Thyristor or Silicon Controlled Rectifier :-

Generally, we use many electrical and electronics componentswhile 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. Silicone control rectifiersconsists 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

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 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 currentfrom 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 begreater than the cathod 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.

V-I characterists of SCR :-

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 electric 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 alternative current to direct current Typically, thyristors are used in ac to ac converter (cycloconverters) which is the most common application of silicon controlled rectifier.

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