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<circuit maker> won't simulate series capacitors? (TINA too)

Discussion in 'Electronic Basics' started by Molokoko, Aug 14, 2003.

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  1. Molokoko

    Molokoko Guest

    All voltage is droped across last in series
    other capacitors are considered shorted.

    I am absolute beginner.
    Where is the catch.
     
  2. John Larkin

    John Larkin Guest

    Spice programs don't like series caps... most insist that every node
    have a DC path to ground. In real life, the voltage at the junction of
    series'ed caps is indeterminate, so that sort of makes sense.

    John
     
  3. Not "sort of makes sense", its "absolutely makes sense". Since, as you
    say, the potential at a capacitor junction can be anything, no equation
    can predict what that voltage is.

    This very common question is really an illustration, with all due
    respect, of the questioner not understanding electrical circuits, rather
    than not understanding spice.

    Kevin Aylward

    http://www.anasoft.co.uk
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
     
  4. CM has the parameter RSHUNT, as per my reply to OP.
     
  5. Any XSpice simulator, which is them all, apart from PSpice and LTSpice,
    has the .option RSHUNT to automatically place 1/GMIN to ground at all
    nodes. Its not usually enabled by default.

    Kevin Aylward

    http://www.anasoft.co.uk
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
     
  6. John Larkin

    John Larkin Guest

    If two caps are in series, the voltage at the junction can have any
    value, even if the end voltages are known. This is the
    initial-conditions problem. Any mathematical integral (the c voltage
    is the integral of i/c) has a 'constant of integration' which just
    means that we can't know the history of a system backwards infinitely
    in time.

    In real life, if caps are discharged and then placed in series, and a
    signal applied, the junction voltage *is* known, at least until
    leakage makes it drift away (which could be years for good caps.) But
    Spice is picky enough to not allow the constant-of-integration thing
    to stand. I think that maybe SPice does DC analysis with all caps
    open-circuited, which leaves such nodes hanging; maybe Kevin can
    elaborate.

    So just plonk a 1 Tohm resistance from all such nodes to ground, or
    shunt the caps with suitably scaled resistances; either makes Spice
    happy.

    John
     
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