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800j coilgun project

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c0nnect3d

Thu Jul 14 2011, 02:00AM

Registered Member #4001 Joined: Thu Jul 14 2011, 12:30AM
Location:
Posts: 3

hi, I was thinking to build a coil gun but I still have few questions before starting to buy the materials.It would use 1 450v 10 000uf capacitor (charged at 400v) and my goals are to make it work and to make it as efficient as possible for an eventual portable use.

Charging: I don't know what would be the best option for charging, but I had in mind to make a voltage tripler, but this would only give me 360v.

Capacitor: I would use one big 450v 10 000uf capacitor as my power source

Switch: I plan to use a scr

projectile: i would like my coilgun to shoot small projestiles at high velocity, something like 2-4mm in diameter

Coil: using Barry's Inductor Simulation I designed a coil using 5 layers of 12awg wire that has an inductance of 0,039 mh an a resistance of 0,026ohms.

My circuit will be the RLC type that will be damped with a 0.100 ohms and will use 2700amps over 3.25ms

Questions :

1- how can I build a 400v fast charging circuit for my caps that is under 80$ ?
2- is there a cheaper or easier way than a 75$ 250a scr ?
3- is 2700amps over 3,25ms too much foa a 250amps rated scr ?
4- if my timing is not correct, can i play with the projectile length to find the best efficiency ?
5- do you have any suggestions of websites to find these parts at cheap prices ?
6- Does this project have a chance to work ?

sorry for having a lot of questions, but i would very like this to work as it's my first "real" project in electronics.Thank you a lot for your support.

vk2fro

Thu Jul 14 2011, 03:31AM

Registered Member #3930 Joined: Sun Jun 05 2011, 07:27PM
Location: Sydney, Australia
Posts: 33

This:

will charge your cap up to a preset voltage (by adjusting the pot). The parts will not cost you more than $30, especially if you scrounge.

The shutoff regulator can be put together on stripboard, and the mazzilli section can be wired point to point style. I personally made a PCB for mine. Waiting on it to arrive.

SCR's have a average rating and a peak rating. The hockey puck SCR i picked up has a peak rating at 11,500 amps for 10 ms. You should look at an SCR's spec sheet for the peak rating. The spec sheet is also invaluable in knowing what voltage and current is required @ the gate of the device to turn it on. They are usually pulse rated at a figure for 10ms. You can find cheap SCR's on ebay.

Saz43

Thu Jul 14 2011, 07:14PM

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

Hi and welcome to 4HV!

c0nnect3d wrote ...

Charging: I don't know what would be the best option for charging, but I had in mind to make a voltage tripler, but this would only give me 360v.

AC voltage is specified in RMS (Root Mean Square) values, where the peak voltage is actually 1.4 times the RMS value. So a voltage tripler would actually give you 120V*3*1.4 = 504V, which would damage your capacitors if you don't shut the charging off early.

c0nnect3d wrote ...

projectile: i would like my coilgun to shoot small projestiles at high velocity, something like 2-4mm in diameter

This will likely result in poor performance. Big fat coilguns (800J designs included) usually need to use big fat projectiles. A lightweight projectile will speed up very quickly and exit the coil before the magnetic field can do enough work to impart any serious kinetic energy. You might try to counter this by using a low inductance coil, which would have a fast current pulse which would impart all of the energy very quickly. However, low inductance is only achievable by using very few coil turns, and few coil turns means weak magnetic field, which goes back to not enough work being done on the projectile.

You could also make a longer coil, but again that means more inductance and more resistance and a longer, weaker pulse, etc...

It follows that there has to be a balance between projectile mass, coil turns, and coil inductance. This typically results in the trend that more stored capacitor energy => more massive projectile required. Everyone who watches a video of your completed gun on Youtube will be pissed off and demand that you use a smaller projectile, but you will just have to ignore them. Ultimately, you'll have to experiment with your own setup to find the right value, and I predict that you'll find that you need a bigger projectile to do any serious damage to your targets.

c0nnect3d wrote ...

Coil: using Barry's Inductor Simulation I designed a coil using 5 layers of 12awg wire that has an inductance of 0,039 mh an a resistance of 0,026ohms.

