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New Coilgunner, need advice and tips!

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Rokko8652

Sun Jun 27 2010, 04:02AM

Registered Member #2940 Joined: Fri Jun 25 2010, 06:36AM
Location:
Posts: 9

Ok, I finished the charging circuit, so that leaves alot of things left to do. I plan to finish my capacitor bank, then do the barrel/coiling. Using the RLC simulator, could I figure out how many coils/layers I would need?
Would that involve testing the inductance of a small segment, then applying that to the total length of the coil?

GhostNull

Sun Jun 27 2010, 07:42AM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

To design your coil you should use this

. It will give a good approximation for your coil. Then you can fine tune it later by adding and taking turns.

Rokko8652

Sun Jun 27 2010, 07:18PM

Registered Member #2940 Joined: Fri Jun 25 2010, 06:36AM
Location:
Posts: 9

Wow, I didn't even see that! Thanks!
So with that, it seems I'll be set. Thanks for the constant help and putting up with my newbie-ness/stupidity, GhostNull.

Barry

Sun Jun 27 2010, 09:48PM

Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301

Another idea for an inexpensive reliable easy charger is to use a transformer and a diode bridge. Of course this works from the mains instead of a battery. Example schematic:

Cheers, Barry

GhostNull

Mon Jun 28 2010, 06:54AM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

So can you post what you plan to do?

Rokko8652

Mon Jun 28 2010, 07:42AM

Registered Member #2940 Joined: Fri Jun 25 2010, 06:36AM
Location:
Posts: 9

Plan to do for what? The entire gun? Or just a few parts?

GhostNull

Mon Jun 28 2010, 08:13AM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

Just the general system, capacitor bank, switching devicing, coil etc. basically wat I asked before. I want to make sure you don't stuff up XD

Rokko8652

Mon Jun 28 2010, 08:46PM

Registered Member #2940 Joined: Fri Jun 25 2010, 06:36AM
Location:
Posts: 9

Ok:
I am using the standard charging circuit ( I know it will be slow, inefficient, etc. but it is small, simple, and cheap
For the capacitors, I am using 12 photoflash capacitors with ~1200 uF at 330v.
I don't have the barrel or coil thought out yet, but I will work on that once I get it all done first, so I can use the pages you linked before with the simulations.
I still don't know what I need to do with switching. Doesn't that just apply to multistage guns?

GhostNull

Tue Jun 29 2010, 06:45AM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

this is becoming a bit chatty so let's take this to PM

Saz43

Tue Jun 29 2010, 07:02AM

Registered Member #1525 Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294

Switching is a very important design aspect for a coilgun (single and multistage), since it will have a large impact on your efficiency. If you want a coilgun that is efficient, safe, and durable, you will want to spend some time and maybe $$ on your switch. Here are some options as I see them:

Spark Gap: This is can be as simple as touching two wires together to complete the circuit between your bank and coil. This is cheap and easy, and is a great option if you have a low power device and simply want it to work. However, a ton of energy is lost in the form of a big spark when the wires touch together, and the mating is usually less than perfect so contact resistance will be high. This results in less energy to the coil and a lower efficiency. Unless your spark gap is made with some tough material, it will erode and need replacement after a while. it may also be dangerous to have these little electrical explosions occurring all the time. My second coilgun (330V, 680uF) was triggered by a simple wire-to-wire spark gap. It worked fairly well, but each shot would come with a loud bang, a huge spark, and the wires would weld together. It also didnâ€™t yield consistent firing results, which bugged me.

Mechanical Switch: You can use a high current switch or even a relay to close your bank/coil circuit. This isnâ€™t very efficient either, but probably better than a spark gap. The main problem here is that the switch mechanism can weld shut with the high current associated with coilguns. My first coilgun (single photoflash cap) was triggered by a 120VAC wall switch, which welded and siezed up after 10 or so shots.

Solid State Switch: This is the best bet for a coilgun. Youll get the highest efficiency since these have no mechanical mechanisms and thus no energy is lost to ionizing air or due to contact resistance. The downside is cost (which I really don't think is too big a deal, you can get some hefty SCRs on ebay for under $20 if you're patient). An SCR is basically a big fat diode which you can close like a switch by applying a small "gate" current. Look these up on google and learn more, it's worth it! SCRs are THE standard for medium to high powered hobbiest coilguns. My current 1.2kJ coilgun (almost done btw!) uses a C185N SCR to switch 4kA and it works beautifully, blasting through several layers of cardboard and wood. The next step up is the IGBT, which is basically a big fat transistor. The advantage of an IGBT is that it can be turned off whenever you want (unlike the SCR which stays on until the current reaches zero) essentially giving you control of your pulse and opening the door to higher efficiencies. The disadvantage is that they cost more than SCRs and are very easy to fry.

Since your coilgun is 65J, you can probably get away with a spark gap since you arenâ€™t switching too much energy. I would caution against a mechanical switch since at that energy probably anything you use is going to weld, save a household circuit breaker. However, if you want to get the best results without adding too much complexity, I reccomend an SCR as your best bet.

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