gEDA-user: getting unbounded output in a basic RC circuit

["H.S." <hs.samix@xxxxxxxxx>]

Hi,

I experimenting by simulating basic first order circuit with gschem and
ngspice. I am trying out this little problem whose netlist file is:
* gnetlist -v -g spice-sdb -o eg.net eg.sch
*********************************************************
* Spice file generated by gnetlist                      *
* spice-sdb version 10.9.2004 by SDB --                 *
* provides advanced spice netlisting capability.        *
* Documentation at http://www.brorson.com/gEDA/SPICE/   *
*********************************************************
V1 1 0 5
R4 0 4 10
R3 3 4 40
Iswt 5 2 0
R2 0 2 50
G1 3 4 4 0 0.375
R1 1 2 75
C1 5 0 .025 ic=2V
Vswt 3 5 0
.END
*--------------------------------------------------------

(gschem file is given in the PS below). I am using this example to
practice following objectives:
1. The simulated switch (using a current source and a voltage source).
In the given circuit, Iswt=0 means the 'switch' between the two nodes is
open and Vswt=0 means that switch is closed.
2. Using initial conditions on stogare elements.
3. Create and use my own VCCS.

If I do the simulation on this circuit and plot v(4), is comes out to be
a negative exponentially increasing voltage value. However, if I do this
analytically, I obtain a relationship of v(at node 4) = 0.4 e^(0.4t)
which is exponentially increasing. I am not sure where my mistake is.
Could anybody confirm either way.

Here are the commands I use in ngspice:
ngspice 330 -> source eg.net
ngspice 331 -> tran .01 1 uic
ngspice 332 -> plot v(4)

The gschem file is give in the PS.

thanks,
->HS

PS:
In the gschem schematic, I use my own VCCS symbol, hs-vccs-1.sym,whose
sym file is:
$> cat ~/gaf/my-gschem-symbols/hs-vccs-1.sym
v 20050313 1
P 0 200 200 200 1 0 0
{
T 100 250 5 8 0 1 0 0 1
pinnumber=1
T 100 250 5 8 0 0 0 0 1
pinseq=1
}
P 925 200 725 200 1 0 0
{
T 725 250 5 8 0 1 0 0 1
pinnumber=2
T 725 250 5 8 0 0 0 0 1
pinseq=2
}
T 600 1000 5 10 0 0 0 0 1
device=CURRENT_SOURCE
L 200 200 250 200 3 0 0 0 -1 -1
L 675 200 725 200 3 0 0 0 -1 -1
L 350 200 550 200 3 0 0 0 -1 -1
L 550 200 500 250 3 0 0 0 -1 -1
L 550 200 500 150 3 0 0 0 -1 -1
T 150 275 8 10 1 1 0 0 1
refdes=G?
L 250 200 460 340 3 0 0 0 -1 -1
L 460 340 675 200 3 0 0 0 -1 -1
L 250 200 460 60 3 0 0 0 -1 -1
L 460 60 675 200 3 0 0 0 -1 -1



And the schematic eg.sch is the gschem file I made in gEDA.
$> cat eg.sch
v 20050313 1
C 51200 58900 1 0 0 resistor-1.sym
{
T 51500 59200 5 10 1 1 0 0 1
refdes=R1
T 51500 58700 5 10 1 1 0 0 1
value=75
}
C 53800 59700 1 0 0 resistor-1.sym
{
T 53900 59900 5 10 1 1 0 0 1
refdes=R3
T 54400 59900 5 10 1 1 0 0 1
value=40
}
C 55500 57500 1 90 0 resistor-1.sym
{
T 55200 58200 5 10 1 1 180 0 1
refdes=R4
T 55000 57800 5 10 1 1 0 0 1
value=10
}
C 52400 57200 1 90 0 resistor-1.sym
{
T 52200 57900 5 10 1 1 180 0 1
refdes=R2
T 52000 57400 5 10 1 1 0 0 1
value=50
}
C 52900 58100 1 270 0 capacitor-3.sym
{
T 53400 57700 5 10 1 1 0 0 1
refdes=C1
T 53200 57300 5 10 1 1 0 0 1
value=.025 ic=2V
}
C 53700 58800 1 0 0 hs-vccs-1.sym
{
T 53850 59075 5 10 1 1 0 0 1
refdes=G1
T 53900 58700 5 10 1 1 0 0 1
value=4 0 0.375
}
C 50800 58100 1 270 0 voltage-3.sym
{
T 50700 57900 5 10 1 1 0 0 1
refdes=V1
T 50800 57300 5 10 1 1 0 0 1
value=5
}
N 51000 57200 51000 56800 4
{
T 51000 57200 5 10 0 1 0 0 1
netname=0
}
N 51000 56800 55400 56800 4
N 55400 56800 55400 57500 4
N 51000 58100 51000 59000 4
{
T 51000 58100 5 10 0 1 0 0 1
netname=1
}
N 51000 59000 51200 59000 4
N 52100 59000 52700 59000 4
{
T 52100 59000 5 10 0 1 0 0 1
netname=2
}
N 53500 59000 53700 59000 4
{
T 53300 59000 5 10 0 1 0 0 1
netname=3
}
N 53700 59000 53700 59800 4
N 53700 59800 53800 59800 4
N 54700 59800 55400 59800 4
{
T 54700 59800 5 10 0 1 0 0 1
netname=4
}
N 55400 59800 55400 58400 4
N 52300 58100 52300 59000 4
N 52300 57200 52300 56800 4
N 53100 57200 53100 56800 4
C 52200 56500 1 0 0 gnd-1.sym
N 54600 59000 55400 59000 4
C 53300 59000 1 270 0 voltage-3.sym
{
T 53600 58200 5 10 1 1 0 0 1
refdes=Vswt
T 53200 58300 5 10 1 1 0 0 1
value=0
}
C 52900 58100 1 90 0 current-1.sym
{
T 53100 59000 5 10 1 1 180 0 1
refdes=Iswt
T 52800 58200 5 10 1 1 0 0 1
value=0
}
N 52700 58100 53500 58100 4
{
T 53100 58100 5 10 0 1 0 0 1
netname=5
}


In the above file, hs-vccs-1.sym symbols is:
$> cat ~/gaf/my-gschem-symbols/hs-vccs-1.sym
v 20050313 1
P 0 200 200 200 1 0 0
{
T 100 250 5 8 0 1 0 0 1
pinnumber=1
T 100 250 5 8 0 0 0 0 1
pinseq=1
}
P 925 200 725 200 1 0 0
{
T 725 250 5 8 0 1 0 0 1
pinnumber=2
T 725 250 5 8 0 0 0 0 1
pinseq=2
}
T 600 1000 5 10 0 0 0 0 1
device=CURRENT_SOURCE
L 200 200 250 200 3 0 0 0 -1 -1
L 675 200 725 200 3 0 0 0 -1 -1
L 350 200 550 200 3 0 0 0 -1 -1
L 550 200 500 250 3 0 0 0 -1 -1
L 550 200 500 150 3 0 0 0 -1 -1
T 150 275 8 10 1 1 0 0 1
refdes=G?
L 250 200 460 340 3 0 0 0 -1 -1
L 460 340 675 200 3 0 0 0 -1 -1
L 250 200 460 60 3 0 0 0 -1 -1
L 460 60 675 200 3 0 0 0 -1 -1