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Solenoid question

  1. May 12, 2005 #1
    Im building a big solenoid for my physics ISU and i was woundering on how it should be designed. I plan on supplying around 100-200 volts DC to it, at very low current, maby 2-7 amps. The question is this, does the solenoid have to have a specific length or number of coils, or a certain resistance or is the whole system just one big short circut, and if it is will i have to worry about heat or power issues with my powersupply.

    as for the powersupply, im planning on building one for myself, very basic, going from 120vac 15a to 100-200 vdc 2-7a. Now will i need any special components added to the powersupply (larger heatsinks ect ect)...the type of powersupply is basic, similar to the one seen here http://www.eleinmec.com/article.asp?16

    i also need to know about the solenoid coil layers, how would i add more layers to make sure that the rod i insert at one end will shoot out the other end completely
    Last edited: May 12, 2005
  2. jcsd
  3. May 12, 2005 #2


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    Colt - I think you are looking to build what most would call a rail gun? Regardless, you sound like you are overly ambitious and playing with voltages that can easily be lethal should something go wrong.

    I would HIGHLY recommend that you use a manufactured power supply with overcurrent protection and at a lower voltage level of something like 35V or less. Then you can research whatever type of solenoid you want to build. Here's a link to a simple set of projects that may be of interest:

    Last edited: May 12, 2005
  4. May 14, 2005 #3
    thanks for the advice, i talked to a friend of mine who is a little more experienced in matters of power supplies than i am, and he was able to supply me a power supply of a more reasonable output, 40 volts at 3 amps,

    But as for the actual solenoid, since i was unable to locate a sutable sized iron pipe to use, i settled for a copper pipe that was handy, but will this affect the strength of the magnetic field by a large amount?, because i know that copper has a lower magnetic permeability than iron.

    As for the object that i want to fire out of the solenoid what material should it be and in what shape?
    Last edited: May 14, 2005
  5. May 14, 2005 #4
    Well, if your working with weak magnetic field(yes the copper will affect the strength of your magnet) then you'll want to use magnets as your projectiles probably. Look into cow magnets because they are strong slender and cheap. If you want more oomph then try Neodymium magnets. You can get the latter in long slender shapes but they cost about 4 to 5x more. You might try black pipe from a hardware store. It's not the best material but it is iron and will have slightly better permeability than copper will.
  6. May 15, 2005 #5
    thx, but as for the magnets, wouldent the positive pole of the magnet be attracted to the side of the solenoid because when it is powered the inside becomes negative?, would a long slender rod with lets say 4 strip magnets attached around it work( neg side facing away from the center of the rod).

    Will this type of object be shot out of a solenoid if used, and are there any other types
  7. May 15, 2005 #6

    If you align the magnet so the same poles are close to each other initially then give a quick pulse of electricity (On/Off) to start the magnet on its way then switch the field when the magnet is a little greater the 1/2 through the solenoid then you should be able to do it. You'd need to detemine an appropriate solenoid length for this to work.

    You need strong magnets because you are using a weak field(relatively speaking). Adhesive magnets wont work and strong magnets will be very difficult to bond as you suggest(get a couple of Neo magnets and try pushing their S-S or N-N poles together).
  8. May 15, 2005 #7

    I have done what you are trying to do..
    i used a plastic pipe from home depot(0.840 OD & 0.615 ID )..& wound #23 gage wire around it and connected it to a 12 V car battery..it shot my neodymium magnet out the end of the tube..
    Now i dont know where you got the idea that using an iron pipe would be better than a copper pipe, but i can assure you , plastic is better than copper & copper is better than iron.. The reason being is you want something that will pass the magnetic field through it, as unobstructed as possible ..
    i used .5 " X .5" Rod magnets about $ 1.50 for one..
  9. May 15, 2005 #8
    how fast/far did the magnet go? (roughly), and where can i get magnets like that.

    Also are there any other good types of objects that would work as a projectile.

    Regarding the powersupply i obtained, i just realized that it is a switching powersupply, like thoes used in PC applications, now if i were to use this powersupply, would it not detect the shortcircut and cut power?
  10. May 15, 2005 #9
    http://www.wondermagnets.com/cgi-bi...;Permanent_Ma\0\0? {P?\0\0\0\0 {e?\0Cstart=0
    it shot about two feet and landed on the floor, but i could imagine that it could and would go much faster and farther..i was testing the windings at the time , and the coils were not wound specifically to create a projectile..as they are just 0.5 '' wide each..
    the magnets are actually $2 each ..i bought over fifty so i paid $1.60 ..
    i dont know wether or not your powersupply will shut down , but if you are using enough wire , it wont see a direct short , it will see a very low resistance..
  11. May 15, 2005 #10
    would anyone else know forsure about the powersupply?

    and thx willib for that site
  12. May 17, 2005 #11
    no one have any thoughts on the powersupply

    also which would be better to create a strong magnetic field; high current low amps, or hight amps low current
  13. May 17, 2005 #12

    Current IS amps.
  14. May 17, 2005 #13
    it is current that you want , when creating a magnetic field ..
    voltage ..Not so much :smile:
  15. May 17, 2005 #14
    my bad about the current got a bit mixed up, ok, so higher current lower volts, but wont i need a thicker wire to carry the higher current? (i currently have very very thin wire), and if i were to lets say run 30volts at 10 amps, what gauge wire would you recomend?
  16. May 18, 2005 #15
    Actually either one will make a good magnetic field. Lower current allows smaller wire which allows more windings in the same physical space which will get you the same thing in the end. Obviously there is an optimum range, but I am not sure what it is.

    30 volts at 10 amps is 300 watts dissipated in heat. WOW.
  17. May 19, 2005 #16
    colt , very very thin wire is .. what??
    AWG 30 , to me, is very very thin..
    if i were to design a rail gun , i would state my maximum estimated current & voltage , right off..
    using the car battery as an example , 12V/10A=1.2 ohms..
    now , at 11 AWG ,1000 feet gives 1.26 ohms..besides being way too thick wire at .090 " way too much is needed..
    lets see what 250 feet at about 1.2 ohms would be..
    17 gauge is 5.064 ohms per 1000 feet and is more reasonable at .045 dia..
    so 250 feet =1.266 ohms.. close enough.
    but still a little hard to get 250 feet of 17 gauge wire..
    20 gauge wire is only .032 " but it is 10.15 ohms per 1000 feet, if we used two strands of 250 feet each should give 1.27 ohms this is a do-able length and a do-able thickness..
    But can two -250 foot strands of 20 gauge wire handle 10 A ??
    For short durations it could..
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