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Op amp help (multisim)

Discussion in 'General Electronics Discussion' started by Chris13, Sep 22, 2014.

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  1. Chris13

    Chris13

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    Sep 16, 2014
    Hi,

    I am trying to design an amplifier which accepts an input from a strain gauge and output 0-5V.

    The problem I am having in multisim, is that as soon as I adjust R4 (one arm of my bridge) to increase my mV output I get a "convergence" error.

    Can anyone take a look and see if they can spot the problem?

    https://www.dropbox.com/s/7i2uborv0dxatex/amp.jpg?dl=0

    Thanks,
     
  2. LvW

    LvW

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    Apr 12, 2014
    One of the reasons I dislike Multisim is that the various "instruments" partly hide the real circuit.
    Please, can you redraw the whole assembly and make it somewhat clearer?
     
  3. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
  4. Chris13

    Chris13

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    Sep 16, 2014
    Hi Adam,

    Yes this is the same circuit - what have I done differently? I thought I had done what you suggested. After I did it it corrected the problem I had (which was that I could not get zero volt output.)

    thanks
     
  5. Chris13

    Chris13

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    Sep 16, 2014
  6. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    LOL :)
     
  7. davenn

    davenn Moderator

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    Sep 5, 2009
    there's a number of things wrong with that layout
    The layout being the biggest ;) ..... you need to spend some time practicing and learning how to layout a schematic so that it is easy to read

    next major problem is ... it looks like you have the -V rail of the supply shorted out to the common rail

    follow the -V and the 0V rails around and they meet up

    amp1a.JPG
     
    KrisBlueNZ likes this.
  8. davenn

    davenn Moderator

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    Sep 5, 2009
    I cannot even tell what the pin numbers on the chip are so I cant tell if its wired correctly or not

    first lesson in circuit layout ....

    put your power supply to the left side ... so your batteries should be shown vertically

    Positive Rail across the top
    Negative Rail across the bottom
    Use the earth symbol for the 0V ( common) Rail
    DONT draw the 0V Rail across the page, it gets too messy, instead every point that connects to the 0V rail
    use the earth symbol

    you have so many lines just zigzagging all over the place that could be shortened up

    An easy circuit to read makes for an easy circuit to diagnose when it doesn't work properly ;)

    Dave
     
    Last edited: Sep 23, 2014
  9. LvW

    LvW

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    Apr 12, 2014
    An easy circuit to read makes for an easy circuit to diagnose when it doesn't work properly

    Yes, yes, yes...
    I must confess that I am not motivated enough (even to lazy) to start analyzing such a confusing wiring.
     
  10. Chris13

    Chris13

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    Sep 16, 2014
    Clearly not lazy enough to comment though. Perhaps if you saved up the energy making those sorts of comments you could multiply it to a useful comment for one person. Thanks for your input.
     
  11. Chris13

    Chris13

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    Sep 16, 2014

    Thanks Dave - Will try your suggestion and redraw it and see how I get on.
     
  12. davenn

    davenn Moderator

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    Sep 5, 2009
    no probs Chris,

    these things take time :) I don't know how long you have been playing with electronics
    I'm guessing not too long ;) its all just practice and some guidance

    cheers
    Dave
     
  13. LvW

    LvW

    604
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    Apr 12, 2014
    Chris - I am somewhat surprised about your response. Why so harsh?
    In post#2 I have kindly asked you to redraw the diagram (without any response).
    I suppose, all of us have to do also something else than to participate in this forum. And therefore, I think, also the questioners should be aware of this and should not make things more complicated than necessary.
    I only was honest in my answer and I told you that I am not "motivated" to redraw his circuit in order to be able to analyse it.
    Don`t you think my answer couldn`t be also a kind of help ?
    Namely, a small indication that one should endeavor to make such diagrams a bit clearer, to the benefit of all of us.
    According to my experience, students and other young people starting/dealing with electronic circuits not only need simple answers to technical questions.
    We also should give them some guidelines (a) how to ask clear questions and (b) how to support those questions with clear and meaningful drawings.
    Don`t you agree?
    Regards
    LvW
     
  14. Chris13

    Chris13

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    Sep 16, 2014
    No I don't think so, because you didn't say you weren't motivated enough to redraw it. You said you weren't motivated enough to analyse it. Which in my opinion meant you weren't motivated enough to try and help, which is fine, but I don't think that frame of mind belongs in a forum where people are asking for help.

    Perhaps what you meant and what I read were different but that was what I took from it.
     
  15. davenn

    davenn Moderator

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    Sep 5, 2009
    OK guys .....
    lets get back to electronics :)

    Chris
    just to repeat and add a few other comments from my above post

    first lesson in circuit layout ....
    put your power supply to the left side ... so your batteries should be shown vertically

    Tho not essential to have power supply coming in on the left, it is conventional
    it is more important for the 2 or 3 (etc) rails to be well separated
    as commented below

    Positive Rail across the top
    Negative Rail across the bottom
    Use the earth symbol for the 0V ( common) Rail
    DONT draw the 0V Rail across the page, it gets too messy, instead every point that connects to the 0V rail
    use the earth symbol

    the basic idea is the schematic should flow across the page so inputs would be on the left, through
    any signal processing stages and to the output on the right side of the diagram

    lay your parts out in a way that allows for minimal line lengths between various pins etc
    this will avoid the need to do long line runs between components unless absolutely needed


    If I think of anything else I will add to the list ;)

    cheers
    Dave
     
  16. davenn

    davenn Moderator

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    Sep 5, 2009
    please call it quits guys and get back to the electronics subject of the thread

    thanks lots

    Dave
     
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