common cathode LED display limiting resistor?

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

  1. jlon

    jlon Thread Starter Member

    Joined:
    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
     
    #1
  2. Mike2545

    Mike2545 Active Member

    Joined:
    Mar 26, 2009
    116
    3
    You will need a separate resistor for each segment.
     
    #2
  3. hgmjr

    hgmjr Moderator

    Joined:
    Jan 28, 2005
    9,030
    210
    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
     
    #3
  4. jlon

    jlon Thread Starter Member

    Joined:
    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
     
    #4
  5. rjenkins

    rjenkins AAC Fanatic!

    Joined:
    Nov 6, 2005
    1,015
    67
    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.
     
    #5
  6. Wendy

    Wendy Moderator

    Joined:
    Mar 24, 2008
    19,940
    1,986
    If you research it they make 7 resistor arrays, in a 14 pin DIP package. Resistors are cheaper though.
     
    #6
  7. jlon

    jlon Thread Starter Member

    Joined:
    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.
     
    #7
Loading...