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Need help on making magnetic launcher

  1. Apr 26, 2009 #1
    Hello again,
    well after finishing the solenoid (https://www.physicsforums.com/showthread.php?t=309671), I'm stuck with a problem again.

    before that, let me give you a little background.

    Well actually I have a project on making a magnetic launcher. This device should be able to launch a small projectile only by using magnetic force. There is no standard for the projectile (it could be paper clip, nail, etc.). If I'm using electirc current, the power source must not over 12V and has to be DC. the farther the projectile fly, the better.

    I'm planning to make something like http://en.wikipedia.org/wiki/Coilgun" [Broken], but with less power. The projectile is paper clip and my power source is 12V DC.
    After googling for a while, I learned that the capacitor that is needed to make coil gun is 200uF. Because I want less power, I bought four 100uF 63V capacitors. And the other reason is the 200uF capacitor is much expensive.

    my rough calculation :
    assume a paperclip is 0.01 kg

    Energy in capacitors :
    1/2*(4x100)*10^-6*12^2 = 0.0288 J

    Velocity of the paper clip :
    1/2*0.01*v^2 = 0.0288
    v^2 = 5.76

    because the energy from the capacitor is also lost as heat,
    I assume that v = 2 m/s

    I'm not really sure if my calculation is correct (maybe I'm missing something?)

    Now here is my problem :

    I'm connecting the solenoids to 12V DC and the magnet attracts the paperclip as expected.
    Then I charge the four capacitors in parallel, waiting for about 30secs. And then I remove the power source and connect capacitors to the solenoid. Nothing happens.
    Then I test whether the capacitor is storing energy or not. I charge the capacitor again, waiting for some secs, then disconnect the power source. I connect the capacitor with LED, LED glows brightly for a sec and off.
    I counclude that the capacitor is able to store energy, no problem in capacitor.

    So I googling again and found a method to know whether a capacitor has full or not.
    It's by using LED in series with power source, connected to the capacitors.
    I tested it, the LED glows bright for just milisecs and then dimly for ever. (I was expecting it gradually glows from bright to dim and then off. And it takes 30secs or so.). It seems that the capacitor is fully charged in no time. but it store much less power.

    some pictures :
    The LED glows brightly if it just connected to the power source. In this pic, the capacitors is not connected to the circuit. FYI, on the left of the LED, there is resistor 470 ohm. I use this resistor to avoid burning my LED.

    The LED glows very dimly when I connect the capacitors in the circuit.

    This is the circuit.

    I need help to make the capacitors to produce energy to launch the paper clip.

    I'm kinda blind in electrical circuit. Advices is much appreciated ;)

    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Apr 26, 2009 #2


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    I have made one of these but it had to have to have AC power. It works on eddy currents induced in a donut shaped aluminium ring.

    In a laboratory, you get a demountable transformer (one which can be pulled to bits) and only put a primary winding on it. It has a U shaped core and a straight piece of core that goes across the top of the U-shape. Put the horizontal top core vertically on the arm that has the coil on it. ie stand it on its end.
    Get a ring of aluminium.
    Put it on the extended core arm above the coil.

    Apply power briefly with the mains switch.

    Watch out, because the ring will hit the ceiling. To avoid ceiling damage put a chair over the apparatus.
    The ring was the type used for cooking eggs on a barbeque.

    On a small scale, you would need a source of AC, maybe about 6 volts for your small coil.
    Forget the paper clip. The ring has to be a non magnetic material and a very good conductor.
    Aluminium is good, but it must be a continuous ring like you would cut off a piece of pipe. Maybe 1/4 inch long. Joining aluminium strip into a circle probably wouldn't be good enough.

    And you need an iron core to go inside the coil. If it extends beyond the coil, it continues to accelerate the ring while it is on the iron core so you get better distance.
    It has to be soft iron and probably a piece of iron rod would be OK on this scale.
  4. Apr 26, 2009 #3
    thx for reply
    Well, the project specification is DC, not AC.

    by the way, this just came in my mind :
    Do you need exactly 63V DC power source to charge 63V 100uF capacitor?
  5. Apr 26, 2009 #4


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    The 63 volts is just a maximum rating. You should not exceed that rating. You can and should always use less than that.
    And the polarity should be right too. Charge with + of the power supply to + on the capacitor.

