555 Multisim Help Needed!

Discussion in 'The Projects Forum' started by ro169, Apr 5, 2015.

  1. ro169

    Thread Starter Member

    Oct 10, 2014
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    Hi guys.

    I seem to be having issues with simulating a 555 in astable mode on multi-sim 13. I need a period of approximately 1 second.

    I used f = 1.44/(R1+2R2)(C)
    Duty cycle is approx. 53%.

    This circuit is just a rough first attempt and resistor and cap values will be tweeked later to more common values.

    However when I simulated this circuit the scope displayed a straight line function which has left me quite puzzled. Since then I've checked my connections and calculations several times and as well as built the simulation circuit from scratch about 3 more times but it doesn't seem to change anything!

    any suggestions??

    //2 attachments are the circuit and the scope reading.
     
  2. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    Your R2 (RA in many descriptions) is a little small:

    upload_2015-4-5_10-1-12.png
    (Source: AN170)
    Maybe your simulator is sensitive to that.

    John
     
  3. ro169

    Thread Starter Member

    Oct 10, 2014
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    Hi John, Thanks for your response.
    I've just tried that.. still the same response from the scope.
     
  4. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    I haven't checked your calculations and have never used multisim. However, if your period is 1 second, and I were using a real oscilloscope, I would increase the time/division. Of course, with a real scope you would still see the output bob up and down.

    Sorry can't be more help. Your circuit looks fine for simulation.

    John
     
  5. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    What happens when you set the timescale of the scope much higher?
    You now only have 10 mS/div.

    Bertus
     
  6. crutschow

    Expert

    Mar 14, 2008
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    Sometimes oscillators will not start in a simulator without a small kick-start (generated by circuit noise in a real circuit). This can be done by adding the Use Initial Conditions command (UIC) or ramping the power up from zero.
     
  7. ro169

    Thread Starter Member

    Oct 10, 2014
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    I've just realized that the simulation isn't running at real time speed. I actually expected it to as I've used 555 timer projects before on the software and it did. so that problem seems to be solved!

    However, I seemed to have encountered another 1, after the 1st pulse the output voltage from the 555 seems to exponentially decrease. Is there anything specific that could be causing this?
     
  8. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    I checked several Multisim simulations for the 555. They had a load (say 1 KΩ) to ground. Have you tried adding a load to your output?

    John
     
  9. crutschow

    Expert

    Mar 14, 2008
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    Is the virtual oscilloscope set for AC (coupled) input? It should be DC.
     
  10. ro169

    Thread Starter Member

    Oct 10, 2014
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    Yes I have tried adding a load.

    You can check the oscilloscope settings in the screenshot i uploaded earlier.

    Basically from the oscilloscope reading (with a 9V input to the 555) the first peak in the square wave represents 9V, thereafter the successive peaks are at approx 900mV
     
  11. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    I had just posted this on another forum, so I put in your values and resimed it. Note the correct start-up behavior, and the timing.

    47b.gif
     
  12. ro169

    Thread Starter Member

    Oct 10, 2014
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    Hi,
    Thanks for all responses.

    I have attached the oscilloscope readings from the simulation showing different times as you can note under "Channel A".
    Also; if anyone has worked with multisim before, is there any reason/setting that makes the scope displays only about 1.3 seconds of data at a time. (if you look at the picture attached you'll see i had to use 2 screenshots to show times before and after approximately 1.3s as the scope only displays a certain period at a time.

    scope.png
     
  13. ro169

    Thread Starter Member

    Oct 10, 2014
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    Hi all.

    I'm still not sure what was causing the problem on the simulation, but I have altered the circuit and changed nothing on the simulator.
    The results seem to be fine now.

    Thanks For The Input.

    mult.png
     
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