This paper describes a fast and reliable circuit-based simulation method of two-dimensional electrical transient characteristics of thyristors and over-voltage protectors (TOVP). Either device is divided into four-layered square prisms, and a one-dimensional PNP–NPN transistor pair model associated to each of them. The transistors are represented with the Gummel–Poon model, complemented with breakdown and quasi-saturation sub-models. The cells in turn are coupled through the base planes, with current-modulated resistors. Plasma-spreading velocities obtained through simulation are comparable to experimentally obtained ones reported elsewhere. Spatial current density instabilities induced by device asymmetries are also presented. The required spatial resolution was attained by representing these four-layer devices with up to 40,000 Spice circuit elements, including 3000 bipolar junction transistors. The associated Spice lists were assembled effortlessly with a Mathematica program. Simulations took from 1 to 3 h in a 650 MHz Pentium III computer, with no symptoms of convergence problems whatsoever. The authors believe this is the first time that Spice based high-resolution transient behavior simulations of TOVPs and thyristors are presented in the literature. 2002 Elsevier Science Ltd. All rights reserved.