Re: [Help-gnucap] Bizarre behaviour -- possibly a bug.

[al davis]

On Sunday 26 May 2013, Orestes Mas wrote:
> I must confess I'm a bit lost here, due to the fact I'm not
> aware of some  simulator internals.

One practical use of UIC is for circuits that have several 
metastable states, preventing convergence.


> I know that to do transient simulations one must use
> transient sources,  perhaps a PWL 0,0 1n,8 in this case. But
> the students tend to use DC source + UIC to simulate step
> responses as they seems to consider this way "conceptually
> easier". This problem arose when I was reviewing a student's
> exercise where she printed out the current through the DC
> source along with the inductor's current.

A PWL is a correct way to do it.  UIC is not.

Especially in teaching.  "At T=0, close the switch ...".  To 
properly simulate, you need to do exactly that.

> I simply used UIC because I wanted to start with zero current
> in the inductor.  If I don't do that the simulator
> calculates the DC operating point on its own and

The purpose of UIC is to prevent the simulator from calculating 
a DC operating point.

To both specify an initial condition AND calculate the DC 
operating point is inconsistent, unless UIC were defined 
differently, such as to substitute a voltage source for the 
capacitor then calculate.  Even that is not real, in the sense 
that real circuits don't do that.

> 
> But wait... Are you meaning that if one wants to do a
> transient simulation  should use only transient sources?
> perhaps a PWL 0,0 1n,8 in this case?

yes.

That one says that it starts at 0, then ramps up to 8 at 1 ns.  
In addition to showing the state change that starts the 
charging, it also shows that the voltage cannot change 
instantaneously.