    Try discharging your capacitor through the coil. You may get enough eddy current induced to make the aluminium ring move. Those capacitors are very small, though. I would look for some bigger ones like 5000 uF in old computer power supplies. Ask at a computer shop if you can have any old power supplies.

    If it doesn't work, at least try the AC method. I know that works on a larger scale.
    Only touch the AC source to the coil briefly because it will be almost a short circuit and it will try to draw a lot of current.
    Last edited: Apr 26, 2009
  6. Apr 26, 2009 #5
    I can't use AC since the project specification is restricted to 12V DC or less. No AC.

    I have charge the capacitor for some seconds and try to discharge it to my made coil, it is 6 layers 100-200 turns each. but nothing happens.

    I also tried to discharge it with LED. I was expecting that the LED would burn because I discharge four fully charged capacitors to this LED. But the LED didn't burn. Actually it glows dimmer than using 12V battery.

    If you have charged 4 100uF capacitors with 12V DC, would it burn LED if you discharge these capacitors to it?
  7. Apr 26, 2009 #6


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    I'd recommend checking the power through the solenoid when you connect it to the 12V supply. Your capacitor bank will have to be able to supply at least as much power as the 12V supply itself if you want it to attract the paperclip the same way. Once you find out how much power the coil needs to attract the paperclip, check to see how long your capacitors can sustain that power. My understanding is that coil guns are extremely inefficient (< 5%), so it may be that your capacitors simply don't have enough energy to create a strong enough effect (for a long enough period of time).

    As for the LED, it shouldn't burn out if you have it set up with the right resistor. However, rather than having the LED in there (which will consume some of your energy, and force you to use lower currents), you can easily check whether the capacitors are charged/discharged by sticking a voltmeter across them. If the voltmeter reads 12V (or close to), then they are charged. If you then attach them to your solenoid and measure the voltage again, the capacitors should have 0V across them if they fully discharge.
  8. Apr 26, 2009 #7


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    Just looking back through your posts, I see you are using a resistor with your LED, so that is good, but I can't see any mention of an iron core for the solenoid.

    This is very important and I don't think the gun has any chance without it.

    And you need to put all four of your capacitors in parallel. That will give you 400 uF which may not be enough, but it is a start.
    The projectile has to be an aluminium ring on the iron core. It works on repulsion, not attraction.

    I see you have a battery pack for your DC supply. Try putting that straight across the coil.
  9. Apr 26, 2009 #8
    Yes I put all my four capacitors in parallel and the batteries in series.

    Iron core? Do you mean for http://repositorium.sdum.uminho.pt/bitstream/1822/6051/1/76.pdf" [Broken]?

    I'm trying to make a coil gun that works on attraction like this picture :
    But I'm using just one coil.

    From what I've learned on the net for making this type of coil gun, I haven't found one mentioning iron core.

    So what I'm trying to achieve is make a strong magnet inside the barrel (I'm using hollow pen case) by conducting current through the coil.
    When the paper clip approach the middle of the solenoid, the magnet should be off immediately. Thats the reason why I use capacitors. I want to make them store much energy and dump it all at once in the coil. So the magnetic field is induced "on and off" in the matter of millisecs.

    And the problem is, after charging the capacitors, the coil doesnt induce any magnetic field (The paper clip doesnt move even a bit!)
    Last edited by a moderator: May 4, 2017
  10. Apr 26, 2009 #9


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    I think we agree that your system isn't working and isn't going to work. Your capacitors are very small and maybe not even capable of large discharge currents. You should certainly be able to attract a paper clip. Why not ask your dad to check the circuit for you?

    What we need to do is try something that has a chance of working. You can use any kind of projectile , why not use an aluminium ring?

    The first link in your last post is the one that has a chance of working although it works best on AC. But it might work a little by discharging a capacitor.
    Putting an iron core in your solenoid will magnify the magnetic effect it has by many times for the same current in the solenoid.

    If you put an iron core just inside a solenoid, it should pull into the solenoid when you apply power. Even a large bolt should do this. This mechanical movement is used in actuators in washing machines etc.
    If you can't do this, maybe your batteries have gone flat.
  11. Apr 27, 2009 #10
    Okay.. I'm not sure what do you mean by this. More descriptive explanation maybe?
    Do you mean to put a nail and the projectile inside the solenoid at the same time?

