# Re: [SI-LIST] : The correct way to short tow nodes in SPICE is?

From: J. Eric Bracken ([email protected])
Date: Fri Oct 20 2000 - 11:35:34 PDT

Bo wrote:

> Hi
> I would like to discuss one of the big SPICE myths. I have used several
> different spice programs (PSPICE, HSPICE, etc) and I have seen two different
> implementations. The problem is that nobody can agree which is the better
> implementation of the two. I would like to hear your opinion on this.
> The two implementations are:
>
> 1) Connect two nodes to each other by putting 0 Ohm resistor.
> 2) Connect two nodes to each other by putting 0 volt DC source between them.
>

Method 2--using a zero-valued voltage source--is the best way.

The reason has to do with the way Spice handles resistors. It writes a KCL
equation at every circuit node. So if you have a resistor R from node i to node
j,
the contribution of the resistor to KCL at node i is something like this:

vi / R - vj / R

For a zero-valued resistor, 1/R is infinity. The software can't permit this
infinite value to enter into its matrix of equations, so it forces R to some
small minimum value. This minimum value varies from one Spice
implementation to the next.

For a voltage source between nodes i and j, Spice actually writes a
separate KVL equation:

vi - vj = 0

and this is enforced exactly. (At least to the numerical precision of the
matrix solver, which is pretty darn good.)

Only reason I can think of for preferring a small resistor is to avoid
problems caused by loops of voltage sources--which you shouldn't
be creating anyway.

--Eric

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