Common Ground For 7 Segment Display?

Discussion in 'The Projects Forum' started by JDR04, Oct 9, 2011.

  1. JDR04

    Thread Starter Active Member

    May 5, 2011
    339
    4
    Hi Folks, could somebody help me with connecting my Common Cathode 7 segments displays up please?

    I have six of them which are driven by 4026B ic's.

    Can I link all the Cathodes together and then have one connection to ground? Or must each one be connected to ground individually.

    I have linked pins 3 and 8 of each display together and then linked them to the next pins 3+8 and so on and then finally the last one thought a 10K resistor and to ground.

    Your help will be greatly appreciated.
    JDR04
     
  2. #12

    Expert

    Nov 30, 2010
    16,355
    6,852
    You're making me crazy here. Need a drawing. Can't imagine why you'd put 10k in series with your ground line.
     
  3. JDR04

    Thread Starter Active Member

    May 5, 2011
    339
    4
    Thanks for your reply.

    As a newbie I rely on other peoples advice and was told to insert a 10K resistor to protect the 7 segment displays?

    I have done a drawing of how I connected the 7 segment displays together. Hopefully it makes things a bit clearer. Your time and advice is appreciated. Many thanks JDR04
     
  4. bertus

    Administrator

    Apr 5, 2008
    15,650
    2,348
    Hello,

    Do you have the specs for the dispalys?
    A datasheet would be perfect.

    Also a COMPLETE schematic is welcome.

    In your other thread ( Anyone Built This CD4033 Counter Circuit? ) you can see that the cathode is bound directly to ground.
    Each segment of the display has a transistor and a current limiting resistor.

    Bertus
     
    Last edited: Oct 13, 2011
    JDR04 likes this.
  5. SgtWookie

    Expert

    Jul 17, 2007
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    No, you cannot connect them like that.

    All of the common cathodes should be connected to ground.

    Each connection from each 4026 output to the 7-segment display inputs requires a resistor.

    You haven't told us what displays you are using, or what voltage you are operating the circuit with - so until we get that information, we can't tell you what resistors you will need to use.

    [eta]
    Bertus beat me to the reply, but I've attached an example of how to connect the 7-segment displays using resistors. You can get resistor networks in 14-pin DIP configuration, which makes them quick to connect up.

    I've shown the resistors as 330 Ohms, but that may not be correct for your 7-segment displays and voltage you're using.
     
    Last edited: Oct 13, 2011
  6. JDR04

    Thread Starter Active Member

    May 5, 2011
    339
    4
    Thanks Bertus and SgtWookie for your replies.

    Your questions made me ponder what I had done and have pointed me in the right direction. I've got displayus up and running and will expand them one by one.

    I'm still unsure of the common ground bit for the dispalys though:confused:

    The two I have up and running are Common Cathode 7 segment displays. First I connected pins 3 and 8 of each display up to ground individually and it worked. Curiosity got the better of me though and I removed each connection from pin 3 to ground and the displays still worked? Oh, I also removed the 10K resistor as well. So, with reference to the common ground situation I done something that will in the long term cause damage to the 7 segment displays or will it be OK?

    http://docs-europe.electrocomponents.com/webdocs/0200/0900766b802001c1.pdf

    Above is the datasheet for the 7 segment display.

    Thanks for your help Guys JDR04
     
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Good news. :)

    Look in the datasheet; you will see that pins 3 and 8 are connected internally. This is not always the case; sometimes they are individual grounds; for example one might ground segments f,a, b, and g, and the other segments c, d, e, and the dp. I had some displays like that.

    Good, otherwise they would be very, very dark!

    They'll be OK.

    Thanks for the link to your displays' datasheet; that always helps a great deal.
     
  8. JDR04

    Thread Starter Active Member

    May 5, 2011
    339
    4
    Thanks SgtWookie, thats' cleared up the last bit for me. Appreciate your time and help - JDR04;)
     
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