Re: Choosing Capacitors for Railgun [was "Greetings..."]
Marko, Sun Feb 19 2006, 08:10PM

This capacitor is car amplifier current booster, and 25V is far too low for high current discharging, resistance will be too high for it to operate normally.
It will give huge energy but slow and most will dissipate into heat, i wonder if it would actually move projectile rather than welding it.

If you have few of such caps maybe a 100 volt bank could be a try, energy is really huge but voltage low so results are questionable...

Im planing to use 350V 6000uF caps for railgun one day...

350V is best choice to charge from mains if its 220 or 200V if its 110V

Re: Choosing Capacitors for Railgun [was "Greetings..."]
Erg, Sun Feb 19 2006, 09:28PM

Thank you. I was trying to be carefull prior to buying something expensive. And since the capacitor deterimines so much about all else in this circuit I am shopping for them first.

Re: Choosing Capacitors for Railgun [was "Greetings..."]
Dave Marshall, Sun Feb 19 2006, 11:11PM

Railguns are EXTREMELY difficult to get working satisfactorally with electrolytic capacitors. Typical railguns operate in the 2-5kv range, and use energy discharge capacitors to give a very high rise time. If you're going to use 'lytics, I would strongly recommend putting several in series to get voltage closer to 1kv atleast.

Most railguns I've seen that did use electrolytic capacitors in the 300-1200v range were really just spark shooters. Made alot of impressive sparks, but didn't have much output energy.

And being that you seem to be new to the hobby, please dont blow your fingers off with one of the caps (yes they will remove appendages).

Dave

Re: Choosing Capacitors for Railgun [was "Greetings..."]
Marko, Sun Feb 19 2006, 11:20PM

yep, but electrolytics are far easier to get (in my countra even in my town you can buy such high energy elko's)
Pulse discharge are more bulky and more pricy, but characteistics are overwhelming...

If you can find (lots) of dead defibrilators or so that is only place where you can get some at low or no cost, or try to catch something on ebay (far lower than market price but still expensive usually)

Maybe someone on this forum may even want to sell you such capacitor, for few hundred $ usually...

Re: Choosing Capacitors for Railgun [was "Greetings..."]
Erg, Mon Feb 20 2006, 03:35AM

First thank you for the safety warning, I can always be more safety concious. Who makes good Pulse discharge capacitors?

Re: Choosing Capacitors for Railgun [was "Greetings..."]
Dave Marshall, Mon Feb 20 2006, 04:39AM

Cornell Dublier and Aerovox are the two that jump to mind immediately.

They can be found surplus, but as Firkragg says, they can be pricey. You wouldn't need more than one or two good size caps for a first try though.

There ARE pulse rated electrolytic capacitors available, and several such capacitors in series ought to be suitable for a first attempt at a railgun. They tend to be rare, and expensive, however. I believe most of the pulse rated 'lytics are inverter grade, and can be had on ebay if you watch. Characteristics to watch for include very large bolt terminals (all big capacitors use bolts, look for abnormally large terminals though).

Dave

Re: Choosing Capacitors for Railgun [was "Greetings..."]
Quantum Singularity, Mon Feb 20 2006, 07:08AM

I was under the impression that Maxwell Labs energy discharge caps were also excellent for this kinda work. I saw 2 at the dayton hamfest several years ago and bought them (actually there were three but I was a poor college kid and couldnt afford all 3). About 6kJ each. They have done excellent in my ring & disk launcher, can crusher, exploading wire... never did finish my railgun though. Surplus Sales of Nebraska has a page specificaly for Maxwells, but I wouldnt pay those prices unless money is of no consequence to you. I also used a G.E. pulse cap on my bigger tesla coil and it has held up good.

Re: Choosing Capacitors for Railgun [was "Greetings..."]
Electroholic, Mon Feb 20 2006, 07:10AM

How about Maxwell? hehe
for lytics, the large photoflash lytics are good, too.
but they are very rare on ebay.
on and incase nobody have said this
"Try a coil gun first!"

