Re: need assistance on my first coil gun
..., Mon Mar 13 2006, 04:07AM

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!

Re: need assistance on my first coil gun
Yohan, Mon Mar 13 2006, 01:30PM

... 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!

Re: need assistance on my first coil gun
rupidust, Mon Mar 13 2006, 04:33PM

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.

Re: need assistance on my first coil gun
Steve Conner, Mon Mar 13 2006, 05:54PM

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.

Re: need assistance on my first coil gun
Yohan, Mon Mar 13 2006, 06:32PM

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.

Re: need assistance on my first coil gun
WaveRider, Mon Mar 13 2006, 08:07PM

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!!

Re: need assistance on my first coil gun
Simon, Tue Mar 14 2006, 12:34AM

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.

Re: need assistance on my first coil gun
Bjørn, Tue Mar 14 2006, 01:19AM

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.

Re: need assistance on my first coil gun
phil, Tue Mar 14 2006, 03:07AM

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!!

Re: need assistance on my first coil gun
williamn, Tue Mar 14 2006, 04:28AM

You can do alot with that 555! Use it in monostable mode to generate an adjustable pulse, I have used them in counltess coilguns with accurate timing down to 10usec. IGBTs are definately something to aim for in your future research. You can use the 555 in astable to drive a simple boost converter, see the archives for a thread I started ages ago about this topic. Find or build a chronograph. This will really help you figure out what you are doing. You will actually be able to tell when something you do makes a difference in performance. All that theory is great to know, but I always learned more when I get my hands dirty. Wind some coils and start experimenting. Also search some of the older threads for actuall coilgun construction, I know there is one thread in there somewhere I addressed a bunch of new commer questions. Coilguns are great fun, I too have been rebitten by the switched reluctance bug again!

Re: need assistance on my first coil gun
Yohan, Tue Mar 14 2006, 07:17PM

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

Rupidust, are you suggesting that we haven't proven our design? Alternately, are you suggesting that our data is just typing and makes no contribution?

Surely you're not. You wouldn't be that naive. I'm certain you lost your way when writing that statement. Because if you had read the archive as mentioned, you would've seen the most efficient SRLA on this forum to be ours; employing the quite successful TTF system along multiple stages with several supporting photographs and video. I've also made numerous configuration details available and welcomed any and all to verify our results.

Read again, Chief.

I'd be more than happy to field your questions.

Re: need assistance on my first coil gun
Quantum Singularity, Tue Mar 14 2006, 10:04PM

Hey lets offer good ideas here without having to cut down on others ideas, thats uncalled for and rude, not to mention possibly against the rules. I think some basic info for those who arent an expert is great for a newb. Breaking into a new field typicaly has a learning curve, and its often more fun, entertaining, and useful to start off with a good simple working design, then experiment from that point. I built my first TC when I was 18 and didnt understand it. I went off a design someone emailed me. It wasnt even that good of design, but it worked, and if I had to learn every aspect and design it from scratch I probably wouyld never have built it. If I had to re-invent the wheel on every project I started I probably would have a lot less projects under my belt. When I build a CG it willmost likely use SCR's because I have some. If I can get some IGBT's for cheap that can pulse the kind of current I'll be using then maybe I'll consider that after I have a working design. I have alot of elect. eng. skills and I even find some of the IGBT circuits quite daunting to understand - definatly not beginner stuff unless your familiar with them. IGBT's appear to be the best solution for switching, but SCR's can be just fine - just need to tune pulse length with other things like cap size and # of turns and layers in your coil.

Re: need assistance on my first coil gun
phil, Wed Mar 15 2006, 04:32AM

How do I go about charging a capacitor. do i hook up the positive side to the positive side of the battery, and the negative to the negative side? or do i just hook up the positive side? sorry to sound like a noob but normally i only use capacitors for timeing applications, not voltage storage

Re: need assistance on my first coil gun
..., Wed Mar 15 2006, 04:56AM

I am kind of reluctant to give information here for fear that Yohan might deem it inappropriate, but to charge a capacitor you need to connect both terminals to a dc source. Because for most coil guns you need several hundred volts batteries get expensive/bulkley, so most people turn to different charging methods. For a beginner coilgun I recommend taking to small 120v->12v transformers and hooking the 12v sides of the first one to the 12v side of the other one, then plugging the first one in so you get a relatively low current 120vac on the output. From there you can full wave rectify it to give about 170v, add a doubler to get 330v, or a tripler to get a little over 500v. Do not think of plugging a cap strait into a wall outlet.
Just be safe about it.

Re: need assistance on my first coil gun
Bjørn, Wed Mar 15 2006, 06:14AM

do i hook up the positive side to the positive side of the battery, and the negative to the negative side?

Yes and it is good that you ask even if it is simple because doing it wrongly and blowing the capacitor up in your face is not very clever.

I am kind of reluctant to give information here for fear that Yohan might deem it inappropriate

Only the moderators are allowed to do that job. If someone sees something that breaks the rules they should report it and not mention it in the thread. If they disagree with the rules they have to use the Suggestion Box to request a rule change.


