common cathode LED display limiting resistor?

Discussion in 'General Electronics Chat' started by jlon, Aug 29, 2009.

  1. jlon

    Thread Starter Member

    Aug 13, 2009
    13
    0
    I'm want to design a circuit with some 7-segment LED displays driven from a 4511 or some variant. Now, I haven't burned up anything in quite some time. :D So before I burn anything up, I'm wondering:

    Is it okay to put a single current-limiting resistor on the common cathode, or is it a necessity to have a separate current-limiting resistor on each LED segment? Can someone experienced in this give me a hint?

    Joe
     
  2. Mike2545

    Active Member

    Mar 26, 2009
    116
    3
    You will need a separate resistor for each segment.
     
  3. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    213
    I too recommend you go with a resistor per segment. If you try to get away with one resistor, you will find that the brightness of the segments will change as a function of the number of segments you have illuminated at any one time.

    hgmjr
     
  4. jlon

    Thread Starter Member

    Aug 13, 2009
    13
    0
    That's going to be lot of resistors, but I don't want the display to change in intensity. So thanks to all!

    Joe
     
  5. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1,015
    68
    If you are multiplexing several digits, you only need one set of segment resistors as only one display is actually drawing current at any time.
     
  6. Wendy

    Moderator

    Mar 24, 2008
    20,732
    2,467
    If you research it they make 7 resistor arrays, in a 14 pin DIP package. Resistors are cheaper though.
     
  7. jlon

    Thread Starter Member

    Aug 13, 2009
    13
    0
    Yes, I've seen the resistor arrays. I'd rather just solder some 0805 packages. Not hard at all and much cheaper, lots of flexibility in positioning.
     
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