Re: Choosing Capacitors for Railgun [was "Greetings..."]
Dr. Shark, Mon Feb 20 2006, 09:10AM

Actually I am under the impression that electrolytice are quite good for railguns, and as a consequence of this I replaced my bank of 35 40uF 2.5kV capacitors by a bank of 50 3300uf 350V electrolytics. The reason is simple: For a 1m railgun that accelerates a projectile to the speed of sound you want the discharge to take something like 5ms, _not_ 50us as you would probably get with a pulse cap.
The reason traditional railguns used those high voltage pulse caps is most likely that they are just what the military had available, and the price tag was not improtant for them. Also they were usually employing pulse transformers to lower the voltage delivered to the gun. Go figure.

I suggest you keep a look out for high voltage electrolytic capacitors, in the range of 300-500V and 3300uF - 12000uF (the higher the better). You want to do a bulk buy on these, keep a look on eBay, they can be had for few bucks / piece, so you will only pay a few 100$ for a decent sized cap bank.

Re: Choosing Capacitors for Railgun [was "Greetings..."]
Dave Marshall, Mon Feb 20 2006, 05:12PM

Joe Doh, your line of reasoning, while logical, doesn't pan out in the real world.

Higher is NOT better. The higher the capacitance of an electrolytic capacitor, the higher the ESR (equivelant series resistance). In very large capacitors, ESR can approach and exceed half an ohm. This isn't the case with inverter grade capacitors, but its still far greater than any energy discharge capacitor.

Capacitor failure is also FAR more likely with electrolytics. Energy discharge caps are constructed to withstand the forces of intense discharges. The plates of the capacitors in electrolytics are typically thin aluminum. When the capacitor is subjected to a high amplitude pulse, similar in principle to a ring launcher, the plates can experience physical stress. This can often lead to a plate being sheared from its terminal, a punch through of the diaelectric, even failure of a capacitor in a nearly explosive manner. I had several such failures in a large coilgun I constructed in high school.

Its true that energy discharge capacitors often can have too brief a discharge, reducing efficiency and turning the railgun into a glorified spark shooter. However, this can be countered with a pulse shaping inductor to make the pulse behave precisely the way you want it to, with minimal overall loss of energy.

Arguing that electrolytics are more desirable is like arguing that a rubber mallet is a better tool for driving nails than a claw hammer. You might still drive some nails with the rubber mallet, but its usable life is dramatically reduced, and its far more effective to use the claw hammer, even if the claw hammer costs a bit more.

Every successful railgun that I've read about has used energy discharge capacitors. I've seen three using electrolytics, and the results were far from impressive. Lytics are more affordable, yes, and they are likely safer for a beginner's first try, but they are in no way the most desirable tool for the job, and I'd imagine Erg would like to know the benefits and disadvantages before sinking his money into components.

Dave

Re: Choosing Capacitors for Railgun [was "Greetings..."]
Dr. Shark, Mon Feb 20 2006, 05:29PM

Well, theory i fine, but I like to check with reality ocasionally. The only working amateur railgun I am aware of is using electrolytics, and the author explains very well why he is doing it (sorry, its in german).

Edit: Link fixed

Re: Choosing Capacitors for Railgun [was "Greetings..."]
Dave Marshall, Mon Feb 20 2006, 05:48PM

I should clarify some. I gave the impression that a good lytic railgun isn't possible. Its been done by a few different individuals/groups. None of them were simple systems though, they required extensive experience and considerable resources.

In any case, big general purpose electrolytics are not in the least bit suitable for the very high peak currents you want for a railgun. They literally tear themselves apart. An ideal bank of electrolytics for a railgun would be a 'cube' constructed of somewhat smaller pulse rated electrolytics. Several banks of caps in series to get the voltage up, then several of those banks in parallel to bring ESR down. This gets rid of most of the major issues with electrolytics in pulse duty, HOWEVER this is no inexpensive proposition. Such a bank of appeciable energy capacity would cost many hundreds of dollars.

General purpose caps can be used, and would probably work to some degree, but in the end, you'll pay more replacing failed capacitors, and suffer the disadvantages of crummy peak current capability.

