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CD4511 Decoder Schematic Help!

Discussion in 'General Electronics Discussion' started by BocajDude, Feb 22, 2016.

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  1. BocajDude

    BocajDude

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    Feb 22, 2016
    Hi I'm working on a relay clock and I've run into a small problem. I'm trying to read the data sheet for the CD4511 decoders I bought (with my limited understanding of electronics), and am not fully understanding the use of the circled resistors (10k Ohm). I already bought a set of 10kΩ resistors, but knowing their purpose will help me down the road. Many thanks!
     

    Attached Files:

  2. Harald Kapp

    Harald Kapp Moderator Moderator

    10,579
    2,360
    Nov 17, 2011
    Welcome to EP.

    These are pull-down transistors, required to ensure logic low at the inputs of the gate when the switches are open.

    Regards,
    Harald
     
  3. dorke

    dorke

    2,342
    665
    Jun 20, 2015
    Like Harald said,
    they are pull-down resistors.
    But you don't need them in all cases, actually mostly you don't;)

    If you need more help with your clock,do share the schematics...
    I can guaranty(!) it will be worthwhile:)
     
  4. BocajDude

    BocajDude

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    0
    Feb 22, 2016
    Of course!! I should have known. Thanks for the fast reply! I'll gladly share my schematics soon. :)
     
  5. Harald Kapp

    Harald Kapp Moderator Moderator

    10,579
    2,360
    Nov 17, 2011
    Actually, in cases as shown in the schematic you do need them.
    The cases where you don't need them are whenever a driving gate is connected.

    A long time ago, when TTL (74xx) parts were common, you could leave unused inputs open and the gates recognized these as logic High. This changed with the advent of CMOS technology. CMOS inputs are high impedance. Left floating (unconnected), a voltage anywhere between 0 V and Vcc can develop on the input, bringing the input stage into the linear region with all the adverse side effects (high currrent consumption, latch-up etc.)
     
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