question regarding lighting LEDs in segment display

Discussion in 'General Electronics Chat' started by bck1990, Apr 20, 2011.

  1. bck1990

    Thread Starter New Member

    Apr 11, 2011
    27
    0
    Hi all, i would like to ask a question again.

    Please refer to my attachment. Why is it that when we want to light up LEDs we have to use circuit 3. Why not circuit 1 and 2. Is there any difference? The voltage across the LED should be the same? [5V times (resistance of LED + resistance of resistor) / (resistance of LED)] shouldn't the voltage be the same in all cases?

    Would love to build the circuit like in circuit 1. Saves a lot of resistors. Thank you.
     
  2. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    You should not connect LEDs in parallel because their forward voltages are all different unless you test them all and sort them to be the same.
    The LED with the lowest voltage will hog most of the current and burn out. Then the next one with the lowest current will burn out. Then the remaining LED gets all the current which is too much for it so it burns out.
     
  3. bck1990

    Thread Starter New Member

    Apr 11, 2011
    27
    0
    Hi thanks. What if they are all the same? Just what if. Are there any differences among the 3 circuits?
     
  4. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    You can test hundreds of LEDs and sort them into groups with the LEDs in each group having the same voltages if you want. I would simply use one resistor in series with each LED instead.
     
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