Dave

Re: Choosing Capacitors for Railgun [was "Greetings..."]
badastronaut, Mon Feb 20 2006, 06:02PM

Dave, you seem to have an obsession with increasing the working voltage of the capacitors. Please explain your rationale for using a higher voltage. For example, explain why it would be better to wire two identical real capacitors in series rather than in parallel.

You're also wrong about the relationship between electrolytic capacitance and associated ESR. A higher capacitance actually is associated with lower ESR for individual capacitors. Just look at some of the datasheets on Cornell Dubilier's electrolytic capacitors.

Yes, individual photoflash caps have high ESR, about 1 ohm, but large can capacitors have ESR's around 50-20 milliohms.

Re: Choosing Capacitors for Railgun [was "Greetings..."]
Marko, Mon Feb 20 2006, 06:44PM

Yes, individual photoflash caps have high ESR, about 1 ohm, but large can capacitors have ESR's around 50-20 milliohms.

Thats what I talked about for long, nobody listened max current depends much more on ESR than actual resistance of entire system (for electrolityc caps).
People told me also to wire all in series and get most voltage, but actually more peak current can be gained by wiring in prallel (to some limit) because then they can give twice max current of 1 capacitor.

Normal dielectric pulse capacitors have incredibly small internal resistance and thus are very good for this kind of use, actually they are more benefical in series as ESR is very small compared to external resistances.
Here we can get real boost in peak current by seriesing caps and getting more voltage
(for railgun pulse can even sometimes be too short so this is not always good)
...

Re: Choosing Capacitors for Railgun [was "Greetings..."]
Dave Marshall, Mon Feb 20 2006, 07:22PM

I'm no railgun expert, but from the research that I've done, the key to getting a railgun to do its thing is to keep the projectile from sticking to the rails, a feat more easily accomplished if you can get a small portion of the projectile to vaporize. This is usually accomplished with an injector of some sort, and high voltage/peak current. Railguns thrive on high peak current, there is no denying that. High peak peak current is achieved with high voltage, and low resistance.

Capacitance has little to do with ESR overall, HOWEVER, physical size has everything to do with it. A 1000uF, 100v capacitor the size of a film canister is going to have light weight plates, thin connections to the terminal, and small terminals. This lends itself to a high ESR. A 1000uF cap the size of a soup can would have the room for heavier conductors, bigger terminals, and more robust connections to the terminal, equating to a lower ESR. You could have a 1000uF 10v capacitor the size of a film canister, and a 5,000uF 500v capacitor the size of a soup can that have the same ESR, but I have never seen a high value capacitor the same size as a lower value cap, with a lower ESR.

Edit: I think maybe some of the confusion stems from the fact that there are infact different types of railguns. Plasma aramatures are the easiest to construct, and require the least specialized resources (precision machining equipment etc). I expect, being his first try at a railgun, Erg would probably lean towards a simplistic topology. Plasma armatures are as I described above. A small portion of the projectile (and rails, though that isn't really desirable) vaporizes, and the projectile essentially rides a cushion of plasma down the rails, preventing your railgun from turning into an expensive tack welder. This is more easily achieved with voltages higher than typical electrolytics can handle repeatedly. Like I said, electrolytics can work, even for a plasma armature, but general purpose electrolytics will not. You need specialized inverter grade electrolytics that are not exactly cheap. Small ones in the 200j range that I've looked at in the last couple hours start at about $30 each, and only go up.

Dave

Re: Choosing Capacitors for Railgun [was "Greetings..."]
Steve Conner, Mon Feb 20 2006, 07:36PM

Here is my $.02 worth. I'm no railgun guru but I know a lot about capacitors:

It seems that electrolytic capacitors can be used to make a good performing railgun. Jason Rollette and Thomas Rapp both use electrolytics and have probably the two best railguns on the net. Rapp's gun shoots no sparks whatsoever and launches projectiles at the speed of sound. BUT......

Not any electrolytics will do. Both of their guns use the best inverter grade capacitors. These are specifically designed to carry heavy currents, and are bigger than ordinary lytics of the same rating, with very big hefty terminals. These are expensive new, but you can usually find them quite cheap on eBay. They are used in DRSSTCs too. If you use smaller capacitors like you find in a camera flash or TV set, their ESR will probably be too high to shoot anything other than their own innards.

