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Strongest Electromagnet using 2 AA bateries

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by Trisorion, Aug 17, 2008.

  1. rahulan999

    rahulan999 New Member

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    audioguru, can u explain how can we wind the magnet wire on such a shape??????
     
  2. audioguru

    audioguru Well-Known Member Most Helpful Member

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    If a magnet has a straight bar core then there is only half the magnetic field at one end of the core.
    If the magnet is a U-shape then it has both ends of the core at its end for double the magnetic force and if the gap is small (like in a speaker) then the magnetic force is concentrated and is much stronger.

    The magnet structure from a speaker has a small gap for concentrated magnetic power power and it is round with lots of area to carry plenty of paper clips.
     
  3. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Wind the wire on a bobbin then mount the bobbin on the core.
     
  4. dave

    Dave New Member

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  5. rahulan999

    rahulan999 New Member

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    The speaker magnet is arranged in such a way that the flux is fully concentrated into the airgap using the steel plate on backside and center steel piece. It's such a mechanism that its strength is limited inside only! If u want to pick plenty of paper clips using that u have to allow the magnetic flux to spread outside by removing that steel plate and piece!! It's my suggestion.
     
  6. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The permanent magnet structure of a speaker (with its backplate) will pickup millions of paper clips. Just replace the permanent magnet inside with a coil.
     
  7. mneary

    mneary New Member

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    It has everything to do with it. The objective is to attract and hold paper clips. A complete magnetic circuit (U or double-U shaped) will probably hold more paper clips than a single ended magnet.
     
  8. Willbe

    Willbe New Member

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    Verily.
    And permanent magnets do better with a keeper.
     
  9. Trisorion

    Trisorion New Member

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    Bump to get back on track.

     
  10. jpanhalt

    jpanhalt Well-Known Member Most Helpful Member

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    I would look at a simple knife switch -- cheap, high current. Although, I don't think the decision on the switch should slow development and can be changed easily. You just need a switch that will handle 20A or so DC. It doesn't need to be rated that high, as durability is not a real issue for a contest.

    You are in the USA. I would recommend good lead/tin electronics solder. You can get it with 2% silver, but the difference will probably be insignificant.

    I would forget about heat sinks for now.

    John
     
  11. Willbe

    Willbe New Member

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    "My concern is that a switch might add too much resistance.
    Do you think 5 switches would add an appreciable ammount? Is there a certain style of switch that is better?"

    Closed relay contacts carrying rated current have a voltage drop of <30 mV if they're good and >100 mV if they're bad.
    With these numbers you can calc. the closed contact resistance of a switch that you'd like to use.
    Contact resistance depends on the force closing the contacts.
     
  12. Trisorion

    Trisorion New Member

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    Someone stated earlier that the general rule for finding the maximum safe current in a winding was to take what the wire is normally rated for and half it. From this AWG table the NEC rates based on keeping the wire below 90° C. ThisMagnet Wire Datasheet shows that the specialized magnet wire is rated for double the temperature of the NEC ratings (180° C). Can I use this wire without any sort of correction factor?
     
  13. Sceadwian

    Sceadwian Banned

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    It's a general rule for a reason, if you're using specialized wire then go with what the maker says, after all they're in a bad spot if they lie to you. Safe current is variable anyways as it's temperature based and you could use a LOT more current for a short period of time.
     
  14. Trisorion

    Trisorion New Member

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    Now I need to make some circuits to test batteries. I dont even own a multimeter, but im sure there are many threads I could read about which ones are good. I would need to test the battery's:

    Maximum Current (when shorted)- without ruining the battery. I understand that when I actually short them in the electromagnet for 20 seconds, I will most likely destroy the battery. I just cant afford to buy 4 of every type because I will blow up 2 just trying to test them. Additionally, the current may be ~20 amps, most multimeters I have seen max out at 10 amps.

    Internal Resistance- Without getting a 4 wire ohmmeter, how could I get a very exact result. Resolution of 1 milliohm or better. Additionally, I think this might only be tested accurately when the battery is under load.

    Any ideas on how to do this?
     
  15. Willbe

    Willbe New Member

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    xxxxxxxxxx
     
    Last edited: Sep 6, 2008
  16. Trisorion

    Trisorion New Member

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    If I had an unlimited power source and I wired some coils in parallel each one would get as much current as ohm's law would allow. But what happens when my power source is less than what ohms law would allow?

    Like this:
    Code (text):

    Power: 1.2 volts 40 amps
    Wire 1: .05 ohms               24 amp max
    Wire 2: .05 ohms               24 amp max
    Wire 3: .025 ohms              48 amp max
    Total:                         96 amps total
     
     
    Last edited: Sep 6, 2008
  17. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The power source has an internal resistance. So when current is drawn the output voltage of the power source drops due to Ohm's Law.

    A 1.2V Ni-Cad battery cell might drop to 1.0V or less when its current is 10A. At high currents its internal resistance rises quickly so its voltage to the load drops quickly.

    The connections to the battery and the wiring have resistance which also reduces the voltage to the load.

    A current meter has a sensing resistor plus its connecting wires that reduce the voltage to the load.
     
  18. Trisorion

    Trisorion New Member

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    I thought that resistance lessened when the battery heated. My calculations have the internal resistance accounted for (albeit I use the flat value stated in the data sheet for the battery). Anyways, the example 2 posts ago is just for the sake of my questions.
     
  19. jpanhalt

    jpanhalt Well-Known Member Most Helpful Member

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    Relative to the aborted thread, maybe a new thread is the equivalent of "reboot."

    I think the OP needs to get his feet wet and get started. He has had plenty of good advice. Then solve problems and improve. Planning is good, but perseveration at the starting point is no way to win a contest.

    John
     
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  20. Hero999

    Hero999 Banned

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    No, the internal resistances increase with temperature.

    I suppose it also increases at really low temperatures as the chemical reaction can't happen fast enough to supply the required current.
     
  21. Trisorion

    Trisorion New Member

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    Im angry that that thread was deleted. It took me an hour to make. None of the questions were repeats. They were magnetism/physics central questions. Not AWG/internal resistance/H bridges to reverse polarity etc. All it deserved was a move to the physics/math discussion section. I want to remake it in the right forum, but I want to ask a moderator first.

    I have bought the materials for the prototype, made spreadsheets from what people have told me. All I need now is the final shape of the electromagnet. Well anyways, here is the spreadsheet, but first some notes:

    The spreadsheet calculations are for parallel batteries only, ill make a series one later.
    The spreadsheet intelligently calculates the score, using the wire's maximum ability when that is the limiting factor or the battery's maximum output when that is the limiting factor.
    There is no correction factor on the wire max current because I found magnet wire that is rated for double the current, so it all equals out.
    Red w/strike-through= wire would burn up (amps through > maximum rated current)
    Green= Highest result for that battery. This sometimes does not appear when the highest number would result in a burn up and is therefore red.
    Italicized= score is limited by the resistance of the wire, however the circuit will not burn up.


    Edit: Removed the excess anger/complaining
     

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    Last edited: Sep 7, 2008

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