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Basic electromagnet question

  1. Jan 5, 2009 #1
    Hey all

    I have constructed a basic electromagnet with a nail and a coil of copper wire. I plugged it up to a 9V battery to make sure it worked, which it did, but the electromagnet drained the battery fairly quickly since the coil is shorting the battery out. I have a 12 V 3A dc power supply would putting a resistor in line with the electromagnet help this so my power supply will not be harmed will current is flowing through the electromagnet. If so how do I tell what ohm resistor to use with the electromagnet? Does the number of windings affect which resistor to use? If a resistor will not help, then how do I protect my power supply so it is not harmed or does not melt when connected to the electromagnet? Thanks. I appreciate the help.

  2. jcsd
  3. Jan 5, 2009 #2
    If you had a Digital Multimeter you could ohm out the coil and plug it into I=E/r ("x"Amps= "12"Volts/"x"Ohms )to find out how much current would be drawn under a given situation, just to make sure it doesnt exceed the 3A dc power supply.

    Otherwise you could just increase the resistance by adding more windings to your nail, and hope nothing melts.
  4. Jan 5, 2009 #3
    Diodes (rectifiers) would be better maybe. You drop at least 0.7 V per rectifier rising to around 1 V with a large current flowing through one. You can play around with how many you put in series. P600 series 6 Amp are around 30 UK pence each. You have to put them the right way round to get current flow.

    But really more turns would be better fo more Amp-turns. You produce the same magnetic field with ten amps through one turn as one amp through ten turns.
  5. Jan 8, 2009 #4


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    I'll suggest, as a next step, using a D cell battery and 10-ohm resistor (rated for at least 1/2 Watt). You'll need higher current and lower voltage than the 9V battery, so trying a D cell is going in the right direction. You can get the resistor at Radio Shack (if you live in the USA).

    The 10 ohms will limit the current to 0.15A -- I am guessing the coil resistance is significantly less than 10 ohms. See if the magnet still works with that current, and feel free to post back here with questions if it doesn't work.

    Regards and good luck!


    p.s. If it does work out, I'd be interested in hearing about that too :smile:
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