# spice modelling of variable resistance

I am trying to simulate a load in SPICE(PSICE). This load is modeled by a resistance that can go from a high value(thereby sinking low current) to a very low resistance that can draw a large amount. How can I model such a variable resistor (w.r.t time) in SPICE ?

• Do you really need the time aspect? If not parameter sweep will work. If you really need a precise curve over time... I don't know! Commented Oct 13, 2015 at 13:42
• A parameter sweep would do just fine. Commented Oct 13, 2015 at 13:42
• moodle.cecs.pdx.edu/pluginfile.php/709/mod_resource/content/0/… Commented Oct 13, 2015 at 13:43
• it asks for a user name and passworsd :( Commented Oct 13, 2015 at 13:43
• I don't know about your specific spice incarnation, but with ltspice it can be done this way: electronics.stackexchange.com/questions/3623 Commented Oct 13, 2015 at 13:43

• if a precise curve of the resistor vs time doesn't matter .step param, which is supported in PSpice, is a good way. Actually you can remain ignorant of the SPICE dot command syntax in PSpice, because parametric sweeps can be configured entirely via GUI menus (Simulation Settings > Primary Sweep) as explained in this tutorial: http://m.eet.com/media/1179065/chapter (see p. 69)
• for a curve vs time: SPICE3 behavioral expressions. I'm not sure exactly what PSpice supports in this regard... It seems the preferred way of doing this in PSpice is to use its Analog Behavioral Modeling (ABM) extension, which allows abstract components that can use TIME as a variable; see http://www.nordcad.dk/download/Forum/PSpice/abm.pdf for an overview. At least for this problem, it seems ABM is actually the same thing as a SPICE3 behavioral expression, with a (PSpice) syntax like R1 1 0 VALUE={10 + 0.01 * TIME}.