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Registered Member #546
Joined: Fri Feb 23 2007, 11:43PM
Location: Albuquerque, NM
Posts: 239
Ok, so I did some homework, but I'm at that point now where theory meets application and I'm a bit leery about all this.
I have a variety of coils, MOSFETS, IGBTs, and a range of capacitors.... but I'm really just looking for a good place to start with turns/capacitor...
So far I have a variety of Styrene and brass tubes for barrels - 1/4, 11/32, and 1/2"
I have a variety of wire to coil from.... 28 and 21 ga magnet wire (and some smaller stuff), 16ga solid PVC wire (Romex), 21,28ga stranded PVC coated wire (hookup wire).. I have access to realy any wire size I need at my shop...but if I can play with this tonight then cool.
switching: I have everything from some tiny MOSFETS (500V 10A@1ms) to a Semikron SKET 400/16E thrysistor (1600V 14KA@10ms), and a Toshiba MG300J2YS40 IGBT (600V 600A@1ms)...if that helps any
for my caps I have just about everything, but my first choice were some 10,000uF 50V electrolytic or a 6000uF 300V electrolytic caps. one would probably be enough, I think.
I also have a closet full of MOCs and various voltage GE motor start caps, and power caps between 1-50uF around 100-1000V
So I got the basics of physical design down, but given what I have on hand I'm worried I might try to push too much power through my coil too soon...last thing I need is to be digging wire shrapnel out of my face...
I was thinking 45 turns (3 layers 15 turns each) of #21 solid PVC wire with the thrysistor and the 10,000uF 50V cap on the 11/32 barrel.
For ammo I am using tension pins (heavy steel tube with a slot cut the length of it) about 1.5" long in the 11/32 size.
So... I guess my question is, in the end here: Could someone with some first hand experience suggest a good starting point for a coil gun? I'd be more than happy to just copy someone elses coil for my first attempt if you have something you think works quite well. I can read the schematics and do the mechanical work, but knowing which formula to apply and when is still eluding me without something to actually get some hands on experience with first.
Registered Member #546
Joined: Fri Feb 23 2007, 11:43PM
Location: Albuquerque, NM
Posts: 239
hrm... how to respond. I can tell by your previous posts that you're primarily here for the heady advanced discussion, and I can appreciate that - but why bother to respond at all? I don't know you, but you've certainly started the conversation off on a standoffish tone, so please pardon any perceived defensiveness on my part. I know it's something this forum is notorious for, but I didn't think I'd get read the riot act for asking where I start.
Anyways, I guess you missed where I said I'd done some homework. I've read barry's site front to back. While it gives the math, IMO I've found it to be lacking in certain aspects and it makes the assumption that the viewer has already constructed a CG, which I have not. it glosses over much of the trial and error which I find incredibly useful in getting started.
I've searched back through several pages of both the new and old forums. mostly what I've come across is people making hand held CG pistols of very low power or very small caliber. The problem is that there is SO much information here and very little of it is in the HV-Wiki (a great idea, horrible implementation) for easy access. The search function works great to find a specific thread you remember the name of, but putting in "coil gun" or "capacitor" returns about 50,000,000,000,000 unrelated results.
Despite googling and searches here I couldn't find a good FAQ style page for coil guns... just lots of people showing off what they've made with poor or no comments on design/construction.
Anyways.... I guess I'm going to go this alone armed with second hand information, unverifiable claims, a mish-mash of schematics, and a pile of parts...
At least wish me luck. Maybe when I've burned up a few hundred dollars worth of parts I will have better questions.
I've got a lot of ideas, and I don't apologize for looking for some guidance/encouragement for my first attempt to actually make this.
If that can't be found here (which I actually know it can be) then so be it....
Registered Member #32
Joined: Sat Feb 04 2006, 08:58AM
Location: Australia
Posts: 549
GreySoul wrote ...
Anyways.... I guess I'm going to go this alone armed with second hand information, unverifiable claims, a mish-mash of schematics, and a pile of parts...
Which is roughly how most people start.
