Re: gEDA-user: SPICE how to?

[Al Davis <ad82@xxxxxxxxxxxxxxxx>]

On Saturday 16 July 2005 12:20 pm, Harold D. Skank wrote:
> I need to see the current in a particular branch of a circuit
> under SPICE simulation.  I understand that a voltage source
> set to zero is supposed to be the mechanism to use, but what
> variable do I query then to view the current?

As Stuart says, in Spice you need to put a voltage source in and 
probe it.

To put in another plug for gnucap ....  There are lots of probes 
in gnucap.  You can simply probe the current in any 2-terminal 
element, or any of the standard controlled sources.  For a 
resistor R523, ...  "print tran i(r523)".

For more complex devices, you need to be more specific.  For 
example,  a BJT,  what current do you want?  IC(Q3), IB(Q3), 
IE(Q3) are the obvious ones.  You can also probe a bunch of 
internal values, including power, currents, internal variables.  
You can also probe lots of stuff about each the elements that 
make up the transistor, 14 of them for a fully loaded BJT.  You 
can even separately probe things like the current through 
internal capacitances, by accessing the internal elements.

Spice-3 lets you do some of these for the operating point 
analysis, with a different syntax.  I don't know if it works in 
ng-spice or not.