Thyristor or Silicon Controlled Rectifier Tutorial basics and Characteristics

Generally, we use many electrical and electronic components while designing electronics projects and general circuits. These basic components include resistors, transistors, capacitors, diodes, inductors, LEDs, thyristors or silicon controlled rectifiers, ICs, and so on. Let us consider the rectifiers which are classified into two types such as uncontrolled rectifiers (diodes) and controlled rectifiers (thyristors). Actually, many engineering students and electronic hobbyists desires to know the basics about electrical and electronics components. But, here in this article let us discuss in detail about the thyristor or silicon controlled rectifier tutorial basics and characteristics.

Silicon Controlled Rectifier

Thyristor or silicon controlled rectifier is a multilayer semiconductor device and is similar to the transistor. Silicon controlled rectifier consists of three terminals (anode, cathode, and gate) unlike the two terminal diode (anode and cathode) rectifier. The diodes are termed as uncontrolled rectifiers as they conduct (during forward bias condition without any control) whenever the anode voltage of the diode is greater than cathode voltage.

Diode and Thyristor
Diode and Thyristor

But, the silicon controlled rectifiers doesn’t conduct even though the anode voltage is greater than the cathode voltage unless until the (third terminal) gate terminal is triggered. Thus, by providing the triggering pulse to the gate terminal, we can control the operation (ON or OFF) of thyristor. Hence, the thyristor is also called as controlled rectifier or silicon controlled rectifier.

Silicon Controlled Rectifier Basics

Unlike two layers (P-N) in the diode and three layers (P-N-P or N-P-N) in transistors, the silicon controlled rectifier consists of four layers (P-N-P-N) with three P-N junctions that are connected in series. The silicon controlled rectifier or thyristor is represented by the symbol as shown in the figure.

Silicon Controlled Rectifier
Silicon Controlled Rectifier

Silicon controlled rectifier is also a unidirectional device as it conducts only in one direction. By triggering appropriately, the thyristor can be used as an open circuit switch and also as a rectifying diode. However, thyristor can not be used as an amplifier and it can be used only for switching operation controlled with triggering pulse of gate terminal.

Thyristor can be manufactured using a variety of materials such as silicon, silicon carbide, gallium arsenide, gallium nitride, and so on. But, the good thermal conductivity, high current capability, high voltage capability, economical processing of silicon has made it to prefer compared to other materials for making thyristors, hence, they are also called as silicon controlled rectifiers.


Silicon Controlled Rectifier Working

The thyristor working can be understood by considering the three states modes of operation of silicon controlled rectifier. The three modes of operation of thyristor are as follows:

  • Reverse blocking mode
  • Forward blocking mode
  • Forward conducting mode

Reverse Blocking Mode

If we reverse the anode and cathode connections of the thyristors, then the lower and upper diodes are reverse biased. Thus, there is no conduction path, so no current will flow. Hence, is called as reverse blocking mode.

Forward Blocking Mode

In general, without any triggering pulse to gate terminal, silicon controlled rectifier remains switched off, indicating no current flow in the forward direction (from anode to cathode). This is because, we connected two diodes (both upper and lower diodes are forward biased) together to form a thyristor. But, the junction between these two diodes is reverse biased, which eliminates the flow of current from top to bottom. Hence, this state is termed as forward blocking mode. In this mode, even though thyristor is having condition like a conventional forward biased diode, it will not conduct as the gate terminal is not triggered.

Forward Conducting Mode

In this forward conducting mode, the anode voltage must be greater than the cathode voltage and the third terminal gate must be triggered appropriately for the conduction of the thyristor. This is because, whenever the gate terminal is triggered, then the lower transistor will conduct which switches on the upper transistor and then the upper transistor switches on the lower transistor and thus the transistors activates each other. This process of internal positive feedback of both the transistors repeats until both gets fully activated and then the current will from anode to cathode. So, this mode of operation of silicon controlled rectifier is called as forward conduction mode.

Silicon Controlled Rectifier Characteristics

Silicon Controlled Rectifier Characteristics
Silicon Controlled Rectifier Characteristics

The figure shows the silicon controlled rectifier characteristics and also represents the thyristor operation in three different modes such as reverse blocking mode, forward blocking mode, and forward conducting mode. The V-I characteristics of thyristor also represent the reverse blocking voltage, forward blocking voltage, reverse breakdown voltage, holding current, break-over voltage, and so on as shown in the figure.

Silicon Controlled Rectifier Applications

Application of silicon controlled rectifier  is used in the circuits dealing with large currents and voltages such as electrical power system circuits with more than 1kV or greater than 100A of current.

Thyristors are specially used to reduce the internal power loss in the circuit. The silicon controlled rectifiers can be used to control the power in the circuit without any losses using on-off switching control of the thyristors.

Silicon controlled rectifiers are also used for rectification purpose, i.e., from alternating current to direct current. Typically, thyristors are used in AC to AC converters (cycloconverters) which is the most common application of silicon controlled rectifier.

Practical Application of Silicon Controlled Rectifier

SCR Based Cycloconverter by
SCR Based Cycloconverter by

The SCR based cycloconverter is the practical application of silicon controlled rectifier in which speed of single-phase induction motor is controlled in three steps. Induction motors are constant speed machines and frequently used in several applications such as washing machines, water pumps, and so on. These applications require different speeds of the motor that can be achieved using this SCR-based technique.

SCR Based Cycloconverter Block Diagram by
SCR Based Cycloconverter Block Diagram by

Thyristor based cycloconverter is used for controlling the speed of induction motor in steps. In this project, pair of switches is interfaced to the 8051 microcontroller and these are used for selecting the desired speed (F, F/2, and F/3) of the motor. Based on the status of the switches, the microcontroller delivers the triggering pulses to the silicon controlled rectifiers of dual bridge. Thus, the induction motor speed is controlled in three steps based on the requirement.

Do you want to design electronics projects based on silicon controlled rectifiers? Then, post your ideas in the comments section below for our technical assistance in designing your engineering projects.