Coilguns are so richly complicated that you have to get a feel for them to be 100% sure of what to do. If you don't want to get this by blowing hundreds of dollars of parts, get it by toying with an old plastic pen barrel, an iron nail, a batch of magnet wire and a reasonably big capacitor. Playing around with this will teach you a lot of things that a website can't.
For many experimenters, this hands-on feature is the best thing about coilgunning.
Registered Member #75
Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711
Hey, since you are not as ignorant as the average n00b, and you seem to have done your homework, I'll jump in here with a few suggestions.
1) Definitely use the 300V 6000uF cap, 50V is way to low for a decent discharge time. All you will do is suck the projectile back in the coil. Otherwise your specs sound good.
Start with 100V or so on the cap and work your way up, while there is little danger for anything to explode (except for bad solder joints maybe), you want to get a feel for where to start the projectile, etc.
2) Don's use big, expensive semiconductors at the start, they require some careful design to catch reverse current, otherwise they blow up and make you a sad n00b. In my experience a little To220 SCR is plenty for a 300V 6000uF cap, you just need to trigger it hard, e.g. by discharging a .33uF 300V cap into the gate. Don't forget to add a freewheeling diode.
Oh, one more thing, don't use PVC wire, magnet wire gives a much better fill factor. I don't know what "21 is (stupid imperial system!), but you should be looking at 1mm diameter or so. It will get warm after a few shots, but anything bigger and your fill factors suffers again.
Registered Member #16
Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554
Rupidust, relax. While the question has certainly been asked many times, it was not asked this time in any way that deserved such a reply.
I'd agree with alot of what Joe said. Stick with the 300v cap. Look for a couple cheap 600v 100A stud SCRs or similar on ebay cheap. Those big SCRs you've got would be a damn shame to waste due to a mistake on a project like this.
In my experience, I've always gone straight to the rated max of the capacitor (Not more than rated!), and stayed there as far as charging. It removes just one more variable that can confuse you. Obviously do a couple test shots at lower voltage to make sure nothing is going to die/explode (including you), but don't bother worrying about really observing the results with those runs, except to note that kinetic energy and efficiency generally drop rapidly as voltage does.
Start with a few different coils. Wind them on whatever barrel material you have handy (metal would be fine in this case, as you aren't really gunning for max efficiency). Make one with thinner wire, one with thicker, one as long as the projectile, one twice as long, etc. The one constant is you should always wind them with more layers of wire than seems reasonable. Then you can just trim off layers to find that sweet spot.
I've found that just fiddling with various coil and system variables in no particular order did more to confuse me than educate me, so come up with a system. Jot down notes and compare the things you change with each shot. I'd also recommend, if you've got the experience, building Bjoern's PIC based velocity trap. You're likely to misjudge just how fast some shots are going compared to others if you just observe damage to the target, which can vary wildly depending on alot of variables that you don't want to add into the mix.
The largest wire I'd use for a coil like that would probably be 13 awg or so. If you drop down to fewer layers, you could probably get away with 22awg or so. I've been surprised before by thinner wire performing better than thicker.
I'd stick with projectiles from 1/4" diameter on down, and from 1.5-3.5" or so in length, given your input energy.
Other than those suggestions, its all trial and error.
Registered Member #90
Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301
GreySoul wrote ... I've read barry's site front to back. While it gives the math, IMO I've found it to be lacking in certain aspects and it makes the assumption that the viewer has already constructed a CG, which I have not. it glosses over much of the trial and error which I find incredibly useful in getting started.
Always glad to hear the web site is useful. I'd have to agree that it does gloss over some basics, and just only introduces circuit theory where it applies to coilguns. It maybe has a little more description of the design process than most sites but it's nowhere near a cookbook. IMHO, coilgun parts procurement is so randomized and spotty that a introductory coilgun recipe is a mail-order frustration. By the way, it seems to me that the web site's "trial and error" is fairly well described as the web site laboriously walks through the Mark 1 through Mark 4 designs.
For a starting point? I'd suggest choosing the projectile first, your 11/32" sounds okay, but no larger. Using a little thinner stock will yield better speeds, although the kinetic energy might be a little less.