For instance: I recently bought 12 3300uF 420V capacitors for a DRSSTC project. They are slightly bigger than soup cans and have an ESR of 0.026 ohm IIRC. Hence each one would deliver about 16,000A into a short circuit, or 8,000A into a matched load. If I connected all 12 in parallel I imagine I could launch something somewhere I paid about $100 for the bank of 12: they were surplus stock from a company that made HF spot welders that went bust.

BTW- Rapp's clever projectile design (a plexiglass bullet with thick copper wire brushes) may help the efficiency since it doesn't shoot sparks. A higher voltage power supply might show more of an advantage with regular projectile designs that lead to the formation of a plasma armature with its extra volt drop.

Re: Choosing Capacitors for Railgun [was "Greetings..."]
Eric, Mon Feb 20 2006, 07:58PM

I think electrolytics can be made to work in a railgun. The thing you have to look at when selecting capacitors is the RC constant of a cap, its ESR times its capacitance. This gives you a measure of its minimum discharge time (when short circuited). Just because a cap has a lower ESR doesn't make it better, the RC measure is what essentially tells you how fast you can extract energy from the cap. There's no way to arrange caps to improve this time limit.

The highest power is available in 2*ESR*C, with a load that has the same resistance as the ESR, although in that case half of the energy is wasted heat. Ideally you want the cap RC to be much smaller than the timescale of your planned discharge, that way more of the cap energy will be delivered to the rails instead of internal heating.

Big oil caps are great but they are actually much higher performance than you need for a railgun. These things are made for *really* short discharges, the railgun timescale is much longer.

Oh, and one other thing, as far as voltage goes, 300-400V is an ok range to be in for applied voltage to the rails, you do not want/need to go higher assuming you have a somewhat optimized design.

Re: Choosing Capacitors for Railgun [was "Greetings..."]
Jasonr, Mon Feb 20 2006, 08:57PM

Well let me jump in here with my 2 cents. You are all correct when only talking about a small portion of the over all system.
First you have to think of the cap bank all by it self. The limiting factor of discharge current is ESR. So the one way to lower this value is to parallel caps.

The next factor is if you are using electrolytics then they do not go much higher than 550volts so if you need a higher voltage then you need to series them. But when you series them you end up with the problem of adding ESR! So now you have the chicken or the egg scenario.

The next is the over all resistance of the entire system. If your system has a .5 ohm resistance from the bank to the to the rail and back. ( this includes the armature as it skips down the rails so this is low!)

So if you have a bank of qty 10, 1500uf 500Vdc caps 1875 joules and your system has a resistance of .5 ohm. Your peak current is around 1000 amps. But if you half your capacitance and series parallel them, in the 2 series and five of those in parallel arrangement then your current goes to 1850 amps. Now you say I did not take into account ESR. Well you are right?!?!?! Because the esr of the first scenario is virtually nothing because the ESR is lowered with every cap you add. So if you double your ESR that was virtually nothing it does not really affect your current too much.

The limiting current tends to be your rails and connections, to over come this you use a higher voltage.

Re: Choosing Capacitors for Railgun [was "Greetings..."]
pulslaser, Mon Feb 20 2006, 09:38PM

Hi
like Steve and Joe told I have made very good experience with electrolytic capacitors and unless you going to use a plasma armature 200.. 500 V charging voltage is sufficient. My results with high voltage pulse capacitors where very dissapointing. Of course you get very high puls currents but only for some 10..100 microseconds, useless to make the rails longer than some centimeters, even with the speed of sound the projectil moves just 3mm to 3 cm. On the other hand the extremly high currents predicts difficulties with arcing, switching. Lower currents but longer duration are easier to handle. I found the most important point ist the switching of the current. Selfswitching with any type of injector results in arcing again, and may change youre railgun in a thermo electric accelerator. So my advise: use electrolytic capacitors as much you can get ( I bought mine in ebay 5 Euro each) and use a switch (ignitron, thyristor) to close the current path. By the way, german scientist build a working rail gun during II world war with a battery of lead acid cells

