My Project -Simulation not working

Discussion in 'The Projects Forum' started by HUMBLE, Feb 17, 2011.

  1. HUMBLE

    Thread Starter New Member

    Oct 24, 2010
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    Dear Professional Colleagues

    I am trying to build an Inverter for my final year project.My Initial concept was taken from a design from the Internet which I have attached but I have changed the multivibrator and introduced an inverting op- amps.I have also included the simulation results for six different points in the circuit

    My problem is that the voltage that I am getting during simulation for the inverting op-amps is very small.I have attached the ORCAD simulation results so that you can give your inputs.

    Thanks

    NB:I am building it in stages and then integrating.I am now battling with the pre driver stage
     
    Last edited: Feb 22, 2011
  2. HUMBLE

    Thread Starter New Member

    Oct 24, 2010
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    I have simulated with orcad but I an getting spikes and flat line on the op amps with yellow voltage marker but I got a square wave from the inverting op amps.The other parts are OK .What could be wrong.

    I need your Kind assistance Please
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    You have the noninverting input of U6 tied to ground instead of Vcc/2.

    If you ever get to the point where you connect a transformer up to this circuit, the driver transistors on one side will probably overheat and burn up due to the input signals not being a 50% duty cycle square wave.

    If you want any hope of avoiding that, either use the original oscillator function, or use a D-type flip-flop, Q\ tied to D input, 555 to clock, and don't use two opamps in series on one side and one on the other. 741 opamps are terribly slow, and really shouldn't be used for these kinds of things.

    There is no reason to use two 741 opamps instead of one 358 dual opamp.
     
  4. HUMBLE

    Thread Starter New Member

    Oct 24, 2010
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    Thank you vrry much for pointing out the errors.I have thought of modifying the circuit but my Supervisorss ays he wants me to work on this one first.So I did make the adjustment suggested and simulated it.-The results included.But the output waveform at the transistors are the same-I thought I would get one inverted_ and not up to 12 V.MY CONFUSION is that the wave form suddenly chnaged without me introducing and inverter again.Please Assist
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    What is the function of your 3rd opamp? It has the same output impedance and the same output swing as its input so it can be replaced with a piece of wire.

    Instead of using proper opamps you added a -12V supply. Where will it come from, a second 12V battery? Why?

    The original project uses a CD4047 oscillator/digital divider because its digital divider makes two perfect square-waves that has one already inverted. Its output swings to the positive supply. It uses an LM358 dual opamp because its inputs work fine at 0V so it does not need an added negative supply.
     
  6. HUMBLE

    Thread Starter New Member

    Oct 24, 2010
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    As for the third op amps,I felt that after inverting the output from the oscillator,I needed to boost it ,that is why I included it.

    For the -12V supply,I was asked to add it to power the op amps but I have been thinking of how I will build that in reality.

    Please,how can I alter,I mean modify the part with the third op- amps?
     
    Last edited: Feb 23, 2011
  7. SgtWookie

    Expert

    Jul 17, 2007
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    I guess you didn't understand what I meant by tying the noninverting input to Vcc/2. You added a 2nd 12v source, and still haven't corrected the noninverting input reference, so it's no wonder that you are not getting the output that you expect.

    You need to look at the inverting and noninverting input signal levels.

    I put together just the first part so you can see how I got an output from both 741's.

    Note that the outputs are no where near a 50% duty cycle, which will cause big problems.
     
  8. HUMBLE

    Thread Starter New Member

    Oct 24, 2010
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    Thanks,I see,now that from your model above,I do not need to add a third op amp.And your advise on my getting more information on inverting and non-inverting op-amps,I will certainly do that because,It will be a place where my supervisor will probe further.

    I will start off working on my design and will get in touch tomorrow again to state my progress.
     
  9. Audioguru

    New Member

    Dec 20, 2007
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    Why don't you understand that the 3rd opamp does not boost anything?
    Its voltage gain is only 1. The 3rd opamp is useless because its output is the same as its input. Use a piece of wire to replace the 3rd opamp.

    Did you know that the non-inverting opamp is also useless? Its output is almost the same as its input even when its voltage gain is 2.
     
  10. HUMBLE

    Thread Starter New Member

    Oct 24, 2010
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    Thanks,I am now working on the suggest- removing the third op amps.I will post my progress later
     
  11. HUMBLE

    Thread Starter New Member

    Oct 24, 2010
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    I am presently working on the project and this is what I have come up with.

    I have adapted your suggestions and still need to ask more questions.

    Why I am getting this kind of output waveform from the output of the second transistor in the pair?
     
  12. HUMBLE

    Thread Starter New Member

    Oct 24, 2010
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    I have worked on the suggestions given and this is what I came up with.

    I am still working on the output waveform of the second transistor and wondering why it produced the waveform shown in my attachment.
     
    Last edited: Feb 24, 2011
  13. SgtWookie

    Expert

    Jul 17, 2007
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    Perhaps it didn't occur to you, but the only path from the 2nd transistors' emitters to ground is through reverse-biased diodes.

    Diodes have a very small amount of capacitance, and also a reverse leakage. You're seeing the charge that was stored on the capacitance of the junctions to bleed off via the reverse leakage.
     
  14. Audioguru

    New Member

    Dec 20, 2007
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    You have R9 and R10 in parallel and have R11 and R12 in parallel.
    Instead, R10 should connect to the emitter of Q7 (not to its base) and R12 should connect to the emitter of Q9 (not to its base).
     
  15. HUMBLE

    Thread Starter New Member

    Oct 24, 2010
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    Thanks again for the constructive advice.I will work on the amendment tomorrow and post the detail of my progress.
     
  16. SgtWookie

    Expert

    Jul 17, 2007
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    Instead of creating an MS Word document, just attach the images instead, .PNG preferred.

    It takes too much time to download the document and start up MS Word. There is also the possibility of transferring viruses/malware via Word documents, which is undesired.
     
  17. HUMBLE

    Thread Starter New Member

    Oct 24, 2010
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    Thanks for the advice and I will do just that.
    But one quick question,Where will it be advisable to include my filter to make it a modified sine wave converter?

    Or will the output transformer do that for me automatically?
     
  18. Audioguru

    New Member

    Dec 20, 2007
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    You are making a very simple square-wave inverter. An inverter with a modified sine-wave doesn't have a filter, it has a different circuit that produces a waveform that is more like a modified square-wave. A filter would waste a lot of power. The modified sine-wave has a peak voltage that is close to the amplitude of a sine-wave.

    Why didn't you look in Google Images to see what a modified sine-wave looks like instead of wrongly guessing? I got this photo a few minutes ago:
     
  19. HUMBLE

    Thread Starter New Member

    Oct 24, 2010
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    Thank you very much.I am happy to have this wonderful community of People.

    I am only looking for what I can add as an extra to beat my expectation.

    Please the next question:can I add a crow-bar to the circuit then?

    If yes where will be the best place of adding it?
     
  20. Audioguru

    New Member

    Dec 20, 2007
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    A crow-bar circuit prevents a DC power supply from exceeding its normal DC output voltage and does not apply to your simple AC inverter.

    Maybe you should think of a way to provide voltage regulation, because the simple circuit will have a voltage too high when the load current is light and/or the battery is fully charged, then the voltage will be too low when the load current is heavy and/or the battery voltage is low.
     
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