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Can someone help me understand parallel current draw?

Discussion in 'General Electronics Discussion' started by Crogdor, Jun 22, 2013.

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  1. Crogdor

    Crogdor

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    Jun 7, 2013
    Noob here. I'm trying to understand what's going on in these circuits, which I've drawn up to represent real-world circuits I've created:

    http://i.imgur.com/ZyyVPHO.png

    I have a 130R resistor wired to an LED. The ammeter shows 22.55mA. When I add a second LED in parallel, it goes to 23.73mA, and when I have four LEDs in parallel, it's 24.69mA. (My real-world circuits have LEDs with a forward voltage of 2.4, and a forward current of 20mA, so my real-world measurements differ slightly on the multimeter, but the principle is the same.)

    My expectation was that each LED would draw a specific amount of current, such that the total current drawn would be:

    Code:
    I = forward current of a single LED * number of LEDs wired in parallel
    That would have meant, however, that the first circuit used ~20mA, while the final circuit used ~80mA. That is apparently not the case, as my ammeter readings are starting around ~20mA and going up ever so slightly each time I add an LED in parallel.

    Why is this happening, and what concepts do I need to learn to understand what is going on?
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    10,543
    2,348
    Nov 17, 2011
    The LEDs currents don't add up because other than paralleling lamps, the LED current is limited by the series resistor. Read the article on driving LEDs in the tutorial section.
     
    Last edited by a moderator: Jun 22, 2013
  3. Crogdor

    Crogdor

    9
    0
    Jun 7, 2013
    Awesome, thanks for that link! Now it makes a lot more sense. I've wired it up with a series resistor in front of each LED, and connected each pair of components in parallel. Now I'm seeing the numbers I expected. Thanks!
     
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