Re: Choosing Capacitors for Railgun [was "Greetings..."]
Quantum Singularity, Mon Feb 20 2006, 10:26PM

I'd like to add to the discussion that myself and maybe some others are of the thinking that I was when I set out to design a railgun in the 90's. Back then it was more unheard of to try to pulse thousands of amps out of electrolytics. When I was researching and designing mine, most designs were in the kilo-volt range and used some kind of spark gap switch. My design had a max of 12kJ, which would be dificult to switch solid state. We tend to push the limits of our components sometimes as experiemtners. I would have to beleive that any railgun design using lytics is probably pushing their limits severely. Will it work, probably. Will it work good, maybe. I have not attempted such things with lytics myself. Using the high esr of lytics to slow down the pulse sounds to me like plugging up your fuel injectors so your car cant go more than 65mph. And everyone is talking esr, what about esl? Most pulse caps which are actually designed for this kind of work have an esl rating. I dont happen to know what kind of esl ratings lytics have to compare though, and havent given it great thought to how it would effect performance ina railgun. Just thought I'd mention it to see if someone has.

Re: Choosing Capacitors for Railgun [was "Greetings..."]
Eric, Mon Feb 20 2006, 11:02PM

I was just looking at this for some CDE computer grade filtering caps. Manufacturers usually don't come out and specify the DCL but by looking at an impedance/frequency graph it looks like it's about 40nH for a 7900uF 400V model (9KHz self resonance). Assuming other electrolytics have similar properties, this wouldn't be a significant issue in a rail gun. Transmission line and rail inductances would be much higher in comparison.

Re: Choosing Capacitors for Railgun [was "Greetings..."]
badastronaut, Tue Feb 21 2006, 12:24AM

When choosing capacitors for railguns, there are two things to consider: stored energy and ESR. It is always better to have higher energy for railguns.

If a railgun has a projectile with enough preloading force to prevent arcing, then the only reason that projectile might not move a millimeter is because of friction. If friction is overcoming the accelerating force, then that means there either isn't enough current or enough action integral or both. Friction force is usually greater than 200N for non-arcing railguns.

If a projectile moves through the rails but stops midway before exiting the barrel, then that means that the current pulse wasn't long enough.

Peak current has nothing to do with railgun performance because peak current is a measure of a single instantaneous value. Assuming negligible friction, which situation is better: constant 1,000,000 amps for 1 picosecond, or constant 100,000 amps for 10 milliseconds?

Yes, force is proportional to the square of the current, but that too is an instantaneous value. For accelerating an arbitrary object of fixed mass m to maximum velocity, which is better, a net accelerating force of 10,000 newtons for 1 millisecond, or a net accelerating force of .1 newtons for 1000 seconds?


All of this leads to the concept of impulse-momentum in railgun applications. It turns out that the amount of impulse due to electromagnetic force on a railgun projectile for solid non-arcing armatures is proportional to a constant times the action integral.

For those of you that don't know, the action integral is a scalar value equal to the integral of the square of the current with respect to time from time zero to t. Action is not a conserved property, and it represents the amount of electrical impulse in a circuit.

In order to maximize the momentum transfer to the projectile, it is necessary to maximize the action integral. It may also be necessary to take into account the amount of impulse that the friction force delivers to the projectile.

Sparing the pain of derivation, it turns out that the value of the action integral is equal to the energy stored in a capacitor divided by the TOTAL circuit resitance. (Action = E/R) This is assuming that the energy stored in the capacitor is discharged completely as time approaches infinity. For railguns, the total resistance of the circuit is equal to the sum of the parasitic resistances and the equivalent resistance due to energy transfer to the projectile from motional EMF.

The conclusion is that to maximize efficiency, it is necessary to reduce the total parasitic resistances as much as possible. It turns out that the majority of resistances in amateur railguns is in the capacitor banks, anywhere from 1 ohm to 1 milliohm. The resistance of the rest of the circuit is usually around less than 1-3 milliohms with the former being the typical for good railguns.

