Strange negative current

[David_R]

Hi, I have an SPICE model for CD4007 NMOS and I'm using a DC sweep analysis varying the gate voltage, but I don't know why Altium is showing me a negative drain current  .

Here some pictures



I just became aware that with this simple circuit the same happens

[BobsURuncle]

I am not familiar with the software, but don't you have to define the direction in which you are measuring the current?  If you don't define it then perhaps it defaults to giving it a sign with respect to the voltage across the element, so for example resistors would always have negative current and voltage sources would be positive - assuming it is delivering power.

[David_R]

I am not familiar with the software, but don't you have to define the direction in which you are measuring the current?  If you don't define it then perhaps it defaults to giving it a sign with respect to the voltage across the element, so for example resistors would always have negative current and voltage sources would be positive - assuming it is delivering power.

no, there are no options to define something like that, and it is supposed that a simulator always follows the passive sign convention so in this case the current should be positive

[T3sl4co1l]

Hover the mouse over R1's pins. Note the pin names (the defaults are 1 and 2, IIRC).

Edit the resistor's simulation model.  Note the pin connections (should be 1 = 1, 2 = 2?).

Consult the SPICE spec on current flow direction in two terminal components (such as resistors, sources and etc.).

Flip the resistor symbol, or swap the pin assignments, or multiply the current reading by -1, to invert it.

TLDR: confusion over the sign of a current flow suggests a poor understanding of circuit fundamentals... 

Tim

[T3sl4co1l]

I am not familiar with the software, but don't you have to define the direction in which you are measuring the current?  If you don't define it then perhaps it defaults to giving it a sign with respect to the voltage across the element, so for example resistors would always have negative current and voltage sources would be positive - assuming it is delivering power.

FWIW, Altium uses a pretty much bog standard XSPICE backend.  Results should be largely identical to pretty much every other simulator (which, in turn, are pretty much bog standard themselves).

Tim

[David_R]

you can't say that men, I understand perfectly the basics concepts of circuits (maybe I'm overreacting with the title), there are a biggest potential at the top of R1 ,so the current should flow from drain to source and should be positive..... the name of the pin does not affect anything it is just a convension

[BobsURuncle]

Maybe you need to reverse the plug from your computer into the wall socket. 

[Cerebus]

you can't say that men, I understand perfectly the basics concepts of circuits (maybe I'm overreacting with the title), there are a biggest potential at the top of R1 ,so the current should flow from drain to source and should be positive..... the name of the pin does not affect anything it is just a convension

Yes it does. SPICE tells you the current flow from pin 1 to pin 2 and sets the sign accordingly, if the current is flowing from pin 2 to pin 1 it will show the reverse sign. The first time you encounter this it seems weird because to the sane mind resistors do not have polarity, but in SPICE's warped mind they do.

[David_R]

you can't say that men, I understand perfectly the basics concepts of circuits (maybe I'm overreacting with the title), there are a biggest potential at the top of R1 ,so the current should flow from drain to source and should be positive..... the name of the pin does not affect anything it is just a convension

Yes it does. SPICE tells you the current flow from pin 1 to pin 2 and sets the sign accordingly, if the current is flowing from pin 2 to pin 1 it will show the reverse sign. The first time you encounter this it seems weird because to the sane mind resistors do not have polarity, but in SPICE's warped mind they do.

Thank you, I just turned the resistor and it works