for a coilgun is it better to have higher voltage rated caps or higher uF rated caps?

should i use caps with higher voltage or farads? i can get caps with like 10V and 1200uf, but idk if thats better or worse than caps that are 330V and 10uF? also could u use something with high voltage to make a coilgun like a taser with 1800000V if u just pulsed it with the right kind of switch?

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asantiago25 years ago
I know this is gonna be a wierd question, cause its not really about capacitors, but can anyone explain what SCRs do and how i should hook them up in the circut of my coilgun?
i just can't fully grasp it by reading it on wiki so if someone could explain it would be much appreciated. :D
the scr has 3 pins the 2 pin will be already connected to the circuit.. and the 3rd pin will have a signal to open the circuit untill there is no current passing thru it...
nicholai4144 years ago
A smaller farad value cap will discharge at a faster rate, allowing the coil to be pulsed quicker. If we consider the resistive properties of your coil and the voltage of the capacitor you choose we can use ohms law to determine the amps that are going to flow through it. With a high voltage and low resistance you get more current. Since the magnetic field strength is stronger with a higher amperage, it is preferable to use a high voltage cap. However there is a point where you will over saturate your projectile and will not get any better performance out of it. Where that point is, is based on what you intend to fire from it. I get fair results with a 400v. 1200uf cap and a coil with 10 ohms resistance.
I have a question then, i have a capacitor that has about 334 J of energy and it doesn't do anything with the coilgun. Doesn't even make the projectile jiggle. It shows on the capacitor 630V 334J.
The idea behind a coilgun is to store up a large quantity of electrical energy in a capacitor, and then release it quickly through a coil of wire.

So you should be wondering about how much energy is being stored in the capacitor bank. Supposing the capcitor bank is just one capacitor, with capacitance C, and you charge it to a voltage V, the energy stored in the capacitor is U = (1/2)*C*V2

Look at the candidate capacitors you mentioned, and ask: Which one stores more energy?

1200 uF @ 10 V gives: U = 0.5*(1200e-6)*10*10 = 0.0600 J
10 uF @ 330 V gives:  U = 0.5*(10e-6)*330*330 = 0.5445 J

Notice that factor of V-squared really has a big impact on the total amount of energy being stored.

By the way, the voltage rating they print on the side of the capacitor is a maximum allowable voltage.  Try to charge a capacitor to a voltage higher than this rating and you risk destroying the capacitor.  It's sort of like if rubber bands came with a maximum stretch rating.  Consider for example a rubber band with a maximum stretch rating of 10 cm.  What happens if you stretch it to a length of 12 cm?  Maybe it snaps back with no problems, or maybe it breaks. There's something analogous happening with capacitors, only they call the breakage by a different name:

By the way, the equation for energy stored in an ideal streched rubber band, or spring, is similar:  U = (1/2)*k*x2

Storing energy is one of the goals. Another goal is releasing it quickly. To do this it would actually help if C were smaller.  I mean if you model this thing as an RC circuit it has a time constant of τ = R*C.  If you model it as an LRC circuit, then it is a damped LC oscillator, with an undamped period of 2*pi*(L*C)^(1/2).  It's not really clear how the discharge is going to play out, but it kind of looks like making C smaller would make it faster.  So if you have the opportunity to trade C for V; i.e. use a smaller capacitance, but charge it to a higher voltage, it looks like this would make the discharge faster.

Of course there are limits to how high you can make the voltage.  Capacitors and switches that work at higher voltages are more tricky to build. There are problems with insulators breaking down, or the air surrounding your apparatus breaking down and arcing. 

I think in practice these coilgun toys usually do not use voltages higher than a few KV.  The capacitor in a typical camera flash circuit is charged to a voltage of about 300 V, or 0.3 KV. The size of this camera flash capacitor is about 300 uF, and the energy stored in it is about: 0.5*(300e-6)*300*300 =  13.5 J

Anyway, I think one of these camera flash circuits, or one using similar voltages, would be a good place for you to get started.
I know this is a fairly old post. But I have a question that I can not SEEM to get answered... If you are still on instructables or anyone else who can answer this, perhaps you wouldn't mind answering.

First: I have an 82 volt, 8200 uF (microfarad) Capacitor. Which gives out 27.568 joules of power. I thought that was quite amazing for a capacitor about the size of your index finger and two fingers wide. So, I am wondering, for such a large capacitor, Can I use a 300 volt charger from a disposable camera to recharge this sucker? Or will I risk frying Sparky.. (the cap). I can charge it up to 9.6 volts with a Nickle Metal Hydride battery and it produces a fairly big spark capable of slightly welding a screwdriver to the terminals..

I desoldered Sparky from a high power amplifier from the 90's or so. Two of them, actually, I just got to find the other.. Anyway..... I could use five car batteries in series to charge this sucker enough to kill someone.... or 6 nine volt batteries in series.

Besides that, is that the only way to power Sparky? I want to use it for a Coil gun project, considering the minimum required joules is 10 or so to shoot the projectile to a decent distance. This is my first time using high voltage things such as this.. But I know all the safety thingys.