Therefore, when selecting capacitors, choose the ones with the lowest ESR. In order to reduce ESR, it is necessary to wire the capacitors in PARALLEL, and any other configuration will only hurt performance, assuming projectile velocity is less than 1km/s, instantaneous current is not much more than 200 kA, and initial cap voltage is >~200V.

One set of capacitors in series doubles the nominal ESR of the bank, one set of capacitors in parallel halves the nominal ESR of bank. The action integral is independent of voltage. The bank in parallel will have the higher action integral, which is why it will perform better, assuming the conditions in the above paragraph are true.

Increasing voltage has almost nothing to do with increasing performance, assuming you aren't using an electro-thermal gun. For example, which would you rather have, a railgun with a constant 100,000 amps flowing through it at 100 volts, or stick a resistor in the circuit and have 100,000 amps flowing through it at 10,000 volts?

Edit:
It is also interesting to note that the action integral does not depend on the circuit inductance. This is because the property of inductance dissipate no energy i.e. it gives the energy back to the circuit. Assuming no friction, a railgun with or without an added ideal inductor will perform the same, also assuming no rail resistance and rails long enough to allow the capacitor to discharge completely. However, this is a different story when friction or parasitic resistances are included. Can you guess which one?

The only reason I can think of to add an inductor is to lower peak current without seriously degrading performance.

Edit:
Also note, that if you're having problems with the projectile welding to the rails, then that means that there isn't enough preloading contact force to prevent arcing. Either way, if welding is a problem, then that projectile isn't going to move no matter how much preloading force you have.

The reason welding happens is that the capacitor ran out of energy way before the projectile had a chance to move; therefore, the molten metal had a chance to cool and fuse together.

If the capacitor had enough energy, and there was enough current to produce a significant net accerlating force, then the projectile would just slide along a layer of molten metal and plasma.

Using injection for the sole purpose of preventing welding indicates that there was no significant projectile energy due to EM energy transfer since it could not overcome welding. However, that does not mean that injection isn't beneficial to a legitimate railgun system, and there is a electro-kinetic reason for that............

Re: Choosing Capacitors for Railgun [was "Greetings..."]
Eric, Tue Feb 21 2006, 01:14AM

The only thing I'd add to what you are saying about the importance of system resistance is that you want to choose the capacitors that give you the *bank* with the lowest ESR for a given capacitance/energy, not necessarily the *cap* with the lowest ESR.

For example, if you have the option to get 4700uF caps, 28.1 mOhm ESR or 15000uF caps, 13.1 mOhm ESR at a certain voltage you would want the former since ESR*C is substantially less than the latter. If you build a bank of a certain capacitance out of those smaller caps with the higher ESR you'll have a lower overall ESR.

Re: injection, it's funny but I was just running some simulations of my prospective system, fairly well optimized without injection and found that injection *decreased* energy transfer out of the system. For reasonably small injection speeds up to several 100s of m/s it actually decreased muzzle velocity... I'm not planning on using injection, just thought it was interesting/counter intuitive.

Re: Choosing Capacitors for Railgun [was "Greetings..."]
Quantum Singularity, Tue Feb 21 2006, 06:00AM

As I dont understand all the details on every aspect of engineering a railgun, I have made a simpler model in my head and am trying to figure a few things out. I am sure if there is a much more complicated answer someone will chime in. Adressing series/parallel caps, esr, and action integral.

Take 4 matched caps and a certain load. Assume the circuit resistence is greater than the cap bank ESR. In series config you will get 4X the peak current for 1/4 the time approximately, as compared to parallel config.

Yes, force is proportional to the square of the current, but that too is an instantaneous value. For accelerating an arbitrary object of fixed mass m to maximum velocity, which is better, a net accelerating force of 10,000 newtons for 1 millisecond, or a net accelerating force of .1 newtons for 1000 seconds?

Isnt this like apples and oranges? The integrated force over time is 10x different between the two. Can we not view the current as an average value over the pulse length not just an instantaneous peak? If so the 4x higher AVG current in the series config would then give 16X the AVG force. This is all in 1/4 the time, but still your integrated force would then be 4x greater wouldnt it? And if the force is close to the friction, a lower amount of force over longer duration might not even get it to move?

