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PSpice and series of capacitors

Discussion in 'CAD' started by effepe, Nov 20, 2007.

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  1. effepe

    effepe Guest

    PSpice get crazy to determine voltages when I have a series of capacitors:

    <http://img213.imageshack.us/img213/1521/ex231yg7.gif>

    After 'googled' a bit, I tried to fix the limitation of floating pin
    (seems that PSpice can't determine DC voltage in such situations) by
    adding two shunt resistors (R2 and R3) of huge value (1 TOhm).

    In option tab, I enabled GMIN (1E-12 S) too.

    Is there a possibility to use PSpice or I have to switch to LT
    SwitcherCAD III ?

    Still was wondering how a software like PSpice can fall down so poorly
    with a simple circuit like this.

    Thanks in advance and my apologies for my (surely poor) English ;-)
     
  2. You are going to see the same problems in LTSpice, and just about any
    other spice there is. Every node needs to have a DC path to ground for
    the differential equations to make sense.

    And then, you blew it by putting in a ridiculous resistance like a
    teraohm. Usually, 100MEG is more than sufficient, occasionally a full
    1G if you need really small shunt currents, but this is a simulator with
    a realisitic dynamic range. There aren't real 1T resistances out there,
    and they blow up the simulators when you have 1T resistances, and then
    really small resistances. Get real, get some experience, and then you
    can critize the simulators for real problems...

    Charlie
     
  3. Ading resistors in parallel to the capacitors is OK.
    GMIN doesn't help anything in this case.
    I don't know whether there is an option in PSPICE to do that automatically.
    LTspice adds a little bit of conductance to avoid this problem.
    SPICE requires a DC-path to every node. This is
    not a limitation. It just remembers you about a
    possible weakness of your design.

    Even in a real circuit, you can't predict the DC-voltage
    on the capacitors. It's detrmined by the insulation
    resistance of the capacitors which may vary many
    decades over production lots.
    And last but not least it's bad design practice to
    connect capacitors in series without an additional
    resistor in parallel.

    Best regards,
    Helmut
     
  4. effepe

    effepe Guest

    Charlie Edmondson ha scritto:
    Also using 100MEG, PSpice puts out exotics voltages.
     
  5. effepe

    effepe Guest

    Helmut Sennewald ha scritto:
    It's just an exercise assigned as homework.
     
  6. Jim Thompson

    Jim Thompson Guest

    Bad professor, bad professor ;-)

    Or maybe GOOD professor thinning the class down to a manageable size
    ;-)

    ...Jim Thompson
     
  7. qrk

    qrk Guest

    I say GOOD professor for beating in to the students heads about DC
    floating nodes and the proper way to deal with them in Spice and real
    life. One can learn from bad design examples.
     
  8. Hal Murray

    Hal Murray Guest

    Especially if one learns something the hard way.

    Suppose you are putting 2 caps in series to get a higher voltage
    rating. What size resistors do you use to ensure a DC ballance?
     
  9. Marra

    Marra Guest

    I would rather use my brain than use a simulator.

    At the end of the day a good engineer doesnt need a less than perfect
    simulator.
     
  10. Jim Thompson

    Jim Thompson Guest

    You would never be a competitor anyway, so who gives a shit. You
    certainly don't qualify for the moniker "good engineer".

    ...Jim Thompson
     
  11. Joel Koltner

    Joel Koltner Guest

    Well, your software looks like it comes out of the '70s or early '80s at best,
    and at that point in time you could arguably still be productive without a
    simulator.
    True, but many a good engineer does feel that he needs a job, and for many
    jobs today simulators are requisite. :)

    This same discussion comes up every now and again on SED, Marra -- Google can
    dig them up and you can read how many jobs have clearly demonstrable
    productivity gains via the use of simulation.

    ---Joel
     
  12. So, as the student, why is that? What is it about that arrangement of
    resistors and capacitors causes those voltages? Learn and understand...

    Charlie
     
  13. effepe

    effepe Guest

    Charlie Edmondson ha scritto:
    LTSPice simulate it correctly.
     
  14. Jim Thompson

    Jim Thompson Guest

    LTspice adds the R's that everyone has mentioned. Likewise series R's
    for inductors.

    "correctly"?... you are an IDIOT.

    ...Jim Thompson
     
  15. Jim,
    I'm afraid you are probably the village I-word here, Jim.
    You might first check out what LTspice actually does there,
    instead of spinning rhetoric.

    LTspice is probably more accurate then without such matrix
    modifications if you are talking real world circuits. In
    any case, it's certainly more accurate then PSpice's
    addition of gmin to current sources, or PSpice's Gear
    integration, both of which have contributed to design and
    analysis errors of silicon, in one case, even by your good
    self.

    Regards,

    --Mike
     
  16. Jim Thompson

    Jim Thompson Guest

    I guess I incorrectly assumed you added shunt resistance around
    floating caps, since the OP said, "LTSPice simulate it correctly".

    Floating caps are floating caps... aren't they???

    ...Jim Thompson
     
  17. Jim Thompson

    Jim Thompson Guest

    Now that I drove to the grocery store and back.

    I don't quite understand your little outburst.

    HOW can you "simulate it correctly" if it's truly a floating node?

    You have to be doing at least some kind of surreptitious node setting,
    or adding R to ground.

    I will continue to call idiots as I see them ;-)

    ...Jim Thompson
     
  18. effepe

    effepe Guest

    Jim Thompson ha scritto:
    you are in my kill file
     
  19. Jim Thompson

    Jim Thompson Guest

    You are ignorant beyond specification... and you will always be ;-)

    ...Jim Thompson
     
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