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need assistance on my first coil gun

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phil

Mon Mar 13 2006, 03:32AM

Registered Member #314 Joined: Mon Mar 13 2006, 03:12AM
Location:
Posts: 52

Hello everyone. I've recently become alot more interested in coil guns, but dont know where to start. I have a mediocre knowledge of electronics, meaning i can solder, design/test circuits, and program microcontrollers. But i have ABSOLUTLY no idea on where to begin with the coil guns. The mechanical aspect is not the problem being i've grown up in machinshops. But i don't know what kind of IC's, capacitors, circuits, or tutorials to use. If anyone can give their 2 cents it will be greatly appreciated. I am also a BIG fan of links, so any websites you know of will be read thouroughly

...

Mon Mar 13 2006, 04:07AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

I would recommend the magnetic gun club, a ton of information, some simulators, etc.

For a first coilgun, I recommend the KISS method. [keep it stupid simple]

1. decide how big of a projectile you want and how fast you want it to go.
2. take those numbers (convert into kg and m/s) and plug them into J=.5mv^2
3. since 1% efficiency is a reasonable goal for a first timer coil gun, multiply by 100 to get the amount of energy you need in your capacitor bank.
4. Decide on the voltage you are going to use. I recommend using 400v capacitors because they are pretty easy to find, or if you are on a budget use the 300v ones out of disposable cameras. Then
5. figure out how much capacitance you need, using j/c=.5v^2, or c=2J/v^2. Then multiply by 1000,000 to get microfarad's.
6. try to find a cap that big or a little bigger
7. once you have the cap, look for a switch. For a beginner, so for a SCR. Get one rated for a little over the voltage your caps are rated at. The current needed is where things get tricky. You have to find a trade off of having too little current and creating too long of a pulse, giving a lot of 'suckback', or too short of a pulse which requires an expensive SCR to switch. I would say for a small coilgun (~1 output joules) go for 100a, for a medium one (10-50J) go for like 500, and bigger ones even more.
8. Then find a switch that meets the requirements. If you are on a budget, try to sample some from littlefuse.
9. Then you need to design a coil. Use the simulators in the link above to do that.
10. then to charge it I recommend using a pair of 12vAC (or whatever) 'wall warts' back to back, then add on a voltage tripler with 10ufd/200v caps and 1n4007 diodes. If you can only find dc wall warts, you can bust them open and bypass the rectifiers in them. You also need to put a resistor across the cap bank to bleed off any voltage so you do not pick up the caps the next day and kill yourself. Use a resistor in the 100k range.

have fun and be safe!

Yohan

Mon Mar 13 2006, 01:30PM

Registered Member #194 Joined: Fri Feb 17 2006, 07:52PM
Location:
Posts: 19

... that has to be the most ignorant post I've seen out of you yet.

This forum is dedicated to those in search of knowledge and you create a "Coilgun for Dummies" post. Shame on you.

Phil, because these devices are intrinsically complicated you should first understand R,L,C response. Specifically focus on how inductors behave in a power electronics atmosphere.
Here is a great site for this:

http://www.powerdesigners.com/InfoWeb/resources/pe_html/contents.htm

After you feel comfortable with the physics of "what's going on" then go into the archive and look at the various applications. There are literally hundreds of posts with a ton of information that will help protect you from "KISS" injuries.

My opinion: SCR's have been proven to be a dead end for coilguns. IGBT's are far superior and becoming more affordable so I would hone my archive search to those who have done work with IGBT triggering schemes.

Good luck and don't be afraid to ask questions if you get stumped!

rupidust

Mon Mar 13 2006, 04:33PM

Banned
Registered Member #110 Joined: Fri Feb 10 2006, 12:23AM
Location: Banned City
Posts: 85

For a beginner an SCR is a good start. Majority of beginners use contact switches, now there is the true dead end. Dead end for an SCR is not only extreme but is false. Dead end means no why out or point beyond progress. There is no superiority comparison also. In respect to current SCR has greater current to size ratio, in respect to on-off control only IGBT truely has these feature. Idealy the switch should have on-off control, high current density, and small package size. High current density IGBTs are bulky, expsensive (still), and still barely match the current density of an SCR 1/3 its size.

most ignorant post...(false)
dedicated to those in search of knowledge...(then step out the way)

I have seen archives. Lots of typing and no proof. Ditto on my many contributions.

