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

Discussion in 'Electronic Basics' started by Molokoko, Aug 14, 2003.

1. ### MolokokoGuest

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

I am absolute beginner.
Where is the catch.

2. ### John LarkinGuest

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

3. ### Kevin AylwardGuest

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

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

4. ### Terry PinnellGuest

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

5. ### Kevin AylwardGuest

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

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

6. ### John LarkinGuest

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