What Are the Differences Between SCR and TRIAC?

In the field of power electronics, thyristors are key semiconductor devices widely used in various power conversion and control applications. Also known as silicon-controlled rectifiers, thyristors are high-power controllable rectifiers that offer advantages such as compact size, high efficiency, long lifespan, and easy control. In the electronic components industry, the two most common types are the unidirectional thyristor (SCR) and the bidirectional thyristor (TