[free-electronic-lab] ngspice .PRINT I(<component>) won't work

[Ashwith Rego]

On Tue, Sep 14, 2010 at 1:55 AM, Brennan Ashton
<bashton at brennanashton.com>wrote:

> On Mon, Sep 13, 2010 at 3:10 PM, Ashwith Rego <ashwith at gmail.com> wrote:
> > Hi
> > I've just begun using ngspice and found that I cannot plot the current
> > output. Also, I cannot specify the name of a circuit component in a
> .PRINT
> > or .PLOT statement. Only the node numbers seem to work. Here is an
> example:
> > Ohm' Law
>
> >
> > ngspice doesn't seem to recognize R1 in the .PRINT statement. I get the
> > following error:
> >
> > $ngspice -b diff.net
>
> > Warning: can't parse 'r1': ignored
>
> SPICE uses nodal analysis so it has the voltages at every node, it is
> up to you to determine what the differential measurement will be, for
> the voltage over the resistor you have to list the two nodes, 1 and 0.
>  This is all normal behavior.
>
> >
> -------------------------------------------------------------------------------------------------
> > The simulation however works if I replace it with
> > .PRINT DC V(0,1)
>
> > Secondly, I can't seem to print current. Using
> > .PRINT DC I(R1) or .PRINT DC I(0,1) gives me this error:
> > $ngspice -b diff.net
> >
> > Circuit: ohm's law
> >
> > Doing analysis at TEMP = 27.000000 and TNOM = 27.000000
> >
> > Warning: can't parse '0#branch': ignored
> > Error: no data saved for D.C. Transfer curve analysis; analysis not run
> > doAnalyses: not found
>
> This is also normal, spice will only calculate the current though a
> voltage source.  The normal way to get around this it to place a 0V
> voltage source in series with the current path.  You can then request
> the current from that.  In this case you already have that, it is vin,
> the vector for current is then vin#branch, so print i(vin)  will give
> you the current.
>
> > I don't see this happening in gnucap however (in this case V(0,1) will
> not
> > work). Am I doing something wrong or is ngspice meant to work this way? I
> > went according to the ngspice manual. I'm using Fedora 13 64-bit with
> Free
> > Electronic Lab groupinstalled.
>
> gnucap will parse spice for the most part, but it is not the same
> especially as it gives more direct access to current power frequency
> and other properties of components in the circuit.
>
> >
> > Thanks! :-)
> >
> > Ashwith J. Rego
>
>
> You might find that this overview of spice will help you better
> understand how it works.
> http://www.seas.upenn.edu/~jan/spice/spice.overview.html
> Especially read the Independent DC sources section for information
> about measuring current and voltage.
>
> I hope this helps.
>
> --Brennan Ashton
>

Hi Brennan

Thanks. That explained it. Never thought about adding the 0V in series
before. Thank you for your help. :)

Regards
-- 
Ashwith J. Rego
-----------------
My Webpage: http://ashwith.wordpress.com/
Find me on LinkedIn at: http://www.linkedin.com/in/ashwith
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