The order matters (or not?)

Hi QS,

simply to confirm an “elementary” doubt.
As you can see in the figure, in the default netlist from the schematic, in the resistor Rc (collector) it shows a negative value of current that makes a little “damage to the eye” (a positive value would be more pleasant!).
In principle it is logical since I deduce that the order of the nodes matters in the netlist.
The first one is considered to be at a higher potential than the second one.
That’s why I made a test by changing the order to check it.
This has made me to write this message to help me to see what maybe is not important and I am giving too much importance to it.
Thank you. Toni

The +ve current direction in spice for resistor, capacitor, inductor and voltage/current source are defined from 1st net to 2nd net.
There is no default order, it is just because you used Ctrl-R to rotate Rc, and in schematic, its 1nd net go to bottom in your schematic.
You can check net order by

  • F3 (View Symbol Properties)
  • Click on component
  • Goto Pin Nets
  • Click pin name you want to highlight


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Therefore, for voltage source, as +ve current direction is from + to -, the current coming out from voltage source is -ve. In Qspice, when you place a current probe at voltage source Vnnn, waveform viewer in default give you -I(Vnnn). But other spice program may give you +I(Vnnn) and you have to change it to -ve in waveform window.

Based on this principal, a common skill in spice is to use 0V voltage source for current measurement. Here is an example

Third party mosfet current measurement - QSPICE - Qorvo Tech Forum

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