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

  1. Feb 21, 2009 #1
    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)

    180V ( 1800uF – 1500uF )


    200V ( 1000uF )

    350V ( 560uF - 470uF )


    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
    380LQ- http://www.newark.com/cornell-dubilier/380lq182m160k032/aluminum-electrolytic-capacitor/dp/63H0353

    What 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.

    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

    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.
    Last edited by a moderator: Apr 24, 2017
  2. jcsd
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