Steve Conner

Mon Mar 13 2006, 05:54PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Yohan, I hope that post was tongue in cheek. I thought it was rather nice of ... to go to the trouble of making a quick coilguns for dummies guide. Remember this forum caters for all levels of experience, not just uber lords of coilgunning like yourself.

I agree with what he says except I wouldn't even bother using a SCR: I would just use a heavy duty relay or mechanical switch. It's not going to be as good as a SCR or IGBT (the mechanical switch arcs a lot and will destroy itself in a high powered application) but if you're a newbie I guess you just want something that does something.

Yohan

Mon Mar 13 2006, 06:32PM

Registered Member #194 Joined: Fri Feb 17 2006, 07:52PM
Location:
Posts: 19

Dead end for an SCR is not only extreme but is false.

Wrong. If only used as the primary trigger, SCR's offer no control over pulse length or energy recovery. Using an SCR alone is a horribly inefficient approach toward the coilgun and leaves little room for configurations. Dead end, it has been proven.

dedicated to those in search of knowledge...(then step out the way)

I suggest you venture into the archives to look at what I've done. Then re-evaluate this comment.

Yohan, I hope that post was tongue in cheek.

No, it wasn't. The forum is becoming a "lets repeat what everyone else has done" venue with little contribution toward the advancement of the technology. I, of course, am not speaking toward the veterans who continue to explore new territories. We need to get off the old and crack open the new.

I agree with Steve. If he just wants to build a quickee, use a light switch, mech relay or even a toggle. But, if he wants to get into the nitty gritty, bypass the damn SCR and move toward the IGBT.

But most importantly: READ! It's all in here.

WaveRider

Mon Mar 13 2006, 08:07PM

Registered Member #29 Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500

I think we all need to remember that we were once beginners too. Ignorance is not a crime, only a temporary state of mind.

Phil, the mechanics of basic reluctance coilguns (the ones with the iron slug) are pretty easy to understand..

1. build up field in coil quickly by pushing as much current thru it in a short time as you can.
2. suck the iron slug into the coil.
3. before the slug begins to exit, switch off the current as fast as you can.

The trick is in how these things are done. Certainly, as Yohan says, IGBTs offer the best control. However, they can be a bit expensive. However, many coilgunners here have got by with relays, diodes, photoflash caps and light-switches. High performance, tho', requires some more careful thought!

Cheers!!

Simon

Tue Mar 14 2006, 12:34AM

Registered Member #32 Joined: Sat Feb 04 2006, 08:58AM
Location: Australia
Posts: 549

My first "coilgun" was simply some wire wrapped around a empty pen, which I touched against a PhFC. There's nothing wrong with that; it quite literally sparked my interest in doing a better job.

My KISS is even more KISS than ...'s. Fill something with magnet wire (sewing machine bobbins are good), find something that's ferromagnetic and fits well inside and discharge a cap into the coil. Experiment.

If you're going to go simple, some warnings: high energy caps are dangerous, never trust them (they can sometimes recharge themselves through dielectric absorption, ouch!), coilguns can fire by accident so always be prepared for the projectile to shoot out (in either direction). Wear goggles and stand to the side at any time there's anything inside the coil.

Even though the cg is easy to make, respect it because it can really hurt you.

Bjørn

Tue Mar 14 2006, 01:19AM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

Here is a coil gun I made a few years ago to show a member the different parts. It is nothing wrong in starting it simple. When it comes to progress, coil guns that were made about 100 years ago were quite efficient and delivered several kJ to the projectile. I am pretty confident that I can count on one finger the number of people that have surpassed that.

phil

Tue Mar 14 2006, 03:07AM

Registered Member #314 Joined: Mon Mar 13 2006, 03:12AM
Location:
Posts: 52

Thanks for all the help guys. I'm going to start working on a single stage CG. I was thinking of useing a 555 timer to control the on/off time because I know my way around these kind of chips. I'm still a little shakey when it comes to the capacitor charging. if anyone could point me in the direction of a diagram or schematic that would be great. So far i have a coil made out of 24 guage hookup wire (only stuff i had). I'm not sure how many turns, i just kept wrapping it :). I pulled some 450 volt capacitors off a circuit board, so i'll probably use one of those. Thanks again!!

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