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Capacitors and their ratings, impact.

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youngcoilgunner

Tue Sept 02 2008, 04:42AM

Registered Member #1633 Joined: Tue Aug 12 2008, 04:21AM
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Posts: 45

first of all, you dont need to get them at a dedicated photoproccessing store. a 1HR photo shop in target, wallmart, or walgreens will give you capacitors which store much more energy than your caps. 20000 Âµf sounds like a lot, but for coilguns, its more the voltage that counts. you need voltages above 100 volts to overcome much of the resistance of the coils, and the capacitance is there only to allow you to keep a very high current pulse for a useful time period. your caps will make a small current pulse for too long, drawing the projectile back into the coil if you make your coils too resistive. Furthermore, using a camera chip to charge your caps will take along time, not to mention you could seriosly damage them. either find another way to charge them or use disposeable flash camera capacitors. I suppose you could use a camera chip to charge them, but you need to monitor the voltage verrry carefully.

Angry_Llama

Tue Sept 02 2008, 03:43PM

Registered Member #1669 Joined: Sun Aug 31 2008, 02:33AM
Location: 127.0.0.1
Posts: 13

youngcoilgunner wrote ...

first of all, you dont need to get them at a dedicated photoproccessing store. a 1HR photo shop in target, wallmart, or walgreens will give you capacitors which store much more energy than your caps. 20000 Âµf sounds like a lot, but for coilguns, its more the voltage that counts. you need voltages above 100 volts to overcome much of the resistance of the coils, and the capacitance is there only to allow you to keep a very high current pulse for a useful time period. your caps will make a small current pulse for too long, drawing the projectile back into the coil if you make your coils too resistive. Furthermore, using a camera chip to charge your caps will take along time, not to mention you could seriosly damage them. either find another way to charge them or use disposeable flash camera capacitors. I suppose you could use a camera chip to charge them, but you need to monitor the voltage verrry carefully.

I know about most of the stuff you have said already, and I understand that it's not the most ideal way to do things, but I'm just using it as a "bench test". Furthermore, in future, more refined coilguns, I would most definitely use better circuitry then the parts I am using now. Actually, since I hadn't considered it until now, what would be a good way to provide a charge to my capacitors?

youngcoilgunner

Wed Sept 03 2008, 10:11PM

Registered Member #1633 Joined: Tue Aug 12 2008, 04:21AM
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Posts: 45

well, if your using 63v caps, you should try to only charge them up to 60v max. a good and fast way to charge would be to get 6 9v batteries in series, and connect it to your cap with a switch. this will provide 54 volts, and charge the capacitor very fast.

Angry_Llama

Fri Sept 05 2008, 08:50PM

Registered Member #1669 Joined: Sun Aug 31 2008, 02:33AM
Location: 127.0.0.1
Posts: 13

Well, I just got a large supply (15-20) of 150Âµf, 330V capacitors from the disposable cameras from CVS. These should be sufficient, but the charging method you posted wouldn't work well. Also, I need to know a safe and quick way to discharge the damn things! They keep sparking giant sparks when I short them. Think "firecracker" and you've got the right idea.
Edit: I did the math, and it works out to about 82 joules of Potential Energy if 10 are run in parallel. How does this translate into velocity of the projectile?

Backyard Skunkworks

Fri Sept 05 2008, 11:55PM

Registered Member #1262 Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451

The velocity thing totally depends on the rest of the gun (the coil). However I can say my 65J pistol shoots a large nail at around 15m/s.

You really should build a boost converter for charging the cap's, however I think you can get away with a camera circuit for 85J, at least at first.

Switching: SCRs are the best method. If the coil is highly inductive you can get away with some high current switch that closes very fast, so long as there isnt sparking it'll work.

Don't forget the protection diodes or your caps will die young.

rp181

Sat Sept 06 2008, 01:25AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
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Posts: 1529

recdetified mains gives you 150v, use a voltage doubler (search crocafft Wallton(not how you spell it) or voltage multiplier)

Angry_Llama

Sat Sept 06 2008, 02:36AM

Registered Member #1669 Joined: Sun Aug 31 2008, 02:33AM
Location: 127.0.0.1
Posts: 13

Skunkworks, you seem to be very well-versed in the technical aspects of this, so I'll direct this towards you, but if anyone else can answer, I'd appreciate that too!
What do you mean by Protection Circuit and Boost Converter? I understand the concepts, yet have no idea on their execution, a link to an explanation or schematic would be appreciated. Additionally, could you suggest an SCR from mouser or another site to purchase? I can't seem to find one suitable for this project...
Also, after tallying, I have 22 330v Capacitors, split pretty evenly between 120 and 80 ÂµF. All of them were received for free, and I could probably get about eight times that by going to other stores as well. So, in theory, my total Potential Energy would work out to about 120 joules. What would be a good coil design to maximize the output? My initial idea was for a much lower energy device, but it seems like it would fail with such a high amount of energy.
I made the questions bold for lack of a better organization method, hopefully it helps.

Backyard Skunkworks

Sat Sept 06 2008, 04:52AM

Registered Member #1262 Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451

I'm by no means the best coilgunner around (see: barry), however I do know a fair amount...

The protection diode is just a diode connected across the coil so it opposes the charge of the capacitors. It should have a high current rating.

For SCRs I'd reccomend TYN640RGs rated at 400A peak and 600V, I'd parallel maybe three to five for overkill sake, to make the thing electrically bullet proof (you could really get away with one or two, but it's a good idea to have some room for max current increases incase you wanna upgrade).

Coil design: I pretty much use TLAR engineering (THAT LOOKS ABOUT RIGHT! ...so lets see what it does), however this is poor practice. Use Barry's induction sim

and RLC sim

to get a nice short pulse time with high current spike and build whatever coil does that. (also take note of the current spike, this plus 25% is the minimum surge rating your SCRs need.)

Uzzors has a schematic for a boost converter floating around, it converts low voltage DC into high(er) voltage DC.

youngcoilgunner

Sat Sept 06 2008, 05:14AM

Registered Member #1633 Joined: Tue Aug 12 2008, 04:21AM
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Posts: 45

I posted a schematic for a Boost converter under the recent boost converter topic. I havent listed all the component specs yet, but I will try and write them down asap

Angry_Llama

Sat Sept 06 2008, 01:28PM

Registered Member #1669 Joined: Sun Aug 31 2008, 02:33AM
Location: 127.0.0.1
Posts: 13

Thanks guys! Between youngcoilgunner, Backyard Skunkworks and Barry, I seem to have gotten all my questions answered. I guess you have to start somewhere, and this is the place for me to do that. Thanks again, I'll update this thread (and it's title) when I have some progress on the actual coilgun

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