Capacitors (coilgun) ???

[freeskier89]

Hey I was planning to build a single stage coilgun and I had a few questions. First of all I don't really want to use a SCR. I could use a SPDT switch to switch between the capacitor charging and discharge circuits. I was wondering when choosing capacitors, should I choose ones with high capacitance and high voltage ratings when using a standard 12V supply? Do the caps charge up to full capacitance always with power, does the source voltage only effect the speed of chaging?
Or does the cap only charge to source voltage. In other words, I want the highest voltage from a low voltage supply.

Thanks for any help!
-freeskier89

ps: Could you just use standard 110VAC without caps to make a decent coilgun?

 

[evandude]

capacitors charge to the source voltage. if you want higher voltage on the caps, you need to step up the input voltage to charge them to a higher voltage.

and, I would not recommend using a mechanical switch instead of an SCR. due to the wonderful nature of the amount of power you'll be dealing with, the switch will probably be destroyed before long.

and don't expect much out of a single-stage coilgun either. ESPECIALLY if you are switching it mechanically. to be truly effective, you have to turn the coil OFF as soon as the projectile reaches the center of the coil. otherwise, with the coil still on, once the projectile passes through the coil, the coil will then be pulling it BACK, which will slow it down... it may still launch, but only go a few feet. to do the switching, you either need some timing circuitry, with a specific projectile and starting location, or you need a photo interrupter type circuit that will detect when the projectile reaches a certain point in the tube. Either one of those is impossible if you're simply switching the device manually.

My guess is that you don't quite realize the complexities of what you're getting into... nor the dangers of large capacitors and high currents being switched through coils... maybe when you first weld the contacts of your switch together you'll see :lol:

 

[freeskier89]

Thanks for the info. My hunch was that they charge to source voltage. By no means do I want it too high of voltage. I was hoping for a simple design it would not need to be firing at awesome speed. There are a few people that have just used a single camera flash capacitor that launches a small projectile.

Thanks.

 

[zachtheterrible]

you're going to need a lot more than 12 volts charging the caps :lol:

 

[freeskier89]

About 12V supply I agree. As all my ideas are, it was only a preliminary one. Would just using 120VAC on the coil create any propulsion even for a paperclip if you didn't use a timer/opto device? I need to do a bunch more research (I know its apparent for all of you) lol.

 

[DigiTan]

To take advantage of the PE = 1/2 (C * V^2) equation, you'll have to look for a high-voltage capacitor bank. As far as size vs. energy goes, I lean toward large photoflash capacitors that are rated in the 1000uF @ 360WV range. The ideal value will depend largely on your coils inductance, coil dimensions, saturation point, projectile mass, projectile composition, and mess of other annoying parameters.

A 120VAC signal will cause the projectile to "stick" inside the coil and vibrate at a constant frequency. This is a lot like how bells worked in those old time telephones. An ideal pulse will drive the projectile into the coil until it's in mid-way, then either cut off or reverse current direction so the shot isn't "sucked" back in.

 

[evandude]

if you only need to launch a very small projectile, you could try just using a disposable camera. just open it up and disconnect the flash, and run the flash wires to the coil instead. that will give you a reasonably high-voltage, short pulse. It doesn't have any feedback as to where the projectile is of course, so you would have to experiment with the placement of the projectile relative to the coil prior to firing, so the coil pulse is ending about the time the projectile gets to the coil...

it won't be an anti-tank rifle or anything, but at least you might be able to launch a small piece of wire across a table or something :lol:

and later on, you could add a simple opto-interrupter circuit to short out the coil with an SCR or something, as soon as the projectile reaches the coil, to try and kill the field as fast as possible at the right point.

 

[Halogrunt1234]

I have had a little bit of experience with this myself. I have created (and am still working on) a coilgun too. I would also definitely recommend a higher volteage power supply. with about 600 feet of coil and 36 volts of power supply (4 9V batteries) I have managed to make a paper clip launch about 5 inches when i time i correctly with a MECHANICAL SWITCH. I have noticed that a 6800 uF electrolytic capacitor with a voltage rating of 35 V has too long of a discharge time to succesfully launch anything. also, with disposable camera capacitors, I have found that many of them are umarked, due to their extreme cheapness, and i would not recommend applying too much more than 1.5 V on those capacitors. also, you do not NEED to have a control circuit for this kind of project, although, they may make life alot easier.

anyways, where can I find schematics for a phototransistor operated trigger-off circuit? I don't feel like working out impossible timing calculations on pencil and paper.

 

[Someone Electro]

Photoflash caps are mostly 100V 100uF electrolics.

I have 4 of 10 000uF 64v capacitors.I have them in paralel to fry stuff.It also acts as a arc welder.

Thats not good for an coil gun since you need an extremly low resistance coil to discharge it fast enugh.

In coil guns you need high voltage.


I think i have 2 of 200V 220uF caps from an switching PSU.That cod make an coil gun.

 

[evandude]

a phototransistor circuit should be pretty simple... basically you have 2 stages:

one stage is the phototransistor circuit, that has a simple digital output that represents whether or not the projectile is detected by the optointerrupter...

and the second stage is something that attempts to short out the capacitor/coil to kill the magnetic field as quickly as possible, when triggered by the logic input. SCR, FET, I don't know what would be best here.

but if you build the phototransistor stage first, it should make it easy to experiment with the best option for the other stage.