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Electromagnet directly connected to power supply not working

  1. Jan 3, 2014 #1
    Recently, I made an electromagnet by winding copper wire (length: 4m) around a metal screw. The power supply was from a mobile phone charger (5V).

    Here's the issue:

    When I connected the electromagnet in series with a 10 ohms resistor, it worked fine (attracted nearby metal objects), but when I connected it directly across the 5V supply, it ceased to function! Just to test the connecttions, I connected an LED in series and it also glowed; however, after removing the LED and then again directly connecting the winding with the power supply, the electromagnet didn't work (tried this a few times).

    - What could the reason be?
    - The energy of an inductor 0.5*LI^2, so increasing the current should have actually made the magnet stronger!?
  2. jcsd
  3. Jan 3, 2014 #2

    Simon Bridge

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    It's broken ... if you return to the original setup (series with 10Ohm resister) does it start attracting stuff again?
  4. Jan 3, 2014 #3


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    I'm guessing that your power supply has short circuit protection, and your coil of wire had a low enough resistance to trigger it.
  5. Jan 4, 2014 #4

    Simon Bridge

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    either that or the coil was damaged by the large currents - or both.
  6. Jan 4, 2014 #5
    It worked again after I re-connected it with the 100 ohms resistor. Did this several times.

    My power supply was a 5V Nokia mobile phone charger. Do mobile phone chargers usually have overload/short-circuit protection?
  7. Jan 4, 2014 #6


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    Yes, to stop them self-destructing, and/or catching fire, when people abuse them.
  8. Jan 6, 2014 #7
    If you measure the resistance of the EM Coil you should be able to calculate the current for the 5V supply and see if it exceeds the supply's ratings. If you do not have a meter - you can look up the wire's resistance per unit length - and calculate the resistance. -- in fact you could/should do both and compare the calculated with the measured results. Lastly - with a voltmeter - measure the voltage from the supply as you connect the EM Coil - does it go to Zero V?
  9. Jan 6, 2014 #8


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    Was it 10 or 100 ohms?

    Either way, you might try reducing the resistor to get more current, if you can. Just make sure the resistor's power rating is sufficient if you do this.
  10. Jan 8, 2014 #9
    Check the amp rating (I) on the charger (likely 1A or 2A). At 5V output, the resistance R it could handle is 5/I (remember V=I*R ? ). Your wire must have a resistance R the same or greater than this. Your R will be r per foot * (length in feet). Ignore the inductance for this. Make sure the wire you use is insulated, otherwise you will short it out to itself when winding it and it will fail. Magnet wire is best, as it has very thin insulation, but normal wire will also work.
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