Need help with rail-gun ( capacitor circuitry and specification questions)

In summary, the conversation revolved around building a rail-gun and the team's questions regarding purchasing capacitors, understanding different capacitor series, and the best way to hook them up. They also discussed whether to prioritize capacitance or voltage tolerance, and the concept of back electromotive force.
  • #1
polosportply
8
0
Hello everyone,

I’m a first time poster here on the PF and I come to you, the community to help me and my team-mate in a little project we’re doing. We’re building a rail-gun and we’ve now encountered quite a few problems. I’ll try to make my questions as simple as possible for easy understanding.1) We’re looking to purchase capacitors and we have a general idea of what we need, but we’re still unsure of which ones would be the best for our rail-gun. We’re looking to have about 12 capacitors, depending on the cost. We are ready to spend only about 60$ ( so only like 5$ each for 12 ). We can charge the capacitors with either a 155V, a 175V or a 195V power supply. We’re also supposed to get our hands on a wall-socket-adapted transformer which can dish out 350V, which is what we’re definitely going to use as a charger when we get it, hopefully not “if”.

We’re only looking to order from a certain website:

Side-scroll to see all the specs on the website. The list on the website goes from highest capacity to lowest capacity.(scroll down to check out only the ones with the right capacity which I indicate next to the voltage)
/--------------------------------
160V ( 1800uF – 1500uF)
http://www.newark.com/jsp/search/br...4187+294548&Ns=ATRB_SORT_001_Capacitance_72|1

180V ( 1800uF – 1500uF )

http://www.newark.com/jsp/search/br...4187+294559&Ns=ATRB_SORT_001_Capacitance_72|1

200V ( 1000uF )
http://www.newark.com/jsp/search/br...94583&No=50&Ns=ATRB_SORT_001_Capacitance_72|1

350V ( 560uF - 470uF )

http://www.newark.com/jsp/search/br...4187+294672&Ns=ATRB_SORT_001_Capacitance_72|1
/---------------------------------------

What do the SERIES: 380LQ or LPX or - or TSUQ or ESMH mean?

The ‘‘LPX’’ and ‘‘-’’seem to always retail for more. Why?
For example, here, the LPX retails for twice the price of the 380LQ, for the same specs :

LPX- http://www.newark.com/cornell-dubilier/lpx182m160h7p3/aluminum-electrolytic-capacitor/dp/62H5922
And
380LQ- http://www.newark.com/cornell-dubilier/380lq182m160k032/aluminum-electrolytic-capacitor/dp/63H0353What do these series tags mean? How would it affect our circuit?

_____________________________________________________________________________

2) We have batteries that can give from 150V-350V. The higher voltage tolerant caps and higher capacitance capacitors are more expensive. To get the most cost-effectice capacitors for our rail-gun, should we lean more towards capacitance or voltage tolerance.

-If we go for a higher capacity, Q(max) = CV will be greater and so will be t= RC. Let’s say we double capacitance. I= dQ / dt , so a 2 times higher Q(max) and a 2 times higher RC means I = 2dQ/2dt = (same as before), but it’ll hold it out for a longer amount of time.

-If we go for a higher voltage tolerance, we’ll be able to get a higher I(max) = V/R . But the energy of a capacitor is U = ½ CV^2 . So technically, we’re way better off getting the capacitor sacrificing capacitance for volt tolerance. Right? But the total amount of charges that one can contain is Q = CV , and since our capacitors are going to have enough time to discharge completely, it doesn’t matter if the Q (or charges) are extracted faster or slower, since they’re all going to have enough time to get out.
Whether we get 2000 amperes for 0.01 seconds or 1000 amperes for 0.02 seconds, we don’t really care, as long as the I * t = ? is the highest as possible.

So how would having a higher voltage be better than having a higher capacitance?
U= ½ CV^2 versus CV =Q , the speed at which the charges are ejected wouldn’t matter, since all the charges will be expulsed from our capacitors anyways.
But isn’t I*t = Q. So we want the highest I*t = Q = CV. So why is energy = ½CV^2?Can someone help us figure what will make our projectile go faster? Only the current which passes through the rail makes the magnetic field, and simply said, F= ma , F= Bil /// F= k1 ( i^2) , a= k1/m ( i^2) , a= k2 ( i^2) , a= i^2 , a=i . Only the current matters here, is my point.
So how does Capacity and Voltage come into play here in a way to make our “I” as great as possible?

_____________________________________________________________________________

3) We’re also wondering about hooking up our capacitors in parallel or in series. We’re leaning parallel btw. But, let’s just take a look to take any kinks out of our minds.

We have 12 capacitors. Specs: 200uF, 250V chargeable, resistance of 1 Ohm.

Let’s compare hooking up those charged capacitors in series or in parallel.

