Pspice question....

Discussion in 'Electronics Resources' started by newton3375, Oct 19, 2008.

  1. newton3375

    Thread Starter Member

    Apr 29, 2006
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    Hi everyone...

    I am trying to model and simulate an astable multivibrator using Pspice but i am having some difficulty....

    Both transistors are coming on at the same time so I never get the oscillations I expect. What do I need to do to make the model act more like a "real life" astable mulivibrator?

    (I have read it is possible for both transistors to come on at the same time in real life as well if the voltage is turned up slowly enough, but this is obviously not what I want if I'm trying to make a multivibrator.)

    Is there some PSPICE trick I am missing?
     
  2. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    The trick is you need to set an initial condition (.ic) on one node. You can pick a node like one of the bases, and set the initial voltage to something like -1V. I'm not sure how you do that in Pspice, but I know it can be done.
    The other way is to make the circuit slightly asymmetrical, such as making one of the caps 1% larger than the other.
     
  3. newton3375

    Thread Starter Member

    Apr 29, 2006
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    0
    Interesting idea....

    I tried changing one of the caps by 1% but it didnt work....I'll try a few more options...
     
  4. newton3375

    Thread Starter Member

    Apr 29, 2006
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    0
    I think I got it....I changed the initial charge on one of the capacitors to .1volts and the circuit appears to be oscillating.

    Now I just have to work on getting a shorter charge up time which means changing the cap or resistor values....

    But at least it works.

    :)
     
  5. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    Indeed....oscillators of any kind...either "linear" or multivibrators are very difficult to model in spice...because of differen startup and run conditions.

    It's usually better to model the AC, the DC and the TRANSIENT models separately.

    eric
     
  6. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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  7. Dave

    Retired Moderator

    Nov 17, 2003
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