Global label and gnd

Schematic Simulation (Ngspice)
1

I’m a complete beginner trying out Kicad 8.0.2 and the new ngspice integration.

If I have this schematic and do an OP Simulation I get these values:

image
image618×502 3.65 KB

If I tie the test-label to the gnd node I get 39.8 kV voltage on the test node (sorry, new user could only post a single image)

Can’t a label be used like this?

– john

Background thread stopped with timeout = 0
Note: No compatibility mode selected!
Circuit: KiCad schematic
Doing analysis at TEMP = 27.000000 and TNOM = 27.000000
Using SPARSE 1.3 as Direct Linear Solver
Reference value : 0.00000e+00
No. of Data Rows : 1

Simulation results:

I(r2): 0A
I(v.xu2.voff): 0A
I(xu2:in+): 0A
I(xu2:in-): 0A
I(xu2:vcc): 0A
I(xu2:vee): 0A
I(xu2:out): 0A
I(r1): 0A
I(v2): 0A
I(v3): 0A
I(r3): 0A
I(e.xu2.eout): 0A
I(e.xu2.elow): 6.35275e-07fA
I(e.xu2.ehigh): -12.01pA
P(r3): 0W
P(v2): 0W
P(v3): 0W
P(r1): 0W
V(net-r3-pad1): 39.8kV
V(test): 39.8kV
V(vcc): 39.812kV
P(xu2): 0W
V(op_in): 39.802kV
V(xu2.lcc): 39.812kV
V(xu2.lee): 39.8kV
V(xu2.2): 39.8kV
V(xu2.int): 39.8kV
V(xu2.inoff): 39.802kV
V(net-u2–): 39.8kV
P(r2): 0W
V(net-r1-pad1): 39.8kV

2

You can use labels like this. KiCad is made on the assumption that you are smarter then KiCad itself, so when you explicitly add a global label to the GND net (Power symbols are also sort of labels), then the GND net gets renamed.

Voltages are always a difference in potential between two points (it’s one of the reasons why your DMM has two probes), and when the GND net is renamed, then ngSpice does not know what voltage to use as a reference.

1 Like
3

Thanks paulvdh for the explanation