SERIES:
C (tot) = 16.6 uF R (tot) = 12 Ohm V ( tot) = 3000 V
RC= 0.0002 sec I = 250A
Q=CV= 0.05 U= ½ CV^2 = 75 j

PARALLEL:
C (tot) = 2400 uF R (tot) = 0.08333 Ohm V ( tot) = 250 V
RC= 0.0002 sec I = 3000A
Q=CV= 0.6 U= ½ CV^2 = 75 j

RC=t stays the same for both in series and in parallel, so they both discharge in the same amount of time. U= Energy also stays the same, which makes sense because, either way the capacitors used are the same and are charged at the same 250V. But in parallel, knowing Q/t = I , we have a higher Q and the t = RC stays the same, which implies a higher I . So why does parallel get a much higher I than series, since they both carry the same energy? Are we right to be leaning towards parallel?

_____________________________________________________________________________

4) I’ve heard something about back EMF or CEMF. Apparently this has to do with fluctuation of current passing in a wire, induced current force in other words. From wikipedia:
<< The counter-electromotive force (abbreviated counter emf, or CEMF ) [1] is the voltage, or electromotive force, that pushes against the current which induces it. [...] Back electromotive force is a voltage that occurs in electric motors [...].>>

So technically, since we’re not building a motor, we shouldn’t need to worry, right? Could someone explain to us, what EMF is exactly, if it concerns us that is.
_____________________________________________________________________________

I know this is one long-as post, but if you're only able to help us with just one aspect, please go ahead. Or maybe if you’ve had some experiences of your own with rail-gun building, you’re welcome to share.

Thanks in advance and I’ll keep checking the thread from time to time, to answer any replies if there are any questions or uncertainties about ours.
 
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  • #2
I suspect that you got no answers because it requires much knoledge and skills to safely design and operate a rail gun. The mere fact that you ask these questions on the Intenet, suggest that you and your friend aren't ready for such a project.

If you want to make a rail gun, get a EE degree from university, then join the navy.

If you want to make a nuclear bomb, get a nuclear engineering degree, then work for Sandia Labs.

Those are not DIY type projects.
 
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Likes davenn
  • #3
anorlunda said:
I suspect that you got no answers because it requires much knoledge and skills to safely design and operate a rail gun. The mere fact that you ask these questions on the Intenet, suggest that you and your friend aren't ready for such a project.

If you want to make a rail gun, get a EE degree from university, then join the navy.

If you want to make a nuclear bomb, get a nuclear engineering degree, then work for Sandia Labs.

Those are not DIY type projects.
I am so surprised you didn't close/lock this one after my report and your post ?

Dave
 
  • #4
davenn said:
I am so surprised you didn't close/lock this one after my report and your post ?

Dave
It is a double standard that I'm using for the workshop.

I'm not going to bother with closures in the Workshop. All threads auto-close when they get old enough.

If it actually contained dangerous info that would contaminate the knowledge base, I would delete it.
 
  • #5
Wait, this is a decade old thread, so do you respond or no?
 
  • #7
So given " In this new workshop forum we are going through the threads and if it's worthwhile, adding a valuable reply, and if not, trashing it. "

The question is, do you answer re technical info on rail guns, I've looked into them quite a bit, and building one is on the list. Then there are quite a few people that have built home hobby level guns, but there is obviously the small issue of helping someone build what is essentially potentially quite a dangerous/lethal thing...
 

1. What is a rail-gun and how does it work?

A rail-gun is a type of weapon that uses electromagnetic force to accelerate a projectile to high speeds. It works by sending a large electrical current through two parallel metal rails, creating a strong magnetic field that propels the projectile forward.

2. What is the role of capacitors in a rail-gun circuit?

Capacitors are an essential component in a rail-gun circuit as they store and release large amounts of electrical energy in a short period of time. This energy is used to create the powerful magnetic field that propels the projectile.

3. What are the specifications for the capacitors used in a rail-gun?

The capacitors used in a rail-gun must have a high capacitance (measured in microfarads) to store a large amount of electrical energy. They must also have a low inductance (measured in nanohenries) to allow for quick discharge of the energy.

4. Can any type of capacitor be used in a rail-gun?

No, not all types of capacitors are suitable for use in a rail-gun. The capacitors must be able to handle high voltage and high current, and must have a low equivalent series resistance (ESR). Ceramic and film capacitors are commonly used in rail-gun circuits.

5. What safety precautions should be taken when working with rail-gun capacitor circuits?

Working with high voltage and high current can be dangerous, so it is important to take proper safety precautions when working with rail-gun capacitor circuits. This includes wearing protective gear, using appropriate tools, and following proper discharge procedures. It is also important to have a thorough understanding of circuitry and to follow strict safety protocols.

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