How powerful can you make a magnet off 3 volts?

Discussion in 'The Projects Forum' started by perfect_imbalance, Sep 18, 2010.

  1. perfect_imbalance

    Thread Starter New Member

    Sep 18, 2010
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    I have a design project coming up and need to make a series of 3 small powerful electromagnets that run off two aa batteries in parallel.

    By small I mean no more than 1 inch long by 1/2 inch wide.

    What sort of core would be best? Are core shape and size going to make a big diffrenece? will using a fine wire with lots of turns make the magnets stronger?

    i've had a look around the net and its been difficult finding info on magnets with low power inputs. Any help would be greatly appreciated.
     
  2. Pich

    Active Member

    Mar 11, 2008
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    The magnetic strength of a electric magnet is determined by amps*turns, in other words the magnetic field will increase by more turns or more amerage or both.
     
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  3. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Smaller wire means more turns per square inch but the AWG size would be limited by how many amps your 3V supply can put through it which will of course be governed by how many ohms per foot the wire has.

    It's one of those things I wouldn't even write an equation for, I'd just pull up a wire table and level it down to the best combination.

    I'll let someone else discuss cores, end gaps, permeability of materials etc.
     
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  4. gerty

    AAC Fanatic!

    Aug 30, 2007
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    That would be 1.5 volts, not 3.
     
  5. Kermit2

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    Feb 5, 2010
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    The pressed metal powder used to make toroid inductor cores and ferrite bead RF suppressors is the best thing to use for making a home built electromagnet...It is hard to come by in large pieces and even harder to find in bar shape.

    Soft steel coat hanger excellent. Coat hanger wire that has been heated red hot in a bed of charcoal and allowed to cool SLOWLY is even better for winding upon. The softer the steel(less carbon) the better it is for making an electro magnet. Hard steels are much harder to magnetize and will remain magnetized to a small degree after the current is switched off.

    For the wire, use 28 gauge copper. (1000 ft will have about 65 ohms resistance). If your average length in a single 'turn' is 4 inches you will get 3000 turns on the core, and the diameter will be just a fraction over 1 inch. It is the smallest wire you can use and still safely pass more than one amp through without overheating. (1.5 amps max I think). Of course it you wind it on to deep and thick the heat will get trapped inside and it could over heat at the maximum current of 1.5 amps. Limit current to about one amp, since that is a realistic amount of current to get from batteries during a short discharge period. Also keep the steel core within reason as far as how long you make it. If you are going to lift things with it, a horse shoe shape is a good one to use. If a bar magnet limit it to about 10 times longer than the short dimension.( a one inch diameter would be 10 inches long max.) The less length the magnetic field has to travel the stronger it will be at the ends. Magnetic fields that travel through a metal will encounter a resistance much like electricty in a wire does. The longer the magnetic circuit the greater the resistance the magnetic field encounters. EMF encounters resistance, MMF encounters reluctance.

    Get some shellac from a home depot type store or wallyworld and after every layer of copper wire you wrap on, coat it with a thin layer of shellac. Wait 15-30 minutes for the shellac to harden then wrap on another layer. limit yourself to about 15-20 layers. This will take 4 to 5 hours. Try to keep it neat and closely packed in each layer. When finished wrap it with a thin layer of First aid cotton bandage wrapping(cheese cloth in a strip) and give a final coat of shellac. let it dry it a warm place over night and it will be ready and strong enough to hold up under modest abuse.
     
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  6. sceadwian

    New Member

    Jun 1, 2009
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    How about the ferrite core from an AM antenna? I have at least a half dozen pieces 1-1.5 inches long floating around, easy to wrap wire around.

    You can get one out of any AM radio. They have several hundred turns of wax insulated copper wire wrapped around them, a rinse in hot water will strip one easily. You could just use the antenna itself as a magnet, but wrapping some more serious enameled wire thickly around it would provide superior results, primarily the current will be higher in a thicker wire, you're gonna drain the batteries FAST though.
     
