From: al davis <ad212@fr...>  20130721 14:55:30

On Sunday 21 July 2013, Soeren D. Schulze wrote: > Adding a series resistance to a voltage source seems both > more effective and more harmless to me  really, who is > going to notice 10u Ohm? It's a tossup. One way to model (I should say approximate) a voltage source is with a small series resistance, which is converted into a current source with shunt. Gnucap does it this way. It messes up the conditioning of the matrix somewhat. Another way is to add another equation, the equivalent of an extra node, representing the current. (Nullor model) Gnucap could do it this way, but doesn't. Another voltagesource plugin would give you a choice. It messes up the pattern of the matrix. Yet another way is to eliminate a node and solve the missing node by adding the voltage source voltage to the nearby node that you keep. All of these have their advantages and disadvantages, which may not be evident unless you look deeper and try to implement it. Similar reasoning applies to the inductor. Spice uses the internal node model of an inductor. Gnucap may use either. It adds the internal node only if there is mutual inductance. The spice method is a disaster in run time for a circuit with lots of inductors, like you get with parasitic extraction, the RLGC model of a system of coupled transmission lines. On Sunday 21 July 2013, Soeren D. Schulze wrote: > It's important for users to understand that leaving out > certain parasitic components sometimes doesn't simplify > things but actually makes them trickier. One of my favorite counterintuitive surprises here is the addition of transmission lines to a circuit to improve convergence. On the other hand, adding stray capacitance usually makes things worse. 