Cheaper induction CG? / Electromagnetic Projectile Accelerators / Forums

Forums

4hv.org :: Forums :: Electromagnetic Projectile Accelerators

« Previous topic | Next topic »

Cheaper induction CG?

1 2 next

Move Thread

LAN_403

GimpyJoe

Tue Mar 14 2006, 08:04PM

Registered Member #316 Joined: Mon Mar 13 2006, 01:30PM
Location: Marietta, GA
Posts: 212

I'm really impressed with the speeds people are getting with these induction CG's and I'm hoping to put one together myself since my local hamfest is coming up in a week and I can get some parts. The trouble is that large pulse caps are expensive and hard to find. My question is: would it be possible to use one pulse capacitor firing multiple times and charge the pulse cap with those cheap surplus energy storage caps? The storage cap would charge the pulse cap through an inductor. Does this sound like it would work?

pulslaser

Tue Mar 14 2006, 11:00PM

Registered Member #156 Joined: Sun Feb 12 2006, 07:04PM
Location:
Posts: 23

Hi,
the caps I used for my last coilgun aren't real pulscaps although they work very well in this application. I think caps which are used in the powersupply of high power shortwave amplifiers should work as well, maybe you will find on this hamfest.
I'm think your idea is a bit difficulte to realize, you have to recharge the puls caps in a short time, some hundert mikroseconds, unless you make the distance between the coils very long, and I' think the aktive spark gap will not quench in this short time to allow recharge. So you have to use quenched spark gaps or hydrogen thyratrons for the switches
But on the other hand it would be an interesting experiment to try

Quantum Singularity

Wed Mar 15 2006, 05:52AM

Registered Member #158 Joined: Sun Feb 12 2006, 09:53PM
Location: Central Ohio
Posts: 282

Hamfests are a great source for stuff. Maybe I am spoiled though because I have only gone to the Dayton Hamfest (in Ohio) and if I am not mistaken its the biggest one in the U.S. HV pulse caps are still pretty rare though, but my first year there I got 2 HV caps that were 10kV 4uF for $10. They guy was selling them cheap because they have PCB's in them and there is not a whole lot of demand for these type of caps. I am not too worried about pcb's so long as they dont leak and I take certain precautions. It was a great way to start of the hobby cheap. I scored several used NST's for free from local neon sign shop (asked for donation for college project) and some HV diodes for $2 each mailorder. I wrapped coils out of regular household wire (14awg-8awg) and used a simple spark gap out of 2 bolts. For under $20 I was flinging disks and copper rings in the air

Plasmaniac

Wed Mar 15 2006, 02:51PM

Registered Member #206 Joined: Sat Feb 18 2006, 03:17PM
Location:
Posts: 72

I scored this cap for only 5â‚¬ on a hamfest:

Cap New

Also, my railgun caps (15x2200ÂµF 400V) are from an other hamfest some time ago: 2DM (1â‚¬) each.

So if you have some luck and/or connections to some guys that go there every month, you can get really cool stuff on hamfest

I use non-pulse-rated caps in my induction coilgun, also. If you can't find some HV caps, you can make a electrolytic capbank, just like FastMHz.

[Edit: Fixed oversized picture]

Dr. Shark

Wed Mar 15 2006, 03:34PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

I suppose some success could be achieved with microwave caps, if they are overdriven hard, like Steve Ward does it.
Or you could try the Plate/Al-foil cap that is described in another thread. It could give you the required energy, however the impedance would be hard to match due to the absurdly high voltage (40kV).

GimpyJoe

Wed Mar 15 2006, 10:44PM

Registered Member #316 Joined: Mon Mar 13 2006, 01:30PM
Location: Marietta, GA
Posts: 212

I've seen the ring launchers, but the reason I want to recharge the cap is to use a multistage design like Pulslaser's.

Yeah, I'm hoping to find some good deals, but as far as I know there are only two or three hamfests in the Atlanta area each year

mike0t4ever

Thu Mar 16 2006, 06:08PM

Registered Member #321 Joined: Wed Mar 15 2006, 03:33AM
Location:
Posts: 14

you mean to use one cap to be used for firing multiple stages?

and recharged in between the firing of each stage?

just think of it this way:
if you rechage the cap in such a short time the pulse energy going in is equal (assuming no losses) as the energy that you will be dumping into the coil.
which would mean that the cap charger would be capable of being connected directly to the coil.
now how do you build such a charger? most likely with caps (unless you have some knowledge of compulsators or similar devices)

GimpyJoe

Thu Mar 16 2006, 07:03PM

Registered Member #316 Joined: Mon Mar 13 2006, 01:30PM
Location: Marietta, GA
Posts: 212

That's exactly what I said in my first post

A nice pulse-rated cap is charged multiple times by energy storage caps.

Quantum Singularity

Thu Mar 16 2006, 09:30PM

Registered Member #158 Joined: Sun Feb 12 2006, 09:53PM
Location: Central Ohio
Posts: 282

Funky, I think what Mike is trying to say is if you can recharge your caps fast enough between firing, then you can just use your 'recharging circuit' to pulse the coils directly. The time between each coil firing isnt much longer than the pulse is, it can even be shorter. Unless you space your coils out about like 20 feet or something, your charger would have to pulse just as fast as your caps the drive the coils. Even spaced out that far you still talking a charge time in the millisecond range or so. So what you propose probably would give much if any benefit and make it alot more complicated. Just look at the graph that pulslaser put in his post "Amateur coil gun approach sound barrier" and youll see just how tight the pusles are. I might be missing something here though, if I am please key me in.

P.S. If you are merely trying to avoid having to buy multiple caps for multiple stages then you can probably use a lower voltage and a switching device that can be shut off like IGBT. That way you only fire a portion of the caps energy during each stage. For example if you can get a cap that holds 1000 joules, and trigger quick pulses around 250J net, then you could run 4 stages. This is a very simplistic example though. Good luck.

P.S.S. As many have pointed out in these forums you dont have to have pulse caps. Many have done well with electrlytic banks. If you can get something cheap enough that you dont care if you eventualy ruin it, then try it!

GimpyJoe

Thu Mar 16 2006, 11:26PM

Registered Member #316 Joined: Mon Mar 13 2006, 01:30PM
Location: Marietta, GA
Posts: 212

Thanks! I didn't realize the time the coil and cap spend ringing down was so long compared to the charging time. I guess I'll just have to see what I can find at the hamfest.

1 2 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob