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Magnetic field and voltage question

  1. Jan 19, 2009 #1
    i have a question i cant come up with the answer for. i have built a large solenoidal coil with # 8 awg magnet wire. i have wound 310 turns around an 8 inch length bobbin and have an iron core inside. the resistance is only .250 ohms for the entire coil. when i connect it to a 12 volt car battery nothing happens except a crackling noise at the battery. i connected it with the north pole to - and south pole to + . i also tried driving it with a pulse width modulator unit that is capable of 100 amps @12 volts and nothing happened, no magnetic field and the coil doesnt even get warm. my questions are: is 12 volts not enough to push the current through the wire even though it has a very low resistance and if so, how much voltage would it take to push 50 amps through the coil?
  2. jcsd
  3. Jan 19, 2009 #2


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    Staff: Mentor

    Jiminy Christmas, son. Don't do that. Don't be hooking stuff to a CAR BATTERY when you do not have a basic understanding of what you are doing.

    Start with a current-limited 12Vdc power supply. Do NOT connect things to a car battery when you don't know what to expect for output current, output power, overcurrent faults in the battery, and so on. Any time you are making "crackling noises" come from a car battery (which is filled with acid, BTW), you are doing bad things and need to stop and re-group.

    What do you want to accomplish with your coil? Are you trying to make an electromagnet to pick things up, or a solenoid to generate large pull forces, or what?

    What is your level of EE education? Do you have a Mentor there locally (or prof or TA) who is (or should be) helping you with this? Especially the safety aspects of this project?
  4. Jan 19, 2009 #3
    Ditto with the above. Putting 1/4 ohm across a car battery likely will make it boil internally and the resulting pressure could make it explode, spraying acid everywhere.
  5. Jan 19, 2009 #4
    I got a wire across the terminals of car battery once. It vaporised.
  6. Jan 19, 2009 #5
    Thanks for the warnings guys, i have a basic understanding of electricity and its laws. I understand about the output currents of a car battery, its capable of pushing around 850 amps of current with its reserve capacity. According to all the electrical engineering and magnetic field calculators i have been using, i need 40 amps of current through my coil to create the large repulsion force i need. The calculators do not ask or tell anything about voltage, just current. Using ohms law: 12V/0.250 ohms=48A. I do understand if a coil without much resistance is connected to a battery, the battery could explode. I only connect the battery for 2 seconds at a time, so i feel like the risk is minimized, also i am shielded from the battery just in case. This is just a test coil i have built, i need to prove my theory works. When current flows through the wire, it should build a very large field inside the coil and core in a few milliseconds causing a very large push. Obviously voltage plays a bigger role than i thought, as just 12 volts from the battery did nothing. I understand that voltage is how hard the current is pushed. Can u point me in the right direction to calculate the voltage i need? if i can prove that my set up works with the test coil, im going to hire an electronics engineer to design a stable power supply, but i need to prove it works before i go in that far. Any help is appreciated. thanks
  7. Jan 20, 2009 #6
    A calculator won't give to you what you need. What you need is to study more on the subjet or ask to your supervisor, because indeed it is dangerous what you are doing, in the best case your battery will have a leakage of a powerful acid.

    This current cannot be supplied by normal methods, you will need specialized current sources because a simple battery won't work, neither something that you can find at home. Once you have it I wonder if you have taken into account how many watts are you sending into a wire, because I guess that you wire will be completely destroyed.
    Just a question how many oesterd do you need?
    Hope this help
  8. Jan 20, 2009 #7
    The wire is enameled, correct?
  9. Jan 21, 2009 #8
    Hi guys, thanks for the replies. Surely, i could pull 48 amps from a 12 volt car battery, isnt the strater motor of a car just a huge coil for creating torque. Also, a car starter pulls around 200 amps from the battery, whats the difference in my coil and a coil in the starter motor of a car? Shouldnt connecting a wire (coil) across the battery without much resistance allow a huge current to flow from the negative to positive? Is 12 volts not enough to push the current through the 400 feet of wire? and the wire is enamaled magnet wire that according to the wire gauge chart is rated @24 amps for power transmission but it also states that the number is very very conservative. how much of a voltage drop is there in 400 feet?any answers are appreciated.
  10. Feb 14, 2009 #9
    ...Put a load in series for safety at least. Light bulb, motor, resistor, something.

    ...Insufficient change in current due to no resistance = insufficient change in magnetic flux. See Faraday's Law. EMF = (N turns of coil) x (change in Flux / change in Time)
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