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  7. perfect_imbalance

    Thread Starter New Member

    Sep 18, 2010
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    What about ferrite beads? they are about the right size. will the hollow make any diffrence?

    The idea of clothes hanger wire is apealing. cheap as chips. Will its thickness be an issue? I read that the diameter after winding should be no more than twice the diameter of the core.

    Extra thanks to Kermit2, very helpful post.
     
  8. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    This is all about making a DC electromagnet. I do not see why one would use a ferrite with relatively low permeability compared to soft iron or iron nickel.

    The suggestion of using a toroid I suspect was to obtain the core material, not to wrap the toroid in the usual way. The external field of a toroid is relatively low, as it is confined mostly to within the toroid. That's one reason they are used.

    Here are some sources and annotations:

    1) A good general introduction: http://info.ee.surrey.ac.uk/Workshop/advice/coils/terms.html

    2) Discussions of magnetic and core materials:
    http://www.mag-inc.com/
    http://en.wikipedia.org/wiki/Permeability_(electromagnetism)
    http://info.ee.surrey.ac.uk/Workshop/advice/coils/mu/ (see particularly for ferrite vs. iron core)

    3) If you like pretty girls with your science: http://www.wisegeek.com/what-is-magnetic-permeability.htm

    My suggestion is to use a soft iron core which is readily available or a nickel-iron alloy core with even higher permeability.

    The limiting factor will be your ability to get high current. Previous student projects have looked at using AA NiCd batteries because of their low internal resistance, if that is within the rules. Batteries in parallel should give more current, as you can increase the wire gauge you use to compensate for the lower voltage.

    You could also charge a capacitor (using batteries in series) and discharge it for a very high current. Unfortunately, the capacitor only provides a short pulse. Is duration an issue in who "wins" the project with the highest magnetic strength?

    John
     
  9. Kermit2

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    Feb 5, 2010
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    My sons science project. we used a powdered iron core and epoxy and packed it around and inside the coil of wire.

    [​IMG]

    [​IMG]

    the material for the core

    [​IMG]

    [​IMG]
     
  10. Kermit2

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    The main reason powdered iron is used in a generator is the powerful eddy currents that can be created in solid core iron prevents a good output and generates too much heat.

    For a DC magnet there would be no eddy currents since the magnetic field is not changing/rotating, therefor the use of solid iron is best for that design.
     
  11. Wendy

    Moderator

    Mar 24, 2008
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    A toroid does not generate an external magnetic field, it is all internal and invisible. Slice a gap in it and that changes, the field between the gap is extra strong. This principle is used by tape recorders.
     
  12. wayneh

    Expert

    Sep 9, 2010
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    The OP hasn't specified the shape or size constraints, or the application. These have an influence on the design choices. A couple key choices are the wire gauge to use and the core dimensions. Transformer steel is far more permeable (think lower resistance) than ferrite, but the latter can be made into varied and small shapes.

    I agree 28 ga. magnet wire is a reasonable starting point and you can often pull a good length of it from anything with an old CRT in it. But once you know the dimensions you're dealing with, sit down with the wire table. You need to find the optimal balance that gives you peak amp-turns. Lower gauge equals thicker copper equals lower ohms, but less turns in a given volume. The table has everything you need to know about the wire but the black box is the battery. I think the optimal coil will have a resistance about equal to or less than the internal resistance of the battery.
     
  13. Kermit2

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    Ferrite cores can be either high or low permeability and there is not an easy way to tell the difference without data sheets. The other High u alloys are WAY expensive and hard to find in less than truckload quantity - SO - needle pulling thread and all that

    iron/low carbon steel is the proper material for civilian, one off, use.
     
  14. wayneh

    Expert

    Sep 9, 2010
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    Well, that depends what you mean by "high". For my applications (alternator coil) they're all way too low. Your chart confirms the point I was making: Transformer steel will give you 8X higher field than soft iron IF (and it's a big IF) you can find the right size and shape piece of it. That's why it's be nice to hear from the OP just exactly what he's up to.
     
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