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Noob questions

Discussion in 'General Electronics Discussion' started by revv, Jun 16, 2012.

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  1. revv

    revv

    23
    1
    Jun 16, 2012
    Noob question

    ok so im trying to learn electronics and am a little confused about this

    I built a really simple circuit that consists of a 6V Battery,2k Potentiometer and LED...

    when I measure the amps with my multimeter and the potentiometer turn on to full resistance I get 2 milliamps

    and when I substitute the LED with a 1k resistor and measure it again the amps is still 2 milliamps...

    I dont get it why do I still get the same amps that I do with the LED am I doing something wrong or is that suppose to happen?

    so... with a LED I have a total of 2k resistance and with the resistor I have 3k total resistance
     
    Last edited: Jun 16, 2012
  2. CocaCola

    CocaCola

    3,635
    5
    Apr 7, 2012
    How accurate is your multimeter? What resolution is it measuring to?

    The reason I asked if it's only down to full mA resolution with no decimal then 1.5 - 2.4 will all be rounded to 2mA and you won't see that there is actually a 0.9mA spread in the readings
     
  3. revv

    revv

    23
    1
    Jun 16, 2012
    im not sure how accurate it is but it is measuring up to 20mA I believe and I get around 1.9

    it should be more current flowing with the LED then with the 1k resistor right?
     
  4. CocaCola

    CocaCola

    3,635
    5
    Apr 7, 2012
    You are or you are not getting a 10th place decimal?

    Is it reading 2mA 2.0mA or 2.00mA?

    Sure, but if your meter doesn't resolve to a resolution that will indicate a change because it is rounding up and down you won't see the small changes as I explained above... The same concept works as you move the decimal around... If your meter resolves to one decimal for example 1.9 then in reality the measurement could be 1.85 to 1.94... There is a change but you meter isn't sensitive enough to indicate it...
     
  5. Harry.k

    Harry.k

    9
    0
    Jun 12, 2012
    Precision and accuracy are totally different. Accuracy is closeness to the real value, while precision is the resolution, ie the no. Of desimals the reading has. Like cola said, if your device is less precise, it will round off the values. The accuracy does not matter if you just want to find the difference.
     
  6. revv

    revv

    23
    1
    Jun 16, 2012
    it reads 1.93
     
    Last edited: Jun 16, 2012
  7. gorgon

    gorgon

    603
    24
    Jun 6, 2011
    The reason is simply that the voltage over the 1k resistor equals the voltage drop over the LED. With a 3k resistance the voltage over the 1k resistor is about 2V(6/3), or 1.93V in your case. This is in the ballpark for a coloured LED, like a red one, for a low current like 2mA.

    In addition to this, the DVM you are using to measure the current may also add an extra resistance into the circuit, and reduce the current a bit.

    TOK ;)
     
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