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Incorrect amps showing on multimeter?

  1. Sep 21, 2010 #1
    Okay so here's what I am dealing with. I created a circuit with 3 volts and used a 330 ohm resistor and a simple red LED, and using my multimeter I tested for the current. Now using ohms law:

    3/330 = 9 mA..... now what I see over the multimeter is 3 mA. I must have calculated something wrong or something is different with in the circuit. Maybe because a load is there, the calculations totally change?
     

    Attached Files:

  2. jcsd
  3. Sep 21, 2010 #2

    vk6kro

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    Science Advisor

    There will be a voltage drop across the LED.

    You can calculate it like this:
    Current in 330 ohm resistor = 3 mA or 0.003 amps
    Voltage dropped across 330 ohm resistor = iR = 0.003 * 330 or 0.99 volts. Call it 1 volt.

    So the voltage across the LED is 2 volts (3 - 1 is 2) . Diiferent coloured LEDs have different voltage drops.



    So, to make the LED light up a lot brighter, you need to send more current through it.

    Let us say 15 mA. So we need a resistor that will drop 1 volt when it has 15 mA flowing in it.

    This is R = 1 volt / 0.015 amps = 66.6 ohms. You can get 68 ohm resistors and that would be OK.
     
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