# Another Spice Trap?

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

1. ### Paul BurridgeGuest

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. ### Mike EngelhardtGuest

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 HellerGuest

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. ### Mike EngelhardtGuest

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 ThompsonGuest

[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. ### Paul BurridgeGuest

C'mon, Mike; fix the problem and make every LT user happy. It ain't no
big deal!

7. ### Mike EngelhardtGuest

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. ### Helmut SennewaldGuest

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