Spice creates time-variant resistor

Vittorio Ricchiuti, Italtel, L'Aquila, Italy -- EDN, 10/28/1999

Figure 1 shows a subcircuit that represents the behavioral model of a linearly time-variant resistor in PSpice (Orcad, www.orcad.com). PSpice libraries include no models of such resistors, which you can use in various applications-for example, to simulate the fallback characteristic of a linear regulator in transient analysis. The suggested model uses the E-device in the PSpice analog-behavioral-modeling parts library. Listing 1 describes the subcircuit in Figure 1 for a time-variant resistor with the characteristic in Figure 2. The function in Figure 2 is the product of an independent voltage source, V_CONTROL. The independent zero-voltage source V_SENSE senses the current I(V_SENSE) through the resistor. The voltage V_R between nodes 1 and 2 of the time-variant resistor is V_R=f(t)*I(V_SENSE), where f(t) is a function of V_CONTROL. (DI #2424)