Building a Coilgun and Capacitor Bank

[360gamer117]

I'm currently constructing a coil gun for a science project, and had a few technical questions that I'm hoping you guys can help with.

First off, I'm building a capacitor bank, consisting of 6 small flash capacitors that are 330V each (I don't recall the Farads off the top of my head and can't access it at the moment, but it should be around 120uF). My main questions for the cap bank are what the exact voltage would be, what the exact capacitance would be, and how I would go about testing the charge of the bank before firing.

From what I understand, two 1.5V batteries linked in parallel provide 1.5V, but when linked serially would equal 3V. My cap bank is in parallel, so my first question is whether or not they would function similarly to a battery.

My next question is similar but concerns Farads rather than Volts. Again, it's my understanding that if I have six 120uF caps, then I should have a bank that stores 720uF. If someone could confirm this for me, that would be much appreciated.

Now, for checking voltage before firing, I'm a little less sure about. I considered hooking an LED to the circuit. I don't know much about them, but I do know that if I do that, then I would either blow the LED or have to install a resistor, which would slow down the rate of discharge. I also though about using an analog voltage meter instead of LEDs.

 

I've also got a few questions about the circuitry. I know I need a diode for the switch, but are there any other locations on the circuit I would need to place one? And what about rectifiers? I have an AC SCR switch, but am running this from a camera circuitboard, and I'm using 3V to charge the caps. Do I need a rectifier between the SCR and circuitboard?

I may have other questions in the near future, but for now this is all I can think of.

Thanks in advance.

[PaulS1950]

Are you using the flash charger to charge the capacitors or just the battery?

 

I am not an engineer but it sounds like you have a lot to learn about the device you are attempting to make.

[360gamer117]

The capacitor bank is made up of flash capacitors. And I have a decent knowledge of what I'm working with, I just need a little help figuring out specific values. I've got it working, but I've only used a single cap without any switches rigged since I don't have a bypass diode.

I'm definitely going to need the bank to fire this thing, because a single cap doesn't do much.

[PaulS1950]

The voltage from the capacitors (and their charge state) depends on the voltage input and the time given to charge. If you are using just the batteries to charge the capacitors you will never end up with more voltage than that of the batteries. If you are using the charger from the flash unit then you will get much more voltage but you will need to charge them for a longer period.

I don't understand what the by-pass diode is used for - do you have a schematic that you could post? For the "fire" switch are you using a high speed SCR or will you be using a UJT?

What do you want for a maximum voltage/charge?

[Double K]

Firstly - my knowledge of electronics is pretty lame, however there is a system in production I believe that already uses this system somewhat, I just wanted to point you to this for inspiration more than anything else (I doubt the product tells you how they did it unfortunately, as I imagine they protect that pretty well) And you've probably already heard about it I'm guessing, but just in case! (Although you may be able to glean some of how they achieved it)

http://www.metalstorm.com/

[JamesNBarnes]

Its never fun when someone says "dont do that", but i recommend you dont do it, yet.

 

Start off with some basic electronics, the internet of full of great tutorials. When you have a basic grasp of everything you can start with more ambitious projects.

 

You CAN just follow schematics, but without a full understanding of what you are doing, you are more prone to accidents. Accidents in electronics can be both expensive and life limiting.

 

Plus if you really understand what you are doing, you will have much more fun in the long run, because you will be able to make bespoke modifications of your new toys!

 

If your using electrolytic capacitors, especially old ones, please be extra careful. Use eyewear when charging them until you know what you are doing.

 

Lastly, having the proper tools always helps! I suggest http://www.dealextreme.com for the basics. You can get yourself set up with multitesters, bread boards, basic components, test hooks etc for very little money.

[HardonColluder]

I once made a design for a coilgun that used a spring loaded metal bolt with male brush terminals underneath , so that when the bolt was fired the brushes would pass over positionable , configurable female terminals . Each terminal would be associated to one of many positionable coils over the barrel . With this type of analog setup you can easily configure/reconfigure the timing of the pulses and the placement of the coils with ease until you settle on the configuration that gives you the most power (Which you can then hardwire into a PCB to replace the spring-bolt mechanism) . I never took it passed the design stage though (I'm too scared of HV) .

[Silent Jack]

I'm currently constructing a coil gun for a science project, and had a few technical questions that I'm hoping you guys can help with.

First off, I'm building a capacitor bank, consisting of 6 small flash capacitors that are 330V each (I don't recall the Farads off the top of my head and can't access it at the moment, but it should be around 120uF). My main questions for the cap bank are what the exact voltage would be, what the exact capacitance would be, and how I would go about testing the charge of the bank before firing.

From what I understand, two 1.5V batteries linked in parallel provide 1.5V, but when linked serially would equal 3V. My cap bank is in parallel, so my first question is whether or not they would function similarly to a battery.

My next question is similar but concerns Farads rather than Volts. Again, it's my understanding that if I have six 120uF caps, then I should have a bank that stores 720uF. If someone could confirm this for me, that would be much appreciated.

Now, for checking voltage before firing, I'm a little less sure about. I considered hooking an LED to the circuit. I don't know much about them, but I do know that if I do that, then I would either blow the LED or have to install a resistor, which would slow down the rate of discharge. I also though about using an analog voltage meter instead of LEDs.

 

I've also got a few questions about the circuitry. I know I need a diode for the switch, but are there any other locations on the circuit I would need to place one? And what about rectifiers? I have an AC SCR switch, but am running this from a camera circuitboard, and I'm using 3V to charge the caps. Do I need a rectifier between the SCR and circuitboard?

I may have other questions in the near future, but for now this is all I can think of.

Thanks in advance.

 

If using 6 of them in parallel, you would add the capacitance and the voltage would be equal to the value of the lowest voltage capacitor in the bank. Optimally, they would all be equal. In your case, with camera caps they should be 330V and between 80uF and 160uF, depending on which cameras you get. While the original charging circuit can be clipped and used, it will be slow and low power. If you use several, you can get away with a bit bigger batteries, but there are limits. It would be better to build a simple charging circuit. There are a variety of open or shared designs available on the web that can be used. A few inexpensive semiconductors and other low cost components and you will be surprised what you can do. Safety is a first though. Always be sure to wear all the proper gear and make sure your work and testing area are clear. If you have more questions let me know. I am eager to hear how your project turns out.