Help! Circuit output not right

Discussion in 'General Electronics Chat' started by peaac, Sep 25, 2011.

  1. peaac

    Thread Starter New Member

    Sep 17, 2011
    11
    0
    So I have this chip... SN74LS47N which I found the datasheet for and it is a TI BCD to 7-Segment driver.

    I have two of these and I have checked the circuit about 10 times and this is what I am getting:

    I have VCC = 5v
    and connected the H to the same VCC for high and to ground for L.

    What I see is that the chip "almost" works but the H and L outputs are at 0.11v and 0.07v!!! And I cannot figure out why!
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Here is a link to the datasheet:
    http://www.ti.com/lit/ds/symlink/sn7447a.pdf

    Note that the very first thing on page 1 of the datasheet after the part numbers, description and revision date, is a bullet that says:

    "Open-Collector Outputs Drive Indicators Directly"

    Open collector means that the outputs can sink current, but cannot source current.
    If you wish to see if they are working as expected, use a ~270 Ohm resistor from +5v to the output; the resistor will source current so that you will then see the voltage on the output pin change.
     
  3. peaac

    Thread Starter New Member

    Sep 17, 2011
    11
    0
    Thanks! That was an aha moment!
     
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