common cathode LED display limiting resistor?

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

  1. jlon

    jlon Thread Starter Member

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    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

    Mike2545 Active Member

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    You will need a separate resistor for each segment.
  3. hgmjr

    hgmjr Moderator Staff Member

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    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

    jlon Thread Starter Member

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    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

    rjenkins AAC Fanatic!

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    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. Bill_Marsden

    Bill_Marsden Moderator Staff Member

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    If you research it they make 7 resistor arrays, in a 14 pin DIP package. Resistors are cheaper though.
  7. jlon

    jlon Thread Starter Member

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    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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