THYRISTORS

Thyristors is the general name given to a family of semiconductor devices having four layers with a control mechanism,although this term is most commonly applied to the SCR(silicon controlled rectifier). This term is derived from thyratron  and transistor because the devices combines the rectification action of thyratron and control action of transistor.

 Thyristor stands for THRYatron transISTOR.

The device was first developed at the Bell Laboratories in the USA and was commercially introduced by the General Electrical Company in the USA in 1957.The device appeared in the market under different names such as SCR, thyristor, thyrode etc.Since its inception the thyristor has come to stay as a basic building block in many industrial and power system applications. Its ability to be controlled,compactness ,fast response,high reliability,better efficiency,large power handling capacity,high voltage and current ratings,good trigger sensitivity,static operation,large power gain,study construction,long life,very little maintenance and low cost of fabrication due to advancement in the field of fabrication-have given the thyristor a colorful reception in every field. Today thyristors are finding applications in the control of dc/ac motors; for the improvement of power factor;and as switching devices.They have become an integral part of HVDC transmission.They are so advantageous that our major steel plants had decided to modernise their plants by replacing MG sets and mercury-arc rectifier by thyristor converts .Thyristor   have helped in further cost reduction and in the development of drive system by changing the emphasis from dc motors to ac motors.With cycle converter and inverters, the  speed of an ac motor can also be controlled with ease and reliability.Apart from these main applications it finds use as a switching devices,particularly in the improvement of power factor of transmission lines and mains. Thyristor  can be used as a power factor of transmission lines and mains.