Need Advice in Building a Coilgun

[Atomizer]

I am trying to build a coilgun for a school project. Its purpose will be to accelerate a ~1cm steel ball as it travels down a ~2.5cm semicircular styrofoam track running through the coilgun. The coilgun itself will consist of several turns of magnet wire around a PVC pipe fitting over the track. I have created a preliminary design, though I have several concerns about it:

The switch on the battery-capacitor circuit controls the battery's connection the the capacitor, and the LED indicates when charging is complete. The gray slip just in front of the ball is aluminum foil, the gun should fire when the steel ball rolls over the foil on the bottom and brushes an identical piece on top.

 

Here are my concerns:

1) I don't think that I am connecting the battery to the capacitor correctly. As I recall reading somewhere, the 9 volt battery should only be able to charge the capacitor to 9 volts. How should I connect the 9 volt battery to charge the capacitor fully?

2) I think that the capacitor may be too powerful and cause the ball to leave the track - am I correct or is this enough to only somewhat accelerate the ball, not fire it at dangerous velocities? If the capacitor is too powerful, should I try instead using multiple 9v batteries, and extending the aluminum foil trigger ~1/3 of the way into the barrel to keep it on the correct length of time? Also, if I use multiple 9v batteries, should I connect them in parallel (more current) or in series (more voltage) for maximum effectiveness?

3) I'm not sure if the aluminum foil switch is the best idea for this. I was thinking of using the microswitch shown here: http://www.coilgun.eclipse.co.uk/coilgun_basics_2.html, but I would need to extend the lever inside the coil. So is my current aluminum foil switch the best option, or is there a better solution to activate the coilgun without a manual switch?

 

I would very much appreciate it if someone would help me find an optimum design for my coilgun.

[Mr Skeptic]

Well I see some problems:

1) You don't want to pass the entirety of your current through an LED; that would be slow.

2) The capacitor shown is designed for high voltage, so you are wasting most of the capacitance using a 9 volt battery.

3) Batteries are a horrible way to power this if it is intended to be anything like a real gun.

 

I'm not sure if the coils will accelerate your metal ball. Haven't done any coilgun designs.

[Atomizer]

Well I see some problems:

1) You don't want to pass the entirety of your current through an LED; that would be slow.

2) The capacitor shown is designed for high voltage, so you are wasting most of the capacitance using a 9 volt battery.

3) Batteries are a horrible way to power this if it is intended to be anything like a real gun.

 

I'm not sure if the coils will accelerate your metal ball. Haven't done any coilgun designs.

 

1) So how do I connect the LED to let me know when the capacitor is charged?

 

2) I am aware of this (as explained in my post), but how else should I charge it? What kind of a circuit would a battery of low voltage need to charge a high voltage capacitor?

 

3) As explained above, the coilgun needs to give the ball a substantial speed boost, but not enough to cause it to fly off the styrofoam track.

[Mr Skeptic]

Well I suppose it would work so long as your capacitor doesn't leak more than the LED takes to charge. Alternately, a wire going around the LED would let some current through, but then making sure the LED lights up would be slightly difficult. Alternately, a lightbulb has a greater range of brightness and would let much more current through (this is normally a drawback but here you want the current to flow).

 

If you get a capacitor designed for a lower voltage you get more juice out of it. Charging a 400 V capacitor to 9V will waste 97.75% of its capacity for charge, or 99.95% of the capacity for energy.

 

I reviewed coilguns and your design should work, so long as powerful bullets are not a requirement. You want your coil to run out of juice by the time the bullet reaches the center of the coil, otherwise it will be decelerated. This is unlikely to be a problem if you use an underpowered capacitor.

[vordhosbn]

I thought capacitors work only in AC circuits. Aren't they supposed to block DC current?

[Mr Skeptic]

I thought capacitors work only in AC circuits. Aren't they supposed to block DC current?

 

The capacitor will eventually block DC current from flowing "through" it when it gets full. This won't be a problem for Atomizer's design, and in fact is how it is supposed to work. When the capacitor is full, he flips the switch and lets all the stored current out at once, very quickly.

[Atomizer]

Well I suppose it would work so long as your capacitor doesn't leak more than the LED takes to charge. Alternately, a wire going around the LED would let some current through, but then making sure the LED lights up would be slightly difficult. Alternately, a lightbulb has a greater range of brightness and would let much more current through (this is normally a drawback but here you want the current to flow).

 

If you get a capacitor designed for a lower voltage you get more juice out of it. Charging a 400 V capacitor to 9V will waste 97.75% of its capacity for charge, or 99.95% of the capacity for energy.

 

I reviewed coilguns and your design should work, so long as powerful bullets are not a requirement. You want your coil to run out of juice by the time the bullet reaches the center of the coil, otherwise it will be decelerated. This is unlikely to be a problem if you use an underpowered capacitor.

 

Thanks for the advice, but I have two more questions now:

 

1) If I wanted to charge the capacitor to full power, how would I do it? And is it possible to do this using a small battery like a 9 volt? I've stubled upon a lot of pages explaining how to use the capacitor (300v) and charging circuit of a disposable camera to charge from a small battery, but nothing about building the actual "capacitor charging citrcuit" from scratch.

 

2) Do you have any thoughts about my method of triggering the coilgun using the steel projectile itself as part of the switch? Would this be a reliable method of triggering the gun?

[Mr Skeptic]

1) If you got the capacitor from a camera, then you can also loot whatever charging circuit it had, which should match the capacitor. I'd suggest going with whatever batteries the camera used.

 

2) There's all kinds of ways to trigger the coilgun using its own projectile, but they would all be much more complicated than a switch. More advanced coilguns have multiple sets of coils that activate one after the other to provide more efficient and continuous acceleration, which would require much testing of the timing, or being triggered by the projectile.

[Atomizer]

1) If you got the capacitor from a camera, then you can also loot whatever charging circuit it had, which should match the capacitor. I'd suggest going with whatever batteries the camera use.

 

If I had taken the capacitor from a camera, I would have done that, but what I am asking is exactly HOW I can build a charging circuit from scratch if the capacitor did not have one originally, preferably to charge from a 9v battery.

[evilgenius]

1) So how do I connect the LED to let me know when the capacitor is charged?

 

2) I am aware of this (as explained in my post), but how else should I charge it? What kind of a circuit would a battery of low voltage need to charge a high voltage capacitor?

 

3) As explained above, the coilgun needs to give the ball a substantial speed boost, but not enough to cause it to fly off the styrofoam track.

If you just want to move the steel ball, you do not need a 400 volt capacitor. If you are building this for some stupid school project you could as well use a 4700uF 18v cap to power it. Use 2 9v batteries in series also make sure that the impedance of the coil is low or else the gun will be very weak. you also need to keep in mind that you have to switch off the current in the coil when the ball travels halfway through it in order to pervent the suckback effect. you can easily control this by using a 555 timer in monostable mode. instead of a mechanical switch i suggest you to use the TIP series power transistors or a relay. you still need to learn a lot in electronics before starting to build a coil gun. as for your fears of dangerous velocities, you do not have the proper capacitors or the SCR's needed to switch the current in the coil. i think that you still don't know how to use a transistor... I'm laughing my head out imagining you building a coil gun. you do not know the basics of a capacitor and a diode. quit trying to build something like a coil gun and concentrate on improving your basics first. your capacitor can store a maximum of 0.5*120*(10^-6)*400*400 joules of energy. that is too weak to build a coil gun that can even accelerate a steel ball to 1 cm/s