Another Spice Trap?

P

Paul Burridge

Guest
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
--

"What is now proved was once only imagin'd." - William Blake, 1793.
 
Paul,

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!
Click to expand...
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.

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!
Click to expand...
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
 
Leon,

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!

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.

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!

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.

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.
Click to expand...
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
 
On Sat, 20 Nov 2004 17:47:35 GMT, "Mike Engelhardt" <nospam@spam.org>
wrote:

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

"What is now proved was once only imagin'd." - William Blake, 1793.
 
On Mon, 22 Nov 2004 19:43:46 +0100, "Helmut Sennewald"
<helmutsennewald@t-online.de> wrote:

"Paul Burridge" <pb@notthisbit.osiris1.co.uk> schrieb im Newsbeitrag
news:7ph2q0lcdh1aloji7b71cqk78t7gmlhp3b@4ax.com...
On Sat, 20 Nov 2004 17:47:35 GMT, "Mike Engelhardt" <nospam@spam.org
wrote:

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.

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

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.
Click to expand...
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.
--

"What is now proved was once only imagin'd." - William Blake, 1793.
 
"Mike Engelhardt" <nospam@spam.org> wrote in message
news:OTKnd.20150$zx1.5539@newssvr13.news.prodigy.com...
Paul,

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!

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.

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!

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.
Click to expand...
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 Heller, G1HSM
http://www.geocities.com/leon_heller
http://www.kasamba.com/viewExpert.asp?conMemID=105725&Catid=1111&banID=2100
 
On Sat, 20 Nov 2004 17:47:35 GMT, "Mike Engelhardt" <nospam@spam.org>
wrote:

[snip]
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
Click to expand...
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
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.
 
Paul,

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.

C'mon, Mike; fix the problem and make every LT user
happy. It ain't no big deal!
Click to expand...
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" <pb@notthisbit.osiris1.co.uk> schrieb im Newsbeitrag
news:7ph2q0lcdh1aloji7b71cqk78t7gmlhp3b@4ax.com...
On Sat, 20 Nov 2004 17:47:35 GMT, "Mike Engelhardt" <nospam@spam.org
wrote:

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.

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