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Multimeter reads different current with LED in series?

  1. May 3, 2013 #1
    I need help. I was under the impression that LED's in series should have the same current running through them and one's in parallel should have different currents because of the voltage drop variation in the individual LED's. When I put them in parallel, my multimeter shows the voltage across each LED changing as I add or remove them to the circuit but the current for each one stays the same. I attached a diagram of my circuit.
     

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  2. jcsd
  3. May 3, 2013 #2
    Calculate the voltage drop across the limiting resistor as the total current changes when you add / remove the LEDs.

    You can then measure the V drop on the resistor to confirm your calculations.
     
  4. May 3, 2013 #3

    AlephZero

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    You are doing something wrong here.Your measurement doesn't make sense.

    if you have 19 mA through each LED, you have 3 x 19 = 57mA through the resistor, so the voltage drop across the resostor = 330 x 0.057 = 18.8 V. But you only have a 6V battery!

    Are you setting the meter to the current range and the putting it in parallel with each LED? If you do that, you are short circuiting the LEDs and you just have the battery in series with the resistor. The current would then be about 6 / 330 = 0.018 A which is close to the 19mA you measured.

    If that is what you did, you should see that all the LEDs go out when you make the current measrement.

    If you want to measure the current through an LED, you have to put the meter in series with that LED, not in parallel.
     
  5. May 3, 2013 #4
    Thanks AlephZero, yeah I was putting the meter in parallel. It makes sense now.
     
  6. May 3, 2013 #5

    AlephZero

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    Well, at least you made the mistake on a low voltage circuit with a fairly wimpy battery power supply.

    On a really bad day, doing that produces a flash and a bang, and vaporizes parts of your meter. (Yeah I know the marketing for digital meters says they have internal protection - but don't believe everything you read in advertisements!)
     
  7. May 3, 2013 #6
    Thanks for the clarification. I'm kind of new to this in case you couldn't tell.
     
  8. May 4, 2013 #7

    Averagesupernova

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    There is usually good protection against simple overload. However, hooking a current meter in parallel with a voltage source is considered misuse/abuse. There is a difference.
     
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