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Elelctrical theory

  1. Jul 28, 2004 #1
    hi i was wondering if any of you ladys and gents out there have fooled around with electronics? if you have i was wondering how to create a homemade solenoid? I have tryed many times before but none of them have ever worked

    Last edited: Jul 29, 2004
  2. jcsd
  3. Jul 28, 2004 #2
    Electronics have more to do with transitors and other solid state components but not with inductor which is basically a coil or solenoid. As far as I know, there is no solid state analogue of the inductor.

    There are now so called chip inductors used in cell phones that can store magnetic energy like the solenoid. But I dont know how it is made.
    Last edited: Jul 28, 2004
  4. Jul 29, 2004 #3


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    One of the things that you should do is to form your ferrite core out of strands, as small as possible, and not touching. This prevents eddy currents from dissapating energy. Then, it's all about optimizing your voltage ability with current supply and flux. Each turn contributes to the flux, so more turns gives a stronger solenoid, ceteris parabis. However, more turns also means higher required voltage.

    I tried to build my very first solenoid recently, as a matter of fact; a total bust. I was so excited to get started that I didn't even bother to make a simple calculation. After $20 of materials and 3 days of winding hair thin wire, it did not work. It barely deflected a compass needle. Then, I decided to do the calculation and kicked myself.
    Last edited: Jul 29, 2004
  5. Jul 29, 2004 #4


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    If you're going to be doing your own winding, you might consider investing in a lathe, and learning how to use it.
  6. Jul 29, 2004 #5


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    Yes, After having done it myself by hand (for the first and last time), I couldn't agree more. Even if the lathe costs a few hundred bucks (I have no idea how much they cost), it would be well worth the time (and frustration) you save.
  7. Jul 30, 2004 #6
    thank you for your comments , but i read in a book that a simple solenoid could be made by just wrapng some wire around a straw and placing a iron nail inside the straw, and applying current to the wireis that so ? if i am repeating myself for give me.

    Thanks regards
  8. Jul 30, 2004 #7


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    When you said that "none of them have ever worked," I took that to mean in the sense that what you have just described would not generally "work well." Do a calculation on the field strength in such a solenoid and you'll see what I mean (i.e. how many windings you would need with say 100 mA in order to have a field strength even as strong as the Earth's). If you want a "serious" solenoid, I suggest being "serious" about the core and making something a little "beefier."
  9. Jul 31, 2004 #8
    so as in make the core beefier i would say like useng a maybe like copper tubing or some type of material like that right?

    thanks regards
  10. Aug 1, 2004 #9


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    Sorry for the confusion; I was not referring to the core but to the wires and overall structure. But going back to the core, I'm no expert, but I don't think that copper is going to do anything for you there. I believe that you want a ferromagnetic material. I think they make iron powder type stuff for the purpose, but you can also throw something together yourself.
  11. Aug 2, 2004 #10


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    You want a soft iron core, preferably laminated (as with Turin's multiple strand idea) so as to reduce the effects of magnetic eddy currents in the core. Not copper, - this would not help your cause!
  12. Aug 2, 2004 #11
    brewnog, have you ever made a solenoid?
  13. Aug 22, 2004 #12
    Skip the higher math and stick to the practical basics

    The amount of confusion and pointless discussion going on here is disturbing.
    Furthermore, average solenoids are dirt cheap and common, so why would you custom build one if you aren't 12 anymore?

    Solenoids are simply a fine-wire choke (wire coil), often with a highly heat- and magnetic- conductive core material, such as transformer-grade silicon steel; some have a fitted steel pin that is mobile, to actuate something else with.
    The technology is strictly 19th century.
    Try the eMagnet site at (http://www.netdenizen.com/emagnet) or the Wheeler site (http://home.earthlink.net/~jimlux/hv/wheeler.htm) for simpler formulas accurate to 2% or so to calculate your coil size with.

    Some basics: the usual solenoid has a length several times (5x+) its diameter. The wire is closely wound around the outside of a cylinder, in the form of a helix with a very low pitch. The magnetic field thus created inside the cylinder is quite uniform, especially far from the ends of the solenoid. The larger the ratio of the length to the diameter, the more uniform the field near the middle.
    The approximate value of the magnetic field is given by B = u0nI, where B is the magnetic field, u0 (u sub 0)the permeability of free space, n is the number of turns of wire per unit length, and I the current through the wire. This relationship would be exact if the solenoid was infinitely long. A more precise calculation shows that the above relationship is within 2 percent of the correct value at the center of a solenoid if the ratio of the length to the diameter is five or greater.
    This equation shows that one way to increase B is to increase I. But, because all wire has resistance, this procedure requires an increase in voltage across the solenoid and results in more heat being generated by the resistance of the wire. Another way to increase B is to increase n. But this increase can only be accomplished by decreasing the wire size (if the solenoid, as is usually the case, has turns wound as closely as possible), resulting in an increase in resistance and an increase in the voltage required for a given current, as well as an increase in heat generated by the resistance of the wire. An alternative way to increase n is to wind several layers of wire. This procedure increases the resistance of the wire, adds insulation problems, and decreases the length to diameter ratio. The selection of the appropriate trade-off is the principle problem that must be solved by the solenoid designer. Sleeving the coil with a ferromagnetic material (iron sleeve) can conduct away excess heat and even out the magnetic field, but will shut down or interfere with actuator-type solenoids.

    If a soft iron rod is placed partly inside a solenoid and the current turned on, the rod will be drawn into solenoid by the induced magnetic field. This motion can be used to actuate a lever, unlock a door, or operate a relay. In this way the operation of a small electric switch can produce a large mechanical action at a remote location. It is worth noting that the iron core has to be placed at the end of the solenoid where the field is non-uniform for it to move. Furthermore, it is not necessary for the current to flow in one direction only. An alternating current will generate a field also.

    The magnetic field of a solenoid can also be used directly with a high degree of accuracy. It is used to deflect the beam of electrons in a television tube. Solenoids are also used to provide the magnetic field for magnetic resonance imaging. Physics textbooks often use solenoids in problems about magnetic fields because the field produced by the solenoid is easily calculated and is easy to visualize.

    On the subject of old technology, consider materials from Lindsay Books (http://www.lindsaybks.com), who carry lots of books on this sort of low technology material.
    A very inexpensive tabletop lathe (plastic and aluminium) to wind coils with can be bought for the task at Harbor Freight Tools for less than $40 (on sale from time to time for $20), and altered quite easily. If you need a wider or deeper bed, build it out of aluminium stock.

    Most of this will be of no use to you if, as I suspect, what you are aiming for is a linear solenoid, rather than just something to ring a doorbell with. Linear solenoids (railguns) use computer-controlled overlapping solenoid fields to generate their motion and high velocity, and the power and forces involved are too substantial for anyone but the most dedicated tinkerer.

    Good Luck.
  14. Mar 30, 2005 #13
    I don't know if I would be considered a dedicated tinkerer, but where could I find more information about linear solenoids, it doesn'st sem to me that railguns were that similar to solenoids other than that they both make a magnetic field. I'm interested in finding out how to use a solenoid design to move somethingthrough a relativly long solenoid, also what kind of affect does a magnetic field have on aluminum.
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