My circuit will be the RLC type that will be damped with a 0.100 ohms and will use 2700amps over 3.25ms

I assume that since your coil is 0.026 ohms and your RLC resistance value is 0.1 ohms, you plan on adding an additional damping resistor? This isn't good design practice, since the idea behind coil design is to maximize both turns and current. More turns of wire means more resistance, which means less current (you can't have both so again you have to find a balance). Adding a damping resistor adds resistance without the corresponding increase in coil turns- all loss and no gain, so it really doesn't make much sense to do this. Just add turns to your coil to increase the damping resistance- that way you get something back.

Other than that, 5 layers of 12AWG sounds about right for your energy and capacitance levels, and your waveform looks good.

c0nnect3d wrote ...

Questions :

1- how can I build a 400v fast charging circuit for my caps that is under 80$ ?
2- is there a cheaper or easier way than a 75$ 250a scr ?
3- is 2700amps over 3,25ms too much foa a 250amps rated scr ?
4- if my timing is not correct, can i play with the projectile length to find the best efficiency ?
5- do you have any suggestions of websites to find these parts at cheap prices ?
6- Does this project have a chance to work ?

1. Prev. answered
2. SCRs generally provide the best balance of cost and performance for hobbyist coilguns. Don't pay $75 though, look on eBay and you can usually find SCRs with similar ratings for $20-$30.
3. Prev. answered
4. Yes!
5. eBay
6. Absolutely. You're off to a great start. It's just going to take patience and flexibility to see the project through to completion.

Forty

Fri Jul 15 2011, 01:37AM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

here's a nicely priced SCR for you

saz beat me to the lecture answering all of your questions :P
if this is one of your first projects with electronics, be very, very careful when you get that capacitor charged up. i've been hit with smaller banks (2400uf, 400v) and it is not a fun or safe experience.

c0nnect3d

Fri Jul 15 2011, 02:25PM

Registered Member #4001 Joined: Thu Jul 14 2011, 12:30AM
Location:
Posts: 3

thank you for all your answers, i know the dangers of this project and i will be extremely careful, that's why i took my time to get the right info to not blow up my capacitors.

1-could i charge my capacitors with 500v and make an auto-shut-off circuit ? ( and how ?) because i tough that the rated voltage on the capacitors was the maximum voltage they could be exposed, including charging.

2-isn't it more dangerous when the coil is not critically damped ? or the diode in the scr is enough to block the huge negative current of an undamped wave ?

3-finally, is this scr able to withstand the 4700amps of an undamped wave , even if it's rated 2830amps for 8,3ms with an half sinusoidal wave ?

thanks for your help

Forty

Sat Jul 16 2011, 12:49AM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

1. I think it's alright to charge with a higher voltage than the caps are rated, so long as you can reliably switch it off when they are charged. however, there are several charging circuits, most notably the boost converter and uzzor's zvs, that can be made to charge exactly to your desired voltage. since you mentioned eventual portable use, i'd look into making a charging circuit. they're pretty fun to build anyway.
2. I'm not too good with damped/undamped circuit stuff, but if you're worried about a back emf from the coil reaching your capacitor, then freewheeling diodes placed antiparallel with the coil are the usual solution (often in series with a low value resistor, but i have not tried such a configuration.) and scr is basically a diode with switching ability, so it should be able to block the negative current you mentioned, but with the freewheeling diode (and possibly resistor) the energy has somewhere to go, as it gets dissipated by the diode and the coil resistance.
3. SCRs are usually underrated a fair bit. your pulse will be much shorter than 8.3 ms, so it should be able to take the abuse.

c0nnect3d

Sat Jul 16 2011, 01:20AM

Registered Member #4001 Joined: Thu Jul 14 2011, 12:30AM
Location:
Posts: 3

I will go for the voltage multiplier, because this option more simple and gives me the best watts/dollar ratio, then I plan to go portable with some lipo batteries and try to charge my caps in Â½sec with a boost converter if i can find the components to hold that much amperage at a descent price.

Thanks for your help, I can now start to order my parts !

Forty

Sat Jul 16 2011, 01:37AM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

well to save trips between your work station and your fusebox, use some sort of load on your voltage tripler, like a lightbulb in series with the mains input.

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