    Okay, I've tried again and now I notice that the paperclip move a bit. The capacitor works, but like you said, it is not powerful enough.
    Will it solve the problem If I buy more powerful capacitor?
    Do you have any suggestion about what capacitor to buy?
  12. Apr 27, 2009 #11


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    If you use the type of magnetic gun that you intend, you have to develop enormous magnetic fields for it to work. If you have hundreds of turns of thin copper wire, it will have a lot of resistance and you will find it difficult to get much current into it at 12 volts, no matter what capacitor you use.

    There are two things that might help.
    One is to charge your capacitors in parallel but discharge them in series. If you do this, you have to be very careful because you can get a small shock off 48 volts. Don't touch both ends of the series string of capacitors at the same time.
    This would let you force more current into the coil.

    The other thing you can do is find an iron bolt that will fit into your solenoid and stick out of one end of it. You then get a short piece of aluminium pipe and put it over the bolt where it sticks out of the coil. This will make the magnetic field a LOT stronger and maybe fire the aluminium ring off the bolt away from the coil.
  13. May 1, 2009 #12
    hey.. are you an itb student?? because we have same assignment and also same rules :)

    i've tried to make a solenoid just like you did. but when i connect it with an adapter of 8,5 V and 1,5 A, it wont give any magnetic field. I tried to attach some paper clip but it wont attracted. But the wire were beeing hot.
  14. May 1, 2009 #13
    @glen.ricky :
    LOL yes. I'm an ITB student :)

    Did you put the paper clip close enough to the coil?
    Based on my experiments the paperclip wouldn't be attracted unless I put about 1/4 part of it or more inside the solenoid..

    I've tried 10,000uF 16V capacitor. It could attract the projectile into the solenoid, but it sucked the projectile back into the solenoid.

    I just found out that discharge time for capacitor = 5*R*C
    the more capacity of the capacitors and the more resistance in the circuit, the more longer the solenoid will be an electromagnet, meaning that the more likely the projectile will be sucked back.
    well it seems the only thing to make the projectile launch is by applying higher volt. Any suggestion?

    By the way I wonder how to solve the suckback effect. The paperclip was attracted and going to leave the coil, but when it was about to leave, it was attracted back into the coil.
    I wonder how to solve this problem.

    I was thinking about lengthen the solenoid length and adds more winding on the farther end, so the farther end has more magnetic fields. (Yes it also adds up the resistance, but who knows it could make the projectile leave the solenoid..)
    I think the projectile will travel to the farther end and still speeding until the electromagnet is off
    Will this help negating the suckback?
    Can anyone give me suggestion on this?
    Last edited: May 1, 2009
  15. May 1, 2009 #14
    Well, this may be obvious, but make sure you're using capacitors with a very low ESR, like a super cap. When you're trying to get huge current spikes out of your capacitors, even a few ohms can add up to a voltage drop and energy loss. This will also affect your time constant.
  16. May 1, 2009 #15


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    Suppose you had a piece of brass (from a brass bolt) and put it in the middle of your solenoid.
    Then you got a piece of Iron, (from an iron bolt) and put it so it was sticking out of one end of the solenoid.

    Then apply your charged capacitor to the coil.

    The iron bolt would be drawn in to the center of the solenoid and hit the brass piece which would go flying out the other end.
    Wouldn't work if both pieces were iron.
  17. May 1, 2009 #16
    oh anak itb juga toh.. hahahahaha..
    fakultas apa lo?? gila niat amat bikinnya ampe pake kapasitor..
    gw buat gitu juga eh tapi magnetnya kecil bangettt... cuma bisa buat gerakin rambut doang.. hahaha.. gw sekarang pengen buat yang gaussian rifle aja deh..
  18. May 1, 2009 #17
    well this project is really interesting I think :)
    I had fun experimenting it.
    My team has decided to to make a gaussian rifle too
    looks so much easier but has few limitations

    How do you know the ESR rating of a capacitor? Is it printed on capacitor's body?

    Thanks, I've got the point.

    By the way http://www.coilgun.eclipse.co.uk/coilgun_basics_2.html" [Broken] works at 12V.
    But I'm using 10,000uF capacitor instead of microswitch.
    Now my problem is timing.
    Last edited by a moderator: May 4, 2017
  19. May 2, 2009 #18
    ESR = equivalent series resistance, and its the parasitic resistance of the capacitor. This is not written on the package usually, and you need to look at the datasheet. The ESR is highly dependent on the technology and materials of the capacitor. Some datasheets will explicitly state the ESR, while others will only give you a dissipation factor or loss tangent which you can use in equations to derive an ESR.
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