Also, another question.. I plan on making Leyden Jars. I thought it was quite amazing of the capacity of them, I was wondering if that is even possible with leyden jars.. I assume that it is, As I can probably make a good high farad capaciter with 5 large barrels in parallel to go outside enclosed in a fenced area...

.... I don't even know if 82 volts is enough to deliver a sudden shock to the electromagnetic coil to push the projectile... but that's my hypothesis.
Here's a (somewhat) bold idea: connect a voltmeter in parallel with your capacitor, so that you can sense/see the voltage across it, increasing as it charges. Then (this step very is important) turn off the charger before the voltage on your capacitor exceeds its maximum value.

Try it first with the capacitor that came with the camera flash charger. Connect the voltmeter across the capacitor. At first the capacitor should have some low voltage across it, just a few volts, or tens of volts. Then turn on the charger, and you should be able to see the voltage reading increasing as the this capacitor charges.

The big assumption that underlies this bold idea, is that you will have several seconds worth of time to see the voltage on the capacitor, and you hopefully will be fast enough on the switch, to turn off the charger circuit before the voltage on your capacitor exceeds the maximum voltage it can hold without breaking.

The reason you first try this trick with the capacitor that came with the flash-charger circuit, is because supposedly there is no danger of overcharging that capacitor.  You can charge it up to like 300 V, or so, without hurting it.  But you want to get a feel for how fast it charges.

I am guessing that a larger (in uFarads) capacitor will charge more slowly anyway, and that you'll have a little bit more time to turn off the charger.  That is to say I'd expect a  8200 uF capacitor to charge more slowly than a 300 uF capacitor.  Am also guessing that the time needed to charge your 8200 uF capacitor from 0 to 75 V, that this will take some number of seconds, probably less than a minute.  That is to say, I expect it should be will be humanly possible to for your eye to see the voltmeter, and your hand to switch of the charger, in time.

Anyway, let me know if this plan makes sense to you.  The trick is to shut off the charger before overcharging the capacitor

If you fail to turn off the charger in time, i.e. you just stand there, and watch the voltage climb over that maximum value of 82 volts, something bad will probably happen.    I'm not sure at exactly what number it will happen. 82? 83?  90? 100?  But I know a capacitor rated for a maximum of 82 V won't go all the way up to 300 V without breaking. 

Also it might explode, so be sure to wear your safety glasses.
Thank you for your reply, I assume you are talking about the camera circuit charger, am I correct? I have not yet tried what you said, I am glad I asked the question as I knew I had to do something with the voltmeter.. That trick is not too much of a problem, as I have a foot pedal power switch I had built. Anyway, So it IS possible to charge the capacitor with the camera charge circuit? Also, I have these capacitors that are about half an index finger wide and long, they are 200 volts, I have two of them, did that math, and comes out to be a total of 480 volts, each capacitor is 680 uF. 1360 total.

To sum it up, I have Three capacitors total, The 82 volt, 8200, And two 680 uF, 200 volt capacitors. (Btw, how dangerous are those charged up, Do they have a bigger risk of killing me or people when charged up with the camera circuit?).. I had taken apart a computer power supply and salvaged these two capacitors.

Is it perfectly safe to wire these two up in series to increase voltage, but keep the uF the same? It comes out to be 482 volts total when the other capacitor is inline, But will it work if I had the two 200 volts in series.. (Or is it parallel to increase voltage?)

Sorry for my noobish questions, I am desperate enough to try this coil gun... I was planning on opening my old CRT monitor to take out that capacitor thats about the size of my hand......

Another question: Can I wire two camera charge circuits together to increase output voltage as such to charge a 600 volt capacitor bank?, I know a guy who wired up to 10 of them to charge his microwave capacitor bank....

Thank you in advance.
CameronSS6 years ago
It's not one or the other. Both are maximum values.

The voltage rating is determined by whatever voltage you're feeding to it. If you're feeding it 300V, the 10V cap will blow, but the 330V one will be barely acceptable. If you're running at 700V, you might need a 1000V cap to handle the higher voltage.

The uF rating is the capacitance, measured in farads -- uF is microfarads. A higher capacitance rating means it has the capability to store more energy. I don't recall the formula for the capacitance and energy right now, so I can't explain with solid numbers. But if your circuit can only put X joules of energy into the capacitor, having a capacitor that can hold more energy won't help you any.

In summary, get capacitors with high enough voltage ratings first, then pick the ones with the highest capacitance you can get. When you make a parallel bank, you're increasing the total capacitance, and you can continue this by simply adding more caps. Too high a voltage will kill them, even if you hook them up in series (I think).
To (partially) answer your second question, I think so. Not at 1.8 million volts, though. That voltage can jump across roughly a foot and a half of air and through about any insulator you can buy, making it impossible to keep everything separated in the device. Tasers start at 50kV, then drop to under 1.5kV, according to The Internet. Also remember that Tasers are designed to deliver highly calibrated pulses continually, while a typical coilgun circuit will deliver a fairly constant voltage. You're more likely to give someone a burn than knock them out.