Circuit works in lab but not in Multisim.

Discussion in 'Homework Help' started by phalanx08, Nov 9, 2011.

  1. phalanx08

    Thread Starter New Member

    Nov 9, 2011
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  2. praondevou

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    Jul 9, 2011
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    What does it do in the simulator?

    Can you post the multisim file as well so the people working with it don't have to redraw the circuit?
     
  3. phalanx08

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    Nov 9, 2011
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    It doesn't do anything. Nothing is displayed on the LED.

    Sorry, I forgot to post the file.
    http://dl.dropbox.com/u/48944649/LEDBCDcounter.ms10
     
  4. praondevou

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    Jul 9, 2011
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    Several things are strange. First I connected an oscilloscope to the three outputs , the two last ones are at about 1V as if the 7473 couldn't drive whatever is connected to it's outputs.

    So I connected something acting as a buffer, the first I found in between. (just a 74f08 with the two inputs tied together)

    Output voltages are at 5V but still nothing on the display.

    Then I replaced the VCC for a DC-power supply and used for the grounds a digital ground symbol...

    Now the display alternates between "0" and "L". :D

    Still working on it.
     
  5. praondevou

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    Ok, the real problem is something else. IMO there is something wrong with the model of the 7473. It is a negative edge triggered FF.
    In Multisim it triggers on the positive edge...

    I replaced it with a 7476 which should be a positive triggered FF. It triggers on the negative edge. The simulation runs with the 7476. ;)

    Not sure what's happening here.
     
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  6. DerStrom8

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    Feb 20, 2011
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    Sometimes you need to adjust the operating current for the LED (usually decrease it). Not sure if that's the case in Multisim or not. I know you sometimes have to change it in ISIS.

    Der Strom
     
  7. phalanx08

    Thread Starter New Member

    Nov 9, 2011
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    I appreciate the help. I'll rebuild it using your modifications (I can't view ms11 files using v.10), and confirm.


    On your recommendation, I lowered the on current and the LED did flicker (showed only seven and the letter F). No matter how low I dropped the current (1e-20 pA haha), it still only flickered between 7 and F. If I could find the correct on current then I'd be in business.
     
  8. phalanx08

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    Nov 9, 2011
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    Well, this is weird. My 7476 are also negative triggered.

    [​IMG]

    EDIT: Even though the 7476N is also negatively triggered, it worked! Many thanks praondevou!
     
    Last edited: Nov 9, 2011
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