used, the shape of the inside electrodes, size of the chamber, the type of aluminum you've used as a "fuel", and so on. Can you perhaps draw these details by hand and post it here?

Ash Small

Thu Jun 13 2013, 10:25AM

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

radhoo wrote ...

Where did you find those ball bearings?

There are some here: (Other sellers are available.)

Wizzup

Thu Jun 13 2013, 02:57PM

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

Yep I got the bearings from that exact same seller Ash posted, i bought a pack of 50.

The mosfets arrived today morning and I assembled the driver and it is working well but I found out that the core I was using for the flyback is not suitable for that kind of operation or something. I get a small arc from it that i can stretch to about 5mm but the output power is only a couple of watts and it won't draw more than 0.35A at 12V so there is obviously something wrong.

I am pretty sure that the problem is the flyback's core as it is not taken from a real flyback, i ordered it from some russian webstore. If I take the core out completely it draws many amps so I guess ill try to find an old TV somewhere and take a real flyback out of it.

Here is a quick diagram of the ETG:
R9n9ms

Barrel ID is 6mm
The copper electrode ID is 8mm, OD 10mm, length 7mm
The insulator is M12 threaded nylon rod with a hole in the middle for the positive electrode
Positive electrode is 6mm steel threaded rod with the head filed to 4mm (basically just threads removed), the filed tip is about 3-4mm long, it varies a bit.

The negative wire from the cap bank is connected to the aluminum and the positive wire is connected to the center electrode. I can't say the exact amount of water because I usually just put a little water in there, shake it and then tap the excess water out so there is a bit water on the bottom but its not nearly full. After putting the water in I take about 1cm x 3cm piece of regular aluminum foil and push it between the copper and center electrodes using a small screw driver.

The copper electrode is just a piece of 10mm copper pipe gently hammered in place, it is there just so that when the shots melt and corrode the surfaces I can just easily take the copper out and clean it or just replace it with a new one.

radhoo

Thu Jun 13 2013, 04:28PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 699

excellent details, thank you.

What are your results on using the etg only with air (no aluminum, no water)?

What about using it only with water?

Wizzup

Thu Jun 13 2013, 05:41PM

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

You're welcome. I have not tested using just air because my bank voltage is just 400V and making a HV "ignition" system is not that easy to make and I don't have much interest anymore on trying it. I tried using just salt water (the salt is to increase water's conductivity) but my bank voltage is too low for that also, what it did was discharge the bank to it in like 5 seconds and create lots of green bubbly cream like stuff.

Also I got my flyback working properly now, the reason it didnt work good earlier today was that the core halves had a little gap between them because the secondary winding was a bit too wide so I grinded about 0.5mm of the winding holding pipe off of both sides so they fit together.

Here is a video of it in action:

EDIT: I also ordered a 3.3uF cap for the driver as I read from uzzors2k's site that if I lower the frequency the charging power will rise a lot. With a 0.68uF cap I get like 10W output, it depends on the bank voltage but he gets about 50W out with a 4.2uF cap. Ill put the new cap and my current one in parallel to get about 4uF, that should improve the charging speed.

radhoo

Mon Jun 17 2013, 02:01PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 699

sure, you can do that. a few more questions if you don't mind. I was actually impressed by the penetration test of that tick 1cm aluminum sheet. Have you done longer range tests (outside)? If not could you try that and post some videos?

Wizzup

Mon Jun 17 2013, 06:35PM

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

No I have not done any longer range tests, Ill see if I will do some someday when its good weather. Also I found a guy in ebay that sells some capacitors with exact same values than the ones I am using now, so I thought I might buy 3 of them to double my bank. The caps are little used so they are pretty cheap also. I would wire them 2 in series and then 3 of those in parallel to get 7050uF and 900V.

I am on my summer holiday atm but I got a 2 week job so I will get some money to buy the caps and stuff. After the 2 week job I will still have about one month holiday left so good time to work on this.

Wizzup

Wed Jun 26 2013, 01:16PM

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

My new resonant capacitor for the ZVS arrived today, the power difference is huge! Compare this to the video I posted previously:

The current in the arc is much larger and the arc is much fatter than what it was so I am really happy with it now, and this is just 12v. With 24v or 36v the arcs would be huge.

EDIT: I cleaned up the desk to get some more space and I cleaned up all the connections on the capacitors and ETG and wired the charger to the capacitors and tested its power. It charged the bank to about 360V in 15s so that is about 60W of charging power.

EDIT2: I didnt want to do a triple post so ill just edit some new stuff here. I organized my setup a bit so it is much cleaner now. I also built a wooden mount for the chamber so it is easier to attach it to the table, here are some pics of the new setup:

Here is a video of a shot with a steel bearing vs aluminum plate:

EDIT3: radhoo requested a longer range shot so here you go:

It is not very long range but much longer than the earlier ones... Aiming this thing accurately is not easy so I wouldnt have hit even that large piece of wood if the range had been greater. Also notice the jet of plasma coming out of the barrel, it looks very cool:

radhoo

Sun Jun 30 2013, 02:11PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 699

impressive results, thanks for making the outside video.

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