Homemade Solenoid - Is my battery strong enough?

  1. Hi Everyone,
    I'm trying to make a homemade solenoid to lift a metal slug, and i have a 2000mah / 7.4 V / 20C lithium polymer battery. Is this battery going to be sufficient?
    Here is a photo of my current setup without the battery:
    side view
    top view

    Thanks in advance!
  2. jcsd
  3. berkeman

    Staff: Mentor

    What is the resistance of the coil? How many turns is that? That's a pretty big coil diameter -- what does your ferrous shaft look like, and how heavy is it and the slug?
  4. The coil 200ft of 30 gauge magnet wire. the outer diameter of the pipe is .8 inches, so the circumference is 2.512in. 200 x 12 is 2400 inches, which makes for 955 total turns.
    I'm not sure what you mean by a ferrous shaft? The slug is slightly smaller than the innerdiameter of the pipe, and it weighs 38.69 grams
  5. berkeman

    Staff: Mentor

    Ah, so the "slug" is the shaft? By ferrous, I meant that whatever you are using for the shaft/slug has to be ferrous (like iron) to be pulled into the coil.

    2400 inches of 30AWG copper wire gives about 21 Ohms (9.5 feet per Ohm from the wire tables). Is that what you measure with your DMM?

    The 7.4V battery will generate about 7.4/21 = 350mA of current through the coil. That's a lot of Amp*Turns. I don't know offhand how to translate that number of Amp*Turns into a lifting force, but this paper has a nice explanation that I think you can use:


  6. I've read elsewhere that the battery should generate around 2000mAh / 1000 x 20C = 40 amps max constant. Is this irrelevant, as the battery only supplies however much the wire draws?
    also, i was looking at this link and i want to clarify: If i copy their design / number of turns / wire, I should get the same result.
    Does magnetic field strength in this application rely on number of amps through the current or the voltage?
  7. NascentOxygen

    Staff: Mentor

    I reckon you'll need as much voltage and current as you can get without the enamel on the coil getting so hot that it starts to smoke, and then you will probably find that's still not enough! (Just my thoughts. :smile:)

    The more voltage and current, the stronger the field. When testing, leave the battery connected only momentarily so the coil doesn't have time to get hot.

    Over how much distance do you want to lift the metal slug?
  8. I'm probably going to change my design into this design: [​IMG]

    The metal slug will be resting at the bottom, where the 1" is marked. The indent that is .5" high will hold the magnet wire.

    So, in effect, I'm not looking to move it much. (around .5 inch) I'm actually going to attach magnets to the top of the shaft, however, so that when a current is run through the wire, it repels the magnet and the shaft is propelled out of the solenoid casing. (I'm not sure if that would work, whats your opinion on it?)
  9. berkeman

    Staff: Mentor

    No, a solenoid works by drawing a ferrous "armature" into the coil, not by repelling a magnet. The bar gets pulled into the area of the coil that has the highest magnetic field density. So you start with the bar/armature only partly in the coil, and when you apply the current, that pulls the bar all the way into the coil. That was explained in the document that I linked for you.
  10. if the ferrous inner material was magnetic it would push it either way depending on direction of turns, no? I've also seen an MIT instructor online "throw" the inner piece (which would normally be sucked in the middle of the inductor coil) out the other side like a bullet. I'm still a bit lost on practical understanding of solenoids myself.... But to push outward is very every day practical and dc voltage can be reversed to have it extend or contract. Must be the placement or magnetism of the inner piece.
  11. berkeman

    Staff: Mentor

    It might help for a little while, but what happens to the magnetic armature as the coil is energized and de-energized (especially if you reverse the direction of the current)...?
  12. Well EMF of course at the instant the voltage stops before reversing. How does that manifest itself in the non-electromagnetic armature case?
  13. berkeman

    Staff: Mentor

    The coil turning on and off (or reversing) has an effect on the permanent magnet. Can you guess what that effect might be? :smile:
  14. ha I'm still staring at this from wikipedia on solenoids and trying to figure it out:

    "The force applied to the armature is proportional to the change in inductance of the coil with respect to the change in position of the armature, and the current flowing through the coil (see Faraday's law of induction). The force applied to the armature will always move the armature in a direction that increases the coil's inductance."

    um...well if it was a permanent magnet armature (which has bound current).... hmmm. I'm not sure.

    depending on the placement of the armature it would shoot out or inward I would guess per the current.
  15. berkeman

    Staff: Mentor

    Hint -- "degauss"
  16. New to me (besides hearing about it per crts). I guess after reading this:

    "The term was first used by (then) Cmdr Charles F. Goodeve, RCNVR, during World War II while trying to counter the German magnetic mines that were playing havoc with the British fleet. The mines detected the increase in magnetic field when the steel in a ship concentrated the Earth's magnetic field over it. "

    ... i'm not sure why the mines wouldn't detect eddy currents. Perhaps to do that would be too "obvious". I'm just tossing around buzz words here without understanding things.

    I get it's a magnetic "bias" (I'm thinking dc bias) but not sure how it affects the solenoid situation?

    And here I had no intention of learning anything today and you ruined it!
  17. berkeman

    Staff: Mentor

    :rofl: Sorry about that! :rofl:

    Here you go: http://www.adamsmagnetic.com/blogs/...t-magnets-to-lose-strength-or-to-demagnetize/

  18. So basically perm mags lose their strength (per the link). So are you simply saying that rather than using perm mags that's why ferrous materials which become temp magnetized by the solenoid "stator??" are used instead? I guess I"m not seeing the degaussing piece in that link. Perhaps I don't have enough foundation yet on induction (that is for sure).
  19. berkeman

    Staff: Mentor

    My reference to degaussing was just about how varying magnetic fields can demagnetize permanent magnets. And yes, I think the variability due to demagnetization is the reason that permanent magnets are not usually used as solenoid stators.
  20. i got the idea from this video: http://www.youtube.com/watch?v=sR3xmof8rZY#t=360 (already timestamped)

    So youre saying that attaching magnets to it would eventually demagnetize the magnets; how fast would that happen usually?
  21. davenn

    davenn 4,356
    Science Advisor
    Gold Member
    2014 Award


    there is no general answer for that....
    depends on at least these things.... the strength of the permanent magnet, the strength of the magnetic field of the solenoid coil, the distance between the 2, and how often the cycling is repeated

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