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Another Spice Trap?

Discussion in 'CAD' started by Paul Burridge, Nov 20, 2004.

  1. HI all,

    Whilst carrying out some tests on oscillators, it transpired that one
    cannot readily establish the phase relationships of the various
    components by means of simulation. To give a simplified example, if
    you hook up a sine source, a capacitor and a resistor in series and
    probe the capacitor current/voltage relationship, you will either find
    current lagging voltage (wrong) or voltage lagging current (right)
    depending on which way around you've placed the capacitor. This
    wouldn't happen in the real world! Worse still, there's no means of
    telling (if you didn't already know) which placement is the correct
    one or even which way around the capacitor is! Whut's goin' on here?

    Thanks,

    paul
     
  2. Paul,
    Oh, yes it does. The ammeter is inside the capacitor, if
    you turn it around, the sense of positive current flow
    reverses, just as when reverse the leads of an ammeter
    on the bench.
    You can make your own capacitor symbol with, e.g., a
    phasing dot. Drawing phasing dots on such components
    isn't widely considered a correct drafting practise,
    so this, in conjunction with the fact that it isn't
    that confusing which way is the sense of positive
    current flow, makes suppling capacitors with phasing
    dots not very common.

    --Mike
     
  3. Leon Heller

    Leon Heller Guest

    The Pulsonix simulator capacitors are actually polarised, with n and p pin
    logic names, so one can check if they are the correct way round. If n is
    connected to 0 the phase relationship is correct, with Paul's circuit.

    Leon
     
  4. Leon,
    The usual SPICE convention is for the direction
    of positive current flow to be from the 1st node
    to the 2nd node of a capacitor. In LTspice, for
    the cap symbol, that's from node "A" to node "B"
    which is downward if the symbol is placed without
    rotating it.

    --Mike
     
  5. Jim Thompson

    Jim Thompson Guest

    [snip]
    ONLY if you loosely specify I(C). If you denote (or marker) in
    PSpice, I(C:1), you get the current INTO device C, PIN 1.

    ...Jim Thompson
     
  6. C'mon, Mike; fix the problem and make every LT user happy. It ain't no
    big deal!
     
  7. Paul,
    I(C1) has a sign convention that LTspice follows.
    You can even refer to that current in behavioral
    expressions -- something missing in most SPICE's,
    e.g., PSpice.

    If you get confused with the direction of current
    in a cap, just add a phasing dot to the symbol,
    something you can do yourself with the graphical
    symbol editor.

    But the program should not ship with phasing dots
    on such symbols.

    --Mike
     
  8. Hello Paul,
    the logic behind the behaviour of LTSPICE regarding pin number
    dependent current is fully correct. That some people can't understand
    the great picture why it is that way, that is not a problem of LTSPICE.
    I hope that Mike never will change the bevaviour of LTSPICE
    as you have suggested.

    Best Regards,
    Helmut
     
  9. I respect your views, Helmut, but the latest version of PSpice does
    actually incorporate the feature I've suggested. It's only a simple
    GUI problem that could easily be fixed-up.
     
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