I'll suggest that coils never use more than two layers; I expect a 3-layer coil can add too much inductance to get the 2-10 msec pulse that you need.
Can you get magnet wire in sizes of 12 AWG or at least no thinner than 16 AWG? Resistance is your enemy; you will be rewarded by making everything as low-resistance as possible. Keep all high-current leads extremely short, solder all connections, etc. Don't even think of using alligator clips in the high-current path.
You'll need a good speed trap to compare results for tune-up. Find something reliable. For low cost, an acoustic trap using a microphone and sound card works well, but it can be a PITB to enter the numbers in a spreadsheet to get the speeds.
Start building. Don't suffer paralysis by analysis. Just try things.
Cheers, Barry www.coilgun.info PS - I know I'm a month late in responding to this thread. You've prolly already built it or abandoned it. But do tell us how it's coming along!
Banned Registered Member #110
Joined: Fri Feb 10 2006, 12:23AM
Location: Banned City
Posts: 85
Dave Marshall wrote ...
Rupidust, relax. While the question has certainly been asked many times, it was not asked this time in any way that deserved such a reply.
I'd agree with alot of what Joe said. Stick with the 300v cap. Look for a couple cheap 600v 100A stud SCRs or similar on ebay cheap. Those big SCRs you've got would be a damn shame to waste due to a mistake on a project like this.
In my experience, I've always gone straight to the rated max of the capacitor (Not more than rated!), and stayed there as far as charging. It removes just one more variable that can confuse you. Obviously do a couple test shots at lower voltage to make sure nothing is going to die/explode (including you), but don't bother worrying about really observing the results with those runs, except to note that kinetic energy and efficiency generally drop rapidly as voltage does.
Start with a few different coils. Wind them on whatever barrel material you have handy (metal would be fine in this case, as you aren't really gunning for max efficiency). Make one with thinner wire, one with thicker, one as long as the projectile, one twice as long, etc. The one constant is you should always wind them with more layers of wire than seems reasonable. Then you can just trim off layers to find that sweet spot.
I've found that just fiddling with various coil and system variables in no particular order did more to confuse me than educate me, so come up with a system. Jot down notes and compare the things you change with each shot. I'd also recommend, if you've got the experience, building Bjoern's PIC based velocity trap. You're likely to misjudge just how fast some shots are going compared to others if you just observe damage to the target, which can vary wildly depending on alot of variables that you don't want to add into the mix.
The largest wire I'd use for a coil like that would probably be 13 awg or so. If you drop down to fewer layers, you could probably get away with 22awg or so. I've been surprised before by thinner wire performing better than thicker.
I'd stick with projectiles from 1/4" diameter on down, and from 1.5-3.5" or so in length, given your input energy.
Other than those suggestions, its all trial and error.
Dave
Dave relax. Relax Dave.
What part of "Wouldn't prior post from this forum be a good starting point? Seriously, why do you even bother." is unrelaxing? The "Wouldn't prior post.." part, the "Seriously" part, or the "why do you even bother" part? If I had left out the "Seriously, why do you even bother" part would you have bothered to mention me in your reply?
Deserve? What a joke. Had I posted what it deserved, the contents would have been far stronger detailing how the combined works of others can so easily be destroyed by queries whose very existence undoes years of prior efforts with just a click of a Submit button. This thread is but 1 of a few hundred equally no efforts go no where threads to suck up keystrokes, bandwidth, and time, yet you behave as if any of this was some form of System International Standard to which all others are measured against.
BTW: Every point of your vague post has been exhaustively written in prior post year after year after year...
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Yeah, what are you on about? They didn't give Nobel Prizes in coilgunning last time I checked. And if they did, the last one would have gone to Kristian Birkeland about 1904.
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ZOMG another n00b w/ CG questions
In my experience a little To220 SCR is plenty for a 300V 6000uF cap, you just need to trigger it hard, e.g. by discharging a .33uF 300V cap into the gate. Don't forget to add a freewheeling diode. 
