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What Thyristor Does?


Welcome to the industry of thyristors, versatile electronic components that wield considerable influence in the domain of high-power electronics. In this exploration, we delve into the fundamental concepts, operational principles, and applications of SCRs, shedding light on their role in controlled power transmission. From their unique structure to critical parameters like VGT, IGT, and IH, we unravel the mysteries of SCRs and their significance in applications ranging from motor speed control to uninterruptible power supplies. Join us on this journey to demystify the power and potential hidden within these electronic workhorses.


1. What is an SCR?

1.1 Basic Concept

The fundamental concept of Silicon Controlled Rectifiers (SCRs) revolves around their role as high-power electronic components designed for switch control in applications demanding efficient power regulation. Abbreviated as SCR or sometimes referred to as a thyristor, these devices serve as robust power switches capable of handling substantial electrical loads. The basic idea lies in their ability to conduct electricity when triggered by a small control signal, known as the gate trigger voltage. In practical terms, SCRs play a crucial role in scenarios demanding precise control over high-power equipment, such as motor speed regulation. The simplicity of their operation, where a small triggering current allows control over a much larger current flow, makes SCRs an integral part of power electronics, especially in applications requiring reliable and efficient switching.

The SCR’s primary function is to act as a unidirectional switch, conducting current only when specific conditions are met. Once triggered, the SCR enters a conduction state and remains in this state until the conduction conditions are no longer satisfied. This inherent one-way conduction property, along with the ability to handle high currents, positions SCRs as ideal components for controlling power flow in various electrical systems. Understanding the basic concept of SCRs involves grasping their role as powerful electronic switches and appreciating how they respond to triggering signals to facilitate controlled power transmission in diverse applications.

1.2 SCR Structure

1.2.1 Unidirectional SCR

The unidirectional Silicon Controlled Rectifier (SCR) functions as a highly efficient electronic switch in electrical circuits. Its basic structure resembles a PN junction, akin to a diode, and its primary attribute lies in its unidirectional conduction capability. When the control gate (G) receives a pulse voltage surpassing a certain threshold and there exists a positive voltage between the anode (A) and cathode (K) greater than the minimum conduction voltage, the unidirectional SCR enters a conduction state. Once triggered, it behaves similarly to a diode, allowing the flow of current in one direction only. This controlled conductivity makes the unidirectional SCR particularly well-suited for applications demanding precision in power regulation, such as in motor speed control. The SCR’s simplicity, wherein a small triggering signal initiates substantial current flow, underscores its significance in high-power electronic systems, providing a reliable means of controlling electrical loads with efficiency.

1.2.2 Bidirectional SCR

The bidirectional Silicon Controlled Rectifier (SCR) represents a versatile electronic component capable of conducting current in both directions, eliminating the need to distinguish between anode and cathode. Structurally, the bidirectional SCR exhibits a configuration allowing bidirectional conduction, making it suitable for applications where current flow needs to be controlled in both polarities. Similar to its unidirectional counterpart, the bidirectional SCR requires a triggering signal at the control gate (G) to initiate conduction, surpassing a specific threshold voltage. Whether exposed to positive or negative voltage, once triggered, the bidirectional SCR permits current flow in both directions. This dual conduction property makes it an ideal choice for scenarios requiring bidirectional power regulation, enhancing its applicability in diverse applications such as AC/DC speed control and other systems where controlling electrical currents in both directions is essential.

2. Working Principles of SCR

2.1 How does an SCR Work?

Conduction Conditions

The conduction conditions of a Silicon Controlled Rectifier (SCR) are critical in understanding how these electronic components facilitate the controlled flow of current. In the case of a unidirectional SCR, conduction is initiated when the control gate (G) receives a pulse voltage that exceeds a specific threshold, and there exists a positive voltage between the anode (A) and cathode (K) greater than the minimum conduction voltage. This triggering signal allows the SCR to enter a conduction state, acting like a diode and permitting the unidirectional flow of current from the anode to the cathode. This process demonstrates the SCR’s ability to serve as a switch that can be controlled through a small input signal, enabling the regulation of substantial power flows in applications like motor speed control.

For a bidirectional SCR, the conduction conditions are somewhat similar to the unidirectional counterpart. The bidirectional SCR also requires a triggering signal at the control gate, surpassing a certain threshold voltage. However, what sets it apart is its ability to conduct in both directions. Whether the voltage applied is positive or negative, once the control gate is triggered, the bidirectional SCR facilitates conduction, allowing current to flow bidirectionally between the anode and cathode. This unique feature makes bidirectional SCRs suitable for applications where controlling power flow in both polarities is essential, providing a versatile solution for scenarios such as AC/DC speed control.

Maintenance Conditions


  • Understanding the maintenance conditions of a Silicon Controlled Rectifier (SCR) is crucial for comprehending its stability during conduction. In the case of a unidirectional SCR, after the initiation of conduction, the SCR will remain in this state as long as there is a continuous positive voltage between the anode (A) and cathode (K) greater than the minimum conduction voltage. Additionally, the flowing current must exceed the minimum maintenance current to sustain the SCR’s conduction state. This maintenance condition ensures that the SCR remains reliably conductive, maintaining a stable and controlled flow of current between the anode and cathode. These parameters are integral in the SCR’s functionality, particularly in applications requiring a sustained and controlled power output, such as motor speed control.
  • For a bidirectional SCR, the maintenance conditions mirror those of the unidirectional SCR. After being triggered and entering a conduction state, the bidirectional SCR requires continuous positive or negative voltage (depending on the current flow direction after conduction) between the anode and cathode, exceeding the minimum conduction voltage. The flowing current must also surpass the minimum maintenance current. By adhering to these conditions, bidirectional SCRs ensure the reliability of bidirectional conduction, making them suitable for applications where the controlled flow of current in both directions is essential, such as in certain types of AC/DC speed control systems..

