Astable multivibrator simulation-floating nodes problem spice

Discussion in 'Homework Help' started by Baydith, Jan 17, 2012.

  1. Baydith

    Thread Starter New Member

    Jan 17, 2012
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    Hey, during our lab we had to solder a simple astable multivibrator, but the supervisor gave us such resistor values that nothing was visible on the oscilloscope. Therefore, I want to make a simulation of multivibrator circuit in PSPICE, but strangely, I have problems with floating nodes. I have NEVER had such problems with PSPICE and I have used it several times to simulate OP-AMPS circuits, etc.I have tried adding a resistor of high value to the ground node, but still the same problem. I have carefully attached all wires, ensuring there is no problem with doubled wiring and/or unconnected nodes.

    I'm posting the screen as well as the .sch file...

    Any help ?
     
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    I don't use PSPICE. But I would think (at least) V1 should be a fixed DC source rather than a waveform generator. Perhaps you also need to set initial conditions for the capacitor voltages. What analysis mode are you using - transient?
     
  3. Baydith

    Thread Starter New Member

    Jan 17, 2012
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    Yep, transient. Well, u're right, since it's an astable multivibrator I don't need pulse source, since it's spontanously switches off and on the transistors...But I don't think it's such a problem, I think that may be the problem is with wiring, but I tried doing it all over again, ensuring the wiring is ok and it still sucks...As for the initial charge on capcitors, yep, it may be true, but I still should be able to run the simulation, just that the traces would be off...
     
  4. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Which version of a software are you using?
     
  5. Baydith

    Thread Starter New Member

    Jan 17, 2012
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    Orcad Capture 16.0
     
  6. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    I cannot help you because I'm unable to open the diagram.
     
  7. crutschow

    Expert

    Mar 14, 2008
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    The Error message says Node GND is floating. Thus, for some reason, the simulator thinks your ground symbol is not actually connected to your circuit or it is an incorrect symbol. That is likely causing the other floating error messages.

    Try deleting and reconnecting the ground.
     
  8. guitarguy12387

    Active Member

    Apr 10, 2008
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    For what it's worth, I've had a lot of trouble getting oscillator circuits to simulate properly using Mentor Graphics tools. I've tried all the suggestions on kick starting it, but I've found its usually not even worth my time.
     
  9. Baydith

    Thread Starter New Member

    Jan 17, 2012
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    I replaced the transistor model with a different one, of similar properties and now it's all working. Guess the library file of 2N3904 was messed, anyway, thanks for help! /close
     
  10. Adjuster

    Well-Known Member

    Dec 26, 2010
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    It is well known that oscillators can be difficult to start in simulations, but I've done it many times, as have many other people, some of whom use this site and have posted examples. I would say it really boils down to a few main subject headings:

    1. Like any other simulation model, the circuit must not contain some mistake that simply stops it working. This is by no means a given.
    2. The DC working point simulation that is the usual starting point for Spice (etc.) transient analysis does not always give a result favourable for oscillator circuits to start. In certain cases, difficulty in getting the simulation to oscillate for this reason may give a clue that real-world starting may not be reliable, particularly if power is applied very slowly. Sometimes this indicates a problem with biasing. A loop gain analysis of the circuit treated as an amplifier may show whether conditions for oscillation to start are present.
    3. Simulations may not supply enough of the randomness (noise) found in real life to start oscillations. An artificial signal source may be required to provide some disturbance
    4. This problem in simulation may sometimes be resolved by skipping the initial bias point analysis, or by setting one or more initial conditions (such as a capacitor voltage), or possibly by providing a pulse to start the oscillator after power-up.
    5. In the real world, the oscillator may need to be changed so that it naturally powers up with good loop gain, or alternatively a deliberate starting "kick" may be applied on every power-up. Personally, I am however not very enthusiastic about circuits which require this, partly because I suspect that some may be subject to non-recoverable stalling due e.g. to a glitch.
    That's not the whole story, but I felt the need to explain because I feel that the OP may be helped by having a more considered view.
    In my opinion, the statement that it is not worth simulating oscillators is a considerable exaggeration.

    I enclose a few examples of oscillating LTSpice circuits.
     
    Last edited: Jan 20, 2012
  11. guitarguy12387

    Active Member

    Apr 10, 2008
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    Haha yeah, true. I should have qualified that with, 'for my situation'

    I appreciate the insight. Very interesting and helpful post!
     
  12. crutschow

    Expert

    Mar 14, 2008
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    I agree with Adjuster's post. If you can't get an oscillator to oscillate in a simulator after using his suggestions, then the oscillator circuit likely has a problem or error. It's usually not a problem with the simulator.
     
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