Explanation of Silicon Controlled Rectifier and Its Applications

In general, we use a lot of electrical and electronic components while making the circuits and electronics projects. The basic components mainly include resistors, capacitors, transistors, diodes, LEDs, inductors, thyristors or SCR (silicon controlled rectifiers), ICs (integrated circuits), and so on. Let us think the rectifiers which are categorized into two types namely diodes (uncontrolled rectifiers) and thyristors (controlled rectifiers). Actually, many engineering students, electronic hobbyists wish to know the basic function of an electrical and electronics components. But, here in this article gives an overview of the silicon controlled rectifier and its characteristics.

What is a Silicon Controlled Rectifier (SCR)?

The SCR (silicon controlled rectifier) or Thyristor is a multilayer semiconductor device and is comparable to the transistor. SCR consists of 3-terminals namely anode, cathode, and gate not like the two terminal diode rectifiers which consist of two terminals namely anode and cathode. The diodes are called as uncontrolled rectifiers as they perform whenever the anode voltage of the diode is superior to cathode voltage (during forward bias condition without any control).

But, the SCR doesn’t perform even though the anode voltage is superior to the cathode voltage unless until the gate terminal is activated. Thus, by providing the activating pulse to the gate terminal, we can manage the operation of the thyristor. Hence, the thyristor is also named as the controlled rectifier or silicon controlled rectifier.

Basics of Silicon Controlled Rectifier

Unlike 2-layers in the diode (P-N), 3-layers in transistors (P-N-P or N-P-N), and the silicon controlled rectifier consists of four layers (P-N-P-N) with three PN junctions that are allied in series. The symbol of the SCR or thyristor is shown in the following figure.

SCR is also a unidirectional device as it performs only in one direction. By activating properly, the thyristor can be utilized as an open circuit switch and also as a rectifying diode. But, the thyristor cannot be used as an amplifier; it can be used only for switching process controlled by triggering pulse of the gate terminal.

The manufacturing of the Thyristor can be done using a variety of materials like silicon, gallium arsenide, silicon carbide, gallium nitride, and so on. But, the superior thermal conductivity, high voltage capability, high capability of current, economical processing of Si has made it choose compared to other materials for manufacturing thyristors, hence, they are also named as silicon controlled rectifiers.

Silicon Controlled Rectifier Working

The working of thyristor can be understood by considering the three states modes of process of SCR (silicon controlled rectifier) such as

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

Reverse Blocking Mode

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

Forward Blocking Mode

Generally, without any activating pulse to the gate terminal, SCR remains switched off, representing no flow of current in the forward direction. This is because; the connected two diodes together will form a thyristor. But, the connection between these diodes is reverse biased, which reduces the current flow from top to bottom. Hence, this state is called as the forward blocking mode. In this mode, even though thyristor is including condition like a conventional forward biased diode, it will not perform as the gate terminal is not activated.

Forward Conducting Mode

In this mode, the anode voltage should be superior to the cathode voltage and the gate terminal must be activated properly for the transmission of the thyristor. This is because, when the gate terminal is activated, then the lower transistor will perform which switches ON the upper transistor and then the upper transistor switches ON the lower transistor and thus the transistors triggers each other. This procedure of inner positive feedback of both the transistors recurs until both get fully triggered and then the flow of current will from the anode to the cathode. So, this form of operation of SCR is called as forward conduction mode.

Characteristics of Silicon Controlled Rectifier

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

The SCR applications include the following

• Application of SCR includes in the circuits which deal with large currents and voltages like electrical power system circuits with more than 1kV or larger than 100A of current.
• These are particularly used to decrease the interior power loss in the circuit.using on-off switching control of the thyristors.
• SCRs are also used for modification purpose, i.e., from alternating current to direct current. Usually, thyristors are used in cyclo converters (AC to AC converters) which is the most general application.

Thus, this is all about Silicon Controlled Rectifier, working, characteristics and its applications. SCRs can be used in AC power control, controlled bridge rectifiers, DC powered transmission, power electronic devices, etc. Do you want to design projects based on thyristors? Then, post your ideas in the comments section below for our technical support in designing your engineering projects.