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Homework Help: The current given from a Car Battery?

  1. Aug 22, 2010 #1
    Hi guys, this is my first thread, and im not sure where i'm supposed to put this so I hope this is ok? if not, please tell me how and where i can move it to
    Ok, just a simple question that i can't seem to find anywhere, what is the current which is given out by a car battery?
    Thanks, Tinger
     
  2. jcsd
  3. Aug 22, 2010 #2

    HallsofIvy

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    Science Advisor

    You can't find it anywhere because there is no such thing as "the current which is given out by a car battery".

    A car battery has a specific voltage (typically 12 volts although some cars have 6 volt batteries). The current produced by that battery depends upon the resistance and load of a particular application (which will vary depending on whether the radio is on, the air conditioning is on, etc.)

    Current= voltage divided by resistance.
     
  4. Aug 22, 2010 #3
    ohh so it would be impossible to make a circuit using the power from a car battery to power an infra-red LED? because the current given would be ever changing?
     
  5. Aug 22, 2010 #4
    A car battery can deliver a few hundred amperes for a short time. This is known as it's cranking current as when you are starting your car. You would need a bank of several hundred IR LEDs in parallel and series in order not to blow them all up instantly.
     
  6. Aug 22, 2010 #5
    Would a high value resistor work to protect the IR LEDs?
     
  7. Aug 22, 2010 #6
    Yes it would. You would decide how much current you wanted to pass through the led. The led will operate at a near constant voltage. You subtract this voltage from 12 for the car battery. What's left is the voltage across the resistor. Using Ohms law you can then work out the value of the resistor.
     
  8. Aug 22, 2010 #7
    yeah i understand that but you said that the battery gave a spike in current, wouldn't that mean that the current received when the car had started would be too low then and the current reaching the IR LED would be almost nothing?
     
  9. Aug 22, 2010 #8

    Borek

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    Staff: Mentor

    Yes, but one LED in series with a correct resistor will do the trick.
     
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