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basic electronics question - help

Discussion in 'General Electronics Discussion' started by HIB, Nov 8, 2012.

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  1. HIB

    HIB

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    Nov 8, 2012
    Hi everyone,

    I'm new to this forum. Could really use a little help on a simple question.

    OK...
    I have four LEDs in series. I connected them to a 12V power supply and they draw 500 mA.
    This is too much current for them, so I am afraid they will break.

    To reduce the current, I put a 10 ohm load resistor in series with the circuit. Now, with 12.5 V, I get 150 mA. This will work perfectly for me, but I don't understand the maths of this.

    Here's how I would work it out - based on easy high school physics:

    4 leds in series; 12V; 500 mA -> LEDs have a combined resistance of 24 ohms
    Adding the load resistor: R = 34 ohm (10 + 24); 12.5 V; I should have 12.5/34 ~ 300 mA.
    I only have 150 mA.

    Why?

    Any help is much appreciated. I can't figure this out.

    Thanks
     
  2. BobK

    BobK

    7,682
    1,686
    Jan 5, 2010
  3. HIB

    HIB

    3
    0
    Nov 8, 2012
    That explains it brilliantly.

    Thanks!
     
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