The subject of power electronics originated in the early part of the twentieth century with the development and application of devices such as the mercury arc rectifier and the thyratron valve. Indeed many of the circuits currently in use and described in this book were developed in that period. However, the range of applications for these early devices tended to be restricted by virtue of their size and problems of reliability and control. With the development of power semiconductor devices, offering high reliability in a relatively compact form, power electronics began to expand its range and scope, with applications such as DC motor control and power supplies taking the lead. Initially, power semiconductor devices were available with only relatively low power levels and switching speeds. However, developments in device technology resulted in a rapid improvement in performance, accompanied by a corresponding increase in applications. These now range from power supplies using a single transistor to high voltage DC transmission where the mercury arc valve was replaced in the 1970s by a solid-state 'valve' using thyristor stacks. Developments in microprocessor technology have also influenced the develop­ ment of power electronics. This is particularly apparent in the areas of control, where analogue controllers are being replaced by digital systems, and in the evolution of the 'smart power' devices. These developments have in turn led to system improvements in areas such as robot drives, power supplies and railway traction systems.