Another Spice Trap?

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

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

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

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

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

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

"Paul Burridge" schrieb im Newsbeitrag news: snipped-for-privacy@4ax.com...

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

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.