If a projectile moves through the rails but stops midway before exiting the barrel, then that means that the current pulse wasn't long enough.

Doesnt that mean that the friction was greater than what the integrated force is? If a 4x longer pulse might have fixed this problem, wouldnt a 4x higher current also have?

Friction force is usually greater than 200N for non-arcing railguns.

What is it for arcing railguns? And what really defines the difference here in terms of how they function?

Re: Choosing Capacitors for Railgun [was "Greetings..."]
badastronaut, Tue Feb 21 2006, 06:51AM

Oh dear...

I hope that your cap ESR isn't lower than your circuit resistance....

When you say you have 4 matched capacitors, do you mean they are all in series?

Regardless, when you add capacitors in parallel or series, you must obey conservation of charge when analyzing them .

Ok, lets consider this problem as averages.

First we must assume that our capacitors have no ESR.
You have 4 capacitors. In parallel, they output 1X current for a time of 1t. In series, the output current (this is average current remember) will be 4X. Now the trick here is that in order for conservation of charge to be observed, the time must be equal to 1/16 t and not 1/4 t.

This is because of conservation of charge. When you wire the capacitors in series, you are reducing the capacitance, and that reduction of capacitance cannot be overcome by the increased working voltage.

In short, two caps wired in parallel charged to 1V will store more charge then the same caps wired in series charged to 2V. This is the origin of the 1/16 time factor in your example.

The only thing that remains constant , assuming the caps have no ESR, is the action integral delivered by the capacitors. In other words, it doesn't matter if they are in series or parallel, the action is the same.

Now this will imply that the performance of either series or parallel is the same. The conclusion is true if and only if the capacitors have no ESR.

When we include ESR in the analysis, it turns out that parallel capacitors always wins because it can deliver the higher action integral since the total resistance is lower in the parallel case.


Apples and oranges???? Yes you're right, the whole point of that is that force and impulse are different. The significance is that impulse is a better measure of performance than peak force in this particular application.

Doesnt that mean that the friction was greater than what the integrated force is? If a 4x longer pulse might have fixed this problem, wouldnt a 4x higher current also have?

It means that the impulse due to friction was equal to the impulse due to EM forces. A 4x higher current might fix the problem if the duration of the current isn't changed. This means you will need to have a bigger capacitor bank. Another way to fix this problem is to shorten the rail length.

I define a non-arcing railgun as a railgun such that the armature never losses contact with the rails in a way as to create electrical arcing. In more technical terms, it means that transition doesn't occur. Arcing and non arcing function the similarly. The difference is the amount of relative friction force between the two, all else constant. Usually the magnitude of the EM force is much greater than the magnitude of the friction force for most of the duration of the pulse. Friction is much more of a serious problem for amateur experimenters than the multi megajoule facilities.

Arcing railguns can be classified as ones that have transition as well as ones that use plasma armatures. With plasma armatures, the physics becomes extremely complex, and there is an addition thermal consideration when trying to analyze them.

Re: Choosing Capacitors for Railgun [was "Greetings..."]
Quantum Singularity, Tue Feb 21 2006, 09:35AM


I was under the impression by the little bit of documented info out that in a railgun (atleast an arcing one) that the circuit resistance will always be higher than your caps ESR... unless your using some crappy lytics with really poor ESR.

ABout the 4 caps... I was just using that as my example, meaning 4 of the same caps compared in a series versus parallel config.

I see my mistake on the 1/16 duration vs 1/4. I flubbered up my calculations.

Are there really non-arcing railguns out there?

And finally, when picking out cap values, is there a fairly easy way to get a ball park figure on the pulse length? It probably isnt quite as simple as a RC time constant because there is some L in there too. Would the whole system resonate for a few cycles dampened by the resistance in the circuit? Or is the L so small that it has no effect?

