This book introduces electric circuits with variableloads and voltage regulators. It allows to define invariant relationships forvarious parameters of regime and circuit sections and to prove the conceptscharacterizing these circuits. The book presents the fundamentals of electriccircuits and develops circuit theorems. Generalized equivalent circuitsare introduced. Projective geometry is used for the interpretation of changesof operating regime parameters. Expressions of normalized regime parameters andtheir changes are presented. Convenient formulas for the calculation ofcurrents are given. Parallel voltage sources and the cascade connection ofmulti-port networks are described. The two-value voltage regulationcharacteristics of loads with limited power of voltage source is considered.  This second edition is extended and contains additional chapters on circuits withnon-linear regulation curves, circuits with non-linear load characteristics,concepts of power-source and power-load elements with two-valuedcharacteristics, quasi-resonant voltage converters with self-limitation ofcurrent as well as the similarity of characteristics of converters andelectronic devices. This book is useful to engineers, researchers andgraduate students who are interested in the basic electric circuit theory andthe regulation and monitoring of power supply systems.