Turn-off Conditions

The turn-off condition of a Silicon Controlled Rectifier (SCR) is a distinctive aspect of its operation. Unlike its capability to be controlled to turn on, the SCR inherently lacks direct control for turning off. Once triggered and in a conducting state, the SCR will continue to conduct until specific conditions are no longer met. In both unidirectional and bidirectional SCRs, the turn-off condition arises when the applied voltage (or current, depending on the type) falls below a certain threshold. Specifically, if the voltage across the SCR drops to a level where it is unable to sustain the minimum conduction voltage, or if the current flowing through it falls below the minimum maintenance current, the SCR will autonomously turn off. This unique feature of self-turn-off provides an inherent safety mechanism, preventing the SCR from conducting when the applied conditions are insufficient to maintain conduction, ensuring controlled and secure operation in various applications.

2.2 Operating Principles

The operating principles of Silicon Controlled Rectifiers (SCRs) hinge on their distinctive structure and conduction characteristics. Structurally, an SCR is equivalent to two interconnected transistors, forming a PNP and an NPN junction. When a small triggering current is applied to the control gate (G), surpassing a certain threshold, the SCR enters a conduction state, allowing the flow of current from the anode (A) to the cathode (K) in a unidirectional SCR, or in both directions for a bidirectional SCR. Notably, once triggered, SCRs lack direct control for turning off and will continue to conduct until specific turn-off conditions are met. This unique property makes SCRs ideal for applications requiring precise control over high-power circuits, such as motor speed regulation. The simplicity of their operation, coupled with the ability to control large currents with minimal input, underscores the significance of SCRs in power electronics, providing a reliable means for controlled power transmission in various electrical systems.

PNP above, NPN below: Equivalent Structure Illustrated
Principle of Thyristor Conduction Control*

*Initially, the silicon-controlled rectifier (SCR) is in a cut-off state. When the G level receives a high voltage, the underlying PNP transistor conducts, causing the voltage at point 2 to be pulled low. This, in turn, leads to the conduction of the upper PNP transistor. Once the upper transistor conducts, the forward voltage is applied to point 1 (on the signal line connected to G), allowing the lower NPN transistor to remain in a conductive state, forming a stable circuit. At this point, the G point does not require an additional high-level signal, and the silicon-controlled rectifier can maintain conduction.

3. Parameters of SCR

The parameters crucial to understanding and characterizing the behavior of Silicon Controlled Rectifiers (SCRs) are VGT (Gate trigger voltage), IGT (Gate trigger current), and IH (Holding current). VGT represents the minimum voltage required at the control gate to trigger the SCR into conduction. This parameter is essential for determining the sensitivity of the SCR to external triggering signals. IGT, on the other hand, signifies the minimum current necessary at the control gate to initiate conduction. It is a crucial measure of the SCR’s sensitivity to triggering signals and provides insight into the minimum control current required for reliable operation. Lastly, IH, or Holding current, denotes the minimum current that must be maintained through the SCR to sustain its conduction state once triggered. Maintaining a current above IH ensures that the SCR remains reliably conductive, while falling below IH will result in the SCR turning off. These three parameters collectively define the operational characteristics of an SCR and play a vital role in designing and implementing reliable and controlled power systems.

LJ-SKKT106 Datasheet

4. Applications of SCR

Silicon Controlled Rectifiers (SCRs) find diverse applications in various electrical systems due to their ability to efficiently control high-power circuits. One of the primary applications is in controllable rectifiers, where SCRs are utilized to regulate the rectification of alternating current (AC) to direct current (DC). This is particularly valuable in applications requiring precise control over the DC output voltage. Another significant application lies in AC/DC speed control systems, where SCRs are employed to modulate the speed of motors or other machinery. The SCR’s ability to control the power flow in both directions makes it well-suited for these scenarios. In contactless switch circuits, SCRs offer an effective means of controlling electrical loads without the wear and tear associated with traditional mechanical switches. Moreover, SCRs are extensively used in applications requiring regulated power, such as uninterruptible power supplies (UPS), ensuring a stable and controlled power output. Overall, the versatility of SCRs in handling high voltages and currents makes them indispensable in applications ranging from motor control to power regulation in various industrial and electronic systems.


Silicon Controlled Rectifiers (SCRs) emerge as essential components in the realm of high-power electronics, playing a pivotal role in controlled power transmission and regulation. Their fundamental structure, characterized by unidirectional or bidirectional conduction, facilitates precise control over large currents with minimal triggering signals. The identified parameters—VGT, IGT, and IH—provide key insights into the SCR’s sensitivity and maintenance requirements, crucial for reliable and efficient operation. The applications of SCRs span a wide spectrum, from controllable rectifiers and AC/DC speed control to contactless switch circuits and uninterruptible power supplies. Their versatility in handling high voltages and currents positions SCRs as integral components in motor control, power regulation, and other industrial applications. As technology evolves, the continued exploration of SCR applications and potential advancements in their usage promises further innovations in the field of power electronics. In engineering design, understanding the principles and parameters of SCRs remains paramount to ensuring safe, reliable, and efficient performance in diverse electrical systems.


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