HD74LS48P Issue

Discussion in 'General Electronics Chat' started by R!f@@, Apr 15, 2015.

  1. R!f@@

    Thread Starter AAC Fanatic!

    Apr 2, 2009
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    Look at the image.
    I bought some CC 7 seg drivers for a student. And per a request I wired a CC 7 Seg circuit to show, I did the same with a 7447 with a CA and it works.
    The 48 did not work. Checked them all 20 pcs. Just one seg lights. All chips the same. Tried the LT pin and does not work. Tried all the combination of RB in out pins and still the same. I gave up. I swapped the 48 and CC to a 47 and CA and the circuit works. So I know the wiring is correct.
    I was wondering as why all the 20pcs are showing the same.
    Could be they are counterfeit ?
    Has any one came up with this issue or am I doing something wrong ?
    datasheets attached for your convenience.
    7448.jpg
     
  2. R!f@@

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    By the way the 7447 is drawing neglige able current when only the supply pins are connected but the 7448 draws 40mA with only Vcc and GND connected.
     
  3. GopherT

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    Nov 23, 2012
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    The '48 chip is a common Anode with internal pullups - see image (not a true common anode, eventhough some datasheets list as common anode) - no external resistors are required on the output if 1.5mA driver current is enough on a red LED display at 5 v chip voltage. Notice the 2k NOM on the '48 drawing.

    '47 is open collector output. LEDs must be connected as Common Anode.

    upload_2015-4-15_10-23-48.png
     
  4. R!f@@

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    I was looking at the Data sheet and noticed the same image and was going to post right now. but you beat me to it...:rolleyes:
    And when both of us agree then I agree I goofed up this time. Woe is me.:(
    I bought it thinking it was a CC driver.:oops:
     
  5. cmartinez

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    If it makes you feel better... at least you didn't get scammed! :D
     
  6. R!f@@

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    Even a 48 using a CA what is the use of the pull ups
     
  7. R!f@@

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    Okay let me ask.
    The circuit should work whether a 7447 or 7448, if they are the same thing with an exception of the pull up
     
  8. R!f@@

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    OK this is confusing :confused:
     
  9. DerStrom8

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    Feb 20, 2011
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    Ok guys, you're confusing me. Here's the schematic for the 7448 with a CC display:

    [​IMG]
    (Ignore the 150K resistors, they should be 150R)

    Have I been looking at 7447s and 7448s wrong all this time?
     
  10. dl324

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    Mar 30, 2015
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    Except that they aren't the same thing. Look at the function tables for the two parts and you'll see that the output polarities are reversed.

    If you need more drive than the 1.3mA, you need to add a resistor from the segment outputs to VCC; taking care not to exceed the maximum sink current.
     
  11. dl324

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    This is just adding 150 ohms in series with the nominal 2K pull up and is unnecessary (and wasting the already wimpy drive current).
     
  12. R!f@@

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    That was the first try and it did not work.

    I do not need more drive. I am buffering the outputs.
    And how can the output be reversed. Look at the output circuitry.
     
  13. dl324

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    Look at the function tables, the segment output polarities are reversed.
     
  14. R!f@@

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    Let's see...!
     
  15. R!f@@

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    Functions table are reversed.
    So I tried without a resistor on a CC. I was thinking that the transistor will sink and turn off the segment, an open output will light the segment.
    The internal pull up was used as the current limiter for the segment.
    ABCD, BI and LT at the state defined by the data...and still only segment "e" lights up. IC does not respond the ABCD pin changes or does not respond to "LT" either. Tried 10 IC's.
    So me thinks the IC's a faulty.
     
  16. MrChips

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    Oct 2, 2009
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    74LS48 outputs can supply 2mA. This is enough to turn on an LED.
    I tested a 7448 with common cathode seven-segment red LED display. Works fine.
    Are you sure you have the right connections on your display?
     
  17. R!f@@

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    I went over the connections more than I can remember.
    Chips, mind posting a picture of your testing or a diagram may be.
    This is getting to be rather frustrating. I never knew I WOULD be stuck with a 7448 and 7 segment. Never did before though.
     
  18. R!f@@

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    You know I tried a 7447 with NOT at output and drove a CC just fine.
    And I replaced the 47 with a 48 on the same circuit and got ZIP.! :confused:
     
  19. MrChips

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    Oct 2, 2009
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    Nothing fancy.

    7448
    pin-16 = +5V
    pin-8 = GND
    pin-3 = GND (LT)

    Take a single LED. Cathode to GND. No resistor required. (Use 220Ω in series if you're scared.)

    Connect anode of LED to 74LS48 to pins 9 to 15 one at a time. Should light each time.
    If not check the output voltage with DMM or scope.

    1.7V with 220Ω series resistor
    2.0V with no resistor (red LED)
    5V with no load
     
  20. R!f@@

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    Will try tomorrow.
    I have a headache.
     
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