The only thing that is on topic here is phils quest for a working coil gun so everyone keep that in mind.

Re: need assistance on my first coil gun
Yohan, Wed Mar 15 2006, 01:14PM

Phil - In a circuit, an uncharged capacitor is considered to be a short at the instant it is connected. Although people do it all the time, it is never a good idea to hook a cap directly to a battery without a current limiting resistor.

I pulled some 450 volt capacitors off a circuit board, so i'll probably use one of those.

uF?

... - What you said was not inappropriate but I feel that you should leave something for phil to figure out on his own. Don't give the roadmap; make him dig a little. That's the only way he'll truly learn. It's ok to nudge people in the right direction. But, a tutorial on voltage triplers and coilguns will do nothing for him if he doesn't understand electricity fundamentals.

I believe you said KISS...so just rectify the AC off the wall. Bridge, resistor, filter cap, done! Probably about $2 worth of components for a 170vdc power supply that'll take him 3 minutes to put together.

Re: need assistance on my first coil gun
phil, Wed Mar 15 2006, 04:05PM

Thanks alot for all the help. And i would prefer that noone gives me a roadmap becuase i like to learn by myself. But i was just asking so i dont blow up a capacitor in my face .


EDIT:
I tried to make a test coil/circuit today. i'm not going to experiment with the 555 timer yet. I already burned out 2 25 volt capacitors . But i have another question. whenever i put power to the capacitor it cahges to the voltage of the battery, do i need a transformer to step up the voltage?

Re: need assistance on my first coil gun
Yohan, Wed Mar 15 2006, 10:07PM

whenever i put power to the capacitor it cahges to the voltage of the battery, do i need a transformer to step up the voltage

If ever there was an opportune time to break out the water tower analogy...this would be it.

Anyone feel up to the explanation?

Re: need assistance on my first coil gun
phil, Wed Mar 15 2006, 10:09PM

What your basicly saying is that i need a higher electrical potential. right? If so then i'll have to get a transformer

Re: need assistance on my first coil gun
Yohan, Wed Mar 15 2006, 10:12PM

Yes...but slow down. Might be premature for a transformer.

What potential would you like to charge to and what is your power supply?
I asked before, but what is the capacity of your capacitors...in uF?

Re: need assistance on my first coil gun
phil, Wed Mar 15 2006, 10:55PM

Right now I am useing 1-2, 9 volt batteries. and i'm experimenting with 450 volt 22 uF capacitors. The coils i'm experimenting with are 12 turn, 4 layer. and made out of 24 AWG multistrand wire. I doubt that their strong enough to do much of anything. when i put power through it it's capable of moveing a wood screw about helf a centimeter. I've been looking online for magnet wire, and better capacitors. But before I buy anything I want to get something that kinda works.

Re: need assistance on my first coil gun
Tipp, Thu Mar 16 2006, 01:43AM

450v @ 22uF isn't really a lot of energy:

Your cap = 2.2275 joules

Basic photoflash cap = 4.356 joules

So even a single modest photoflash cap would store more energy. How many of your caps do you have? A few in parellel would be perfect for shooting a small screw or nail.

Coils sound about right for a first CG.



Thats pretty much how I was introduced. People gave me basic information and I did all the research and homework until I felt confortable with it

Re: need assistance on my first coil gun
phil, Thu Mar 16 2006, 03:07AM

so i'm not so far off!!! thats good to hear. I found a transformer from an old CNC machine i used to have and it converts 24 volts to 12 volts, so hopefully i can use this to either A. provide a steady power supply for the 555 timer and charging circuit. or B. use it to step up the voltage. Right now i can get the screw to move a little, and thats only with 3 volts, cant wait to see what it'll do with 400!! :)

Re: need assistance on my first coil gun
Simon, Fri Mar 17 2006, 01:25AM

Your 24V:12V transformer won't work at 400V. The core will saturate but that's a topic for elsewhere.

The easiest way to into coilgunnery is with something with a camera flash in it. Then you have a cap and ready-made charger.

I think Barry made a number of low-voltage coilguns. Check the wiki coilgun page - there's a link to his site there too.

Re: need assistance on my first coil gun
Sebastian, Mon Mar 20 2006, 09:07PM


Here in Germany where we have 230VAC @ 50Hz I just use a 60W Light Bulb and a diode to charge my caps. The 60W bulb as a limiting resistor and the diode to get the positive part of the sine wave and therefor the RMS of 230VAC which is ~ 1,4 * 230VAC

Re: need assistance on my first coil gun
Maz, Tue Mar 21 2006, 12:38AM

the only thing i can say about giving him all the answers is,
when i started, i had no resources to go by. i experimented, and shocked myself enough to stop for a while. while i am still rather new at this, dont let your egos get in the way of helping him out. for instance help hime with a boost converter. even the theory is confusing to me still and i have made a few.