| Matrix or Vector | Shape Functions | Integration Points |
|---|---|---|
| Stiffness Matrix | None (lumped) | None |
| Damping Matrix | None | None |
| Load Vector | None (lumped) | None |
CIRCU125 has two highly nonlinear electric circuit element options:
Common Diode (KEYOPT(1) = 0)
Zener Diode (KEYOPT(1) = 1)
The I-V characteristics of common and Zener Diodes are plotted in Figure 13.19: I-V (Current-Voltage) Characteristics of CIRCU125.
As can be seen, the characteristics of the diodes are approximated by a piece-wise linear curve. The common diode has two sections corresponding to open and closed states. The Zener diode has three sections corresponding to open, blocked, and Zener states. The parameters of the piece-wise linear curves are described by real constants depending on KEYOPT(1) selection.
The behavior of a diode in a given state is described by the Norton equivalent circuit representation (see Figure 13.20: Norton Current Definition).
The Norton equivalent conductance, G, is the derivative (steepness) of the I-V curve to a pertinent diode state. The Norton equivalent current generator, I, is the current where the extension of the linear section of the I-V curve intersects the I-axis.
The element matrix and load vectors are obtained by using the nodal potential formulation, a circuit analysis technique which is perfectly suited to coupling lumped circuit elements to distributed finite element models.
The stiffness matrix is:
 | (13–194) |
The load vector is:
 | (13–195) |
where:
| G and I = Norton equivalents of the diode in the pertinent state of operation. |