Re: [Help-gnucap] Short circuit current and resistance?

[Al Davis]

On Wednesday 04 February 2004 05:09 pm, Philip Jägenstedt wrote:
> I recently used gschem and gnucap to verify my homework in
> electronics (yes, I got it right). However there are two things I
> can't really figure out how to do, I suspect they are not terribly
> difficult and I'd like to know. I've done some reading in the manual,
> but it didn't help me a great deal. Attached is the gschem file and
> the file generated by gnetlist that I used to verify my homework.

First, your circuit has no ground.  You must designate one node as a 
reference, node 0.  (zero)  Usually, in gnucap it will still work but 
with an "open circuit" warning, and you don't know what it picked as a 
reference.

> 1) If I want to see what the current through a branch which short
> circuits the open section to the right in my figure (i.e. short
> circuiting nodes 8 and 6 in the netlist), how would I do that? What I
> did now was I hacked it by adding a 0 ohm resistance, but I didn't
> think that was a very pretty solution. Since a short circuit doesn't
> show in the netlist as a component, I can't measure the current
> through it by any obvious method.

That works.  In Spice, you do this with a zero volt voltage source.  
Spice only lets you measure current through voltage sources.  Gnucap 
lets you measure current through almost anything.

> 2) I'd like to measure the resistance over the two nodes in the open
> section to the right in my figure. I could get this by dividing the
> open circuit voltage over short circuit current (and this is what I
> did to get the answer for my homework) but I'm wondering if it's
> possible in some other way to measure the resistance between any two
> nodes in the circuit).

That works with a linear circuit.  That is probably the intent of the 
lesson.  There are many ways to measure this.  Gnucap has a "Z" probe 
that will tell you directly.  Put a component across where you want to 
measure, for example "Rload", then probe Z(Rload).  That will tell you 
what that component sees looking in.  The best choice is an open 
circuit, like a "Y" component with admittance of zero, but others work 
too.