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

[Molokoko]

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

I am absolute beginner.
Where is the catch.

 

[John Larkin]

On 14 Aug 2003 14:41:38 -0700, maeltre@yahoo.com (Molokoko) wrote:

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

I am absolute beginner.
Where is the catch.

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

 

[Kevin Aylward]

John Larkin wrote:

On 14 Aug 2003 14:41:38 -0700, maeltre@yahoo.com (Molokoko) wrote:

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

I am absolute beginner.
Where is the catch.

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.

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

 

[Terry Pinnell]

John Larkin &lt;jjlarkin@highlandSNIPtechTHISnologyPLEASE.com&gt; wrote:

Kev,

Doesn't Spice have hooks for a default resistor from every node to
ground? I know that ECA did. That would result in a successful DC
analysis of a series string (the OP didn't note any error messages!)
with all the voltage across the top cap.

I use Circuit Maker, and it does bitch about floating nodes.

John

CM has the parameter RSHUNT, as per my reply to OP.

--
Terry Pinnell
Hobbyist, West Sussex, UK

 

[Kevin Aylward]

John Larkin wrote:

On Fri, 15 Aug 2003 06:52:01 +0100, "Kevin Aylward"
kevin@anasoft.co.uk&gt; wrote:

John Larkin wrote:
On 14 Aug 2003 14:41:38 -0700, maeltre@yahoo.com (Molokoko) wrote:

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

I am absolute beginner.
Where is the catch.

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.


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.


Kev,

Doesn't Spice have hooks for a default resistor from every node to
ground? I know that ECA did. That would result in a successful DC
analysis of a series string (the OP didn't note any error messages!)
with all the voltage across the top cap.

I use Circuit Maker, and it does bitch about floating nodes.

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

 

[John Larkin]

On Sun, 24 Aug 2003 12:06:35 +0200, "Molokoko" &lt;maeltre@yahoo.com&gt;
wrote:

"John Larkin" &lt;jjlarkin@highSNIPlandTHIStechPLEASEnology.com&gt; wrote in
message news:5s8ojvcvhmdtmplqatf2ljsescg0487tn5@4ax.com...

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.

I see that junction is not connected to anything else in the circuit, so
voltage cannot be determined.
But I always measure (in real life) voltage inversely proportional to
capacitance of capacitor at such junctions.
How comes than that it is indeterminate? (Maybe because we don't know prior
state (charged) of capacitor?)
(I don't have any practical expirience in building circuits yet).

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