Re: Choosing Capacitors for Railgun [was "Greetings..."]
Eric, Tue Feb 21 2006, 07:07PM


I've found the only way to understand the circuit is to simulate it. It's not hard to do, just write a differential eqn integrator for an RLC circuit and then include the term for the back emf from the armature. That'll give you a pretty good idea of what to expect, minus friction and some rather nasty-to-calculate AC effects.

You can sort of model the circuit as a simple RLC circuit but it falls pretty short becuase R and especially L are changing during the shot. Still it might give you some ballpark values for pulse length. L is very important to the calculations, moreso than R (in decent systems where R is small).

Re: Choosing Capacitors for Railgun [was "Greetings..."]
pulslaser, Tue Feb 21 2006, 08:56PM

It's very interesting to read all this theoretical discussions but I think "ERQ" want's to build a real railgun and not to simulate one. So I think it would be more helpfull for him to read about practical experiences and not plans about somebody want's to do or somebody should do. Once again, arcing and big bang may impress spectators, but are harmful for the effiency and the speed of a gun. High currents, short time are not the same than lower currents longer time. At high currents you can't apply Ohm's law because of nonlienarities of arcing.
Forget about injectors, if you once the seen, the magnetic pressure of high currents ( even whith relatively small banks, 12kJ 350V, eletolytic)distroying well solded connections and even open bolted ones, you don't care about static friction. Use a switch to start the shot and uses some serial augmentation if you don't get enough farad in your bank to lengthen the puls.
By the way, I should like to get information about the speed of railguns of other builders, not estimated ones, but measured, to compare if my way of low voltage, high capacity is right

Re: Choosing Capacitors for Railgun [was "Greetings..."]
Quantum Singularity, Thu Feb 23 2006, 02:27AM


As I will probably like to finish my railgun project someday, I thought it would be better to understand how it really works not just try to copy someones elses project. BTW, anyone with real world results is more than welcome to post... I dont beleive anyone banned such info from this thread

Re: Choosing Capacitors for Railgun [was "Greetings..."]
Erg, Thu Feb 23 2006, 08:46AM

Holy cow! I leave a bit to study for a fuel system test and when I come back my quetion has exploded into an extreamly facinating debate/knowlege exchange. I have learned a lot from these posts. Thank you very much and and please dont stop. I will have to blame you all for my new addiction (HV & rail guns). Thank you again. I think that I will need to do a bit more work but I need to find a decent schematic to work off of. Any help there would be greatly appreciated.

As for the self built adjustable inductor. I was hoping that I could use this to learn more about the workings of the circuit by testing its effects on the projectile and rails at different settings to see if I can get improved/decreased performance with it. I was inspired by the fly wheel effect that is often taken advantage of in class C amplifiers. The idea of it helping or not having any real effect is exciting. I would like to start simulations to see which it may be.

I think that I will start with the lytics. Price point being the main driver for this. As for them breaking I hope at least one does - it will then be usefull as a classroom visual aid. Thus I will acheive a net gain either way.

I love the math that was mentioned in some of the earlier posts. The more I can get the better.

I am also brain storming ideas for the construction of the barrel. I have a fair idea of how to reduce friction and possibly avoid the use of injectors (or use a solenoid to start the movement and use opto electronics to trigger).

But before I can do that I need to get two things decided fairly firmly: Cap bank size/strength and mass/composition of the projectile. Once I can get the rough idea of a good mass for the projectile I can make it aerodynamic and have a good CG for flight. Then I can design the barrel arround it. It seems to me to be an area that there is not a lot of discussion about that can make a difference in the performance of the overall design. I see some fun times in the machine shop soon.

Thank you all again.
Erg

Re: Choosing Capacitors for Railgun [was "Greetings..."]
robert, Thu Feb 23 2006, 03:49PM

Electrolytic caps do work well, if the right ones are chosen.
Epcos makes some nice low ESR/ESL capacitors, others probably too.
These are made for high current, as can be seen from the almost 2cm thick connection bolts with about 20x the connection surface compared to typical 36D Capacitors.
Im having 25 pcs around, specified 12000µF 350V, 91x150mm, 17miliohm ESR 13nh ESL but i dont have any time for making the railgun i bought these for, so im selling these.