questions about my first coil gun

This is my first coil gun, it powered by 11x 200V 470uf, 1x 400V 330uf, 1x 400V 220uf, 1x 400V 150uf and 1x 400V 80uf capacitors (they are wired in parallel) . when i use 48 volts from 4 12V car batteries charge up those capacitors, and launch a screw, it didn't even shoot through a sheet of thin news paper. Would 250V from a disposable camera's flash circuit make those 200V capacitors explode ? And is there any problem with my coil gun's launch tube?

That's one cool little coilgun. The 200V capacitors would most likely explode or become damaged if 250V are applied to them. From looking at Justin's coilgun, it seems that it is better to keep the capacitors all the same than have a mixed bunch, but I'm not sure.

My coilgun is small, it shoots cotter pins.

Ok, I just typed up a long explanation but then accidentally click back and it all vanished so here it goes again...

It does not matter if you mix different caps. You just get the most out of each one if the voltages are all the same.

NEVER go over the rated voltage of the caps. Bad things happen.

Here is probably your biggest problem why it does not perform as you would like.
Your gun as you have it now has a 5950uf 200V bank of capacitors that equates to a max power of 119 joules (V^2*uf*10^-6*0.5=joules). With 48 volts you are only getting 6.8544 joules, ONLY 5.76% the power!

To charge your gun use a camera flash circuit (I am guessing that is where you got the capacitors). The camera flash may have a max voltage of 250V but you can use it to charge your capacitors to only 200! Just attach a multimeter and wait until it is near 185V then disconnect the charger. This may take a long time. With my coilgun using a camera flash it takes about 3-10 minutes depending on the battery charge. That is why I made my own charger with auto cut off. I now get 3-5 second charges!

Now for your launch tube. You may want a little longer one but it may be fine. The placement of the projectile in the barrel makes a HUGE difference. Just play with it until you find the sweet spot. Generally it is where the tip of the projectile is in the center of the barrel. Also the projectile should fit the tube as close as possible with out getting stuck.

Remember that coilguns are tricky things to get right. A lot of tweaking to get good performance and always be careful with the capacitors charged above 50V (anything above 50V is considered high voltage). Even if they are lower be careful. I shocked my self from hand to hand when trying to solder a wire onto ONE of my capacitors charged at around 100V. I felt like I got hit in the chest very hard and that was only 1.65 joules. Only use one hand when handling high voltage. Getting shocked on two fingers on one hand is no where near as bad as from hand to hand.

Justin

lol, my coilgun only gets .2376 joules, this thing is wimpy!
I'm only using two 3300uF 25V capacitors, and a 12V Power supply, though this thing gets about 3.5 feet when shooting cotter pins .

At least your safe

One of my capacitors is 26.4 joules and I have eight. My hand held gun only uses one though. All eight easily blow through a soda can. That is where I got the logo for the site. It is from a picture of the exit hole of the can.
Justin

this time I added 7 more capacitors, so now it 18x 200V 470uf, 1x 400V 330uf, 1x 400V 220uf, 1x 400V 150uf and 1x 400V 80uf capacitors (total of 9240uf) =D

This time I was shooting at a soda can, and it made a small hole, but the projectile didn't shot all the way through the can. I don't why when I pressed the button to fire the projectile, I heard a loud spark, I thought it must be the button that took too much current and failed but then I test out the button's continuity, it was fine. Im not sure where the spark came from because I wasn't looking at it. After I fired the projectile the capacitors were still charged at 43V so I discharged it with a screw driver, the sound of the spark scared the crap out of me.

here's some pictures














would it shoot through the can if I have a better projectile?

I'm not sure what to say about the projectile. But you stated that 18 of the capacitors are 200V, try and get the charge of 150V and move it up close to 200, like maybe 180V or 190V. Possibly a bigger coil, not longer, with thicker wire. I've made 4 coilguns, 1 works, 1 kinda, the other don't shoot. The reason is because on the 2 that don't work, one has too much wire windings, and the other one is too long. Both of these coils are using very thin wire, which is causing the electric current to continue passing through the the coil before it reaches the end.

More capacitance is not always better. You need a balance between voltage capacitance and coil windings.
More capacitance = longer pulse
More voltage = shorter but stronger pulse
More windings/thin wire = longer pulse
Less windings/thicker wire = shorter but stronger pulse

The whole trick is to get the pulse to stop perfectly when the projectile reaches the center. If I were you I would take 9 of the 200V caps in parallel and the other nine in parallel and then put those two groups in series. You can then pump them up to 400V with out any problems. I had a 800V gun for a while but it took to long to charge and my fast charger does not go up to 800V.

When you put capacitors in series the formula for the combined power is V1+V2=Vnew, (F1+F2)/4=Fnew. So putting two of your 470uf caps in series you get a 400V 235uf capacitor. At first glance this looks like it has less power then before but if you use the formula for joules you fine out it is the same as voltage is squared.

Problems with a mechanical switch...
Large power loss
Problems of wear
Loud
Mechanical switches, like the one you have, create a large spark when the contacts close. This is because the current passing through them is hundreds of times their rating. You can only use a mechanical switch for so long. In my early coilgun days I had a switch like that. It ended up fusing it self together so you could never open it. The solution IGBT or MOSFET. I use IGBT just because I have a lot of them. If you read the coilgun page you would know I got a lot of them for cheap. Good thing, I burned out 5 before I realized I needed a diode anti parallel to the coil. If you want an IGBT from me I will send you one to you for $2 + shipping. You will also need a HV diode to protect them. You should have one on your gun now to protect the capacitors but it is not that big of a deal. I may have an extra that I could throw in, if I can find it .

You can still use a mechanical switch with an IGBT, but the IGBT takes all the large current the switch just tells it when to.

Justin

EDIT: For my projectiles I take a steal rod and cut it in to pieces the length of my barrel then sharpen the tips on a sander.

so, last time I shot a hole on the can, it was at 103.95 joules? forumla is V^2*uf*10^(-6)*0.5 = joules. so, in my case it was 150^2*9240*10^(-6)*0.5 = 103.95 joules ?

No not even close. 103.95 joules would be enough to probably shoot through a wall. Coilguns are at best 2-3% efficient. You are storing 103.95 joules in the capacitors. Voltage, charge, pulse length, projectile, barrel, and the coil all play a role in the efficiency.

Justin

is uf and amperes somehow related?

Farads (uf=micro farads) is a measure of charge, how many electrons can be stored. Amperes is a measure of how many electrons flow past a single point in a conductor. The higher uf the more electrons stored meaning you can give the same amount of amps for a longer time then a capacitor with a lower uf rating. Voltage determines the max amount of current as well as inductance. That is because of Ohm's Law, V=IR. With a set resistance, if you increase the voltage you increase the max amount of current. In a coilgun inductance also plays a large role but I do not remember the formula. I have a LCR meter to measure inductance (L), capacitance (C), and resistance (R). You can then calculate the current. That is useful when selecting an IGBT with a rating higher then the max current.

Justin

If you put a tiny digital watch right beside the coil gun's copper coils, when you charge up those capacitors and hit the gire button. can the coil gun produce electro-magnetic paulse strong enough to disrupt the watch's circuit, like freeze the time displaying on the screen

Possible but I doubt it. My watch has withstood a lot of magnetic fields with no problem. I have 3/4" NeFeB magnets, 50lb of pulling force. I have ruined gift cards but never a watch. The pulse of the coilgun is so short that even if it did do something it would probably not be noticeable.
Justin