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Electromagnets not working!

  1. Apr 18, 2008 #1
    Hello everyone,

    I am writing with a strange problem. I have been attempting to make an electromagnetic coil that will have enough power to pick up say an average 9v battery in weight, the small kind found in the back of telly remotes and smoke alarms.

    I am using iron bolts 5cm long by 0.5cm diameter, and winding these with 0.4mm Enameled copper wire, it is rated 26BS and 25G if that helps any???. I made a small coil to test this and ran it from a 9v battery (not sure of the amps) but it did nothing!

    I started by leaving about 2cm of unwound straight wire at each end and wound from about 0.5cm down from the head of the bolt and wound it in a clockwise direction for about 2cm down the bolt. This did absolutely nothing still.

    Is my problem that I need more turns on my coil before I will see the effect. Also, will 9v be enough to lift a small 9v battery in weight, or will I need a larger voltage input.

    Looking at my bolt I would estimate that in a single layer I could get maybe another half of the original windings on before needing to start a new layer. Also the approx winds without actually counting would be around 35 winds at present.

    Also I have not insulated the bolt from the winds, should I wrap the bolt in paper first and then begin winding onto the paper (and in saying this if I use rubber tube to insulate the nail, would 2mm thick tube be too thick or would that be trial and error?

    Lastly, can anyone give me any idea of a good source of information for optimising electromagnets, so I can make the best possible magnet from the start.

    Thanks in advance for any help!
  2. jcsd
  3. Apr 18, 2008 #2
    A quick further comment, I just used the same bolt as mentioned in the post above and this time wound it 330 turns, in 3 layers of 110 turns each, and then connected it to a computer power supply of 12v @ 8.0 Amp, and it heated up instantly to an incredible heat and began melting the coke lids I used as makeshift washers to hold the winds in position. It did not seem to attract the nut I had sitting beneath the coil though, even though it was only on for a few seconds. The nut was approx. 0.5cm below the coil itself, I did not test it against the bare ends of the bolt core, though I wouldnt imagine that matters.

    So I am assuming now that I fed it too much amperage, does that sound right. I also have a 5v line at 20A but again I would assume this is too much power for a single coil this size. Also I am not sure how to find the saturation of the bolt, would this play an effect in this?

    Any advice or help would be greatly appreciated as I am still only learning and I would appreciate the help.
    Last edited: Apr 18, 2008
  4. Apr 18, 2008 #3


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    Two things come to mind:

    1. Wrapping the bolt with a layer or two of paper sounds like a good idea. If the insulation abrades you can easily short the wire to the bolt.

    2. Do you have a DVM to measure the resistance of your electromagnet? If so you can then get an idea of how much current it is using.

    If you can't measure the resistance, try putting a resistor in series with the magnet, in order to limit the current. You can get resistors at Radio Shack if you're in the USA. I'm not sure what the appropriate value is, but I'd start with a 100-ohm, 1/2-Watt resistor on the 5V source. At least that won't burn out anything, but I suspect you might need a smaller resistance (and higher wattage rating) than that.
  5. Apr 18, 2008 #4
    You don't need more voltage, you need more current. A Single 1.5 D battery can produce much more current than a 9 V battery that has the same size. If you use a power supply, use the one with the lowest voltage you can get.
  6. Apr 19, 2008 #5
    Thank you for the advice freinds,

    I have now determined from what you told me and also some further research that i have built my coil correctly (except for the soft plastic part) I have replaced the coke lid makeshift washers at each end of the coil with Neoprene washers with far more durability. I have also scrapped running it from the pc power supply as 8 amps and also 20 amps seem to be too large to run through a single coil with no resistor. Also I am unsure of whether the output from the power supply is ripple current or AC, the labels on the power supply were half torn and all I have is that it is a 200W psu with 8 Amps on the +12v rail and 20 amps on the +5v rail, and as such I am going to opt for a large 12v battery instead like the kind in the bottom of electric lamps, look like a brick with two springs in top for terminals. 5cm x 5cm x 15cm approx.

    However, I am a little stuck on my mathematical formula's and want to know from anyone who can tell me:

    Is there an equivalency chart anywhere for Teslas, or Henries? What I mean is this, are there any guides that say 1kg of push power from the magnet = x amount of Tesla/Henries? I know the approximate weight I want to push with my electromagnet but I need to use this figure with the input voltage to calculate the turns in my coil. I hope that makes sense.


    I have an input voltage of say 12v @ 2a (on a recharger circuit that keeps the 12v constantly at no lower than 75% charged) and it will run to a coil with 0.4mm SWG22 enameled copper wire of 200 turns. What is my push power in kg or grams?

    For that fact does anyone have any links that will explain what parts of magnetism can be measured and what formulas to use. For istande in straight wire we can measure Resistance using Ohms Law, when using gravity, what can we measure and what formula do we use to measure it? Any links would be greatly appreciated.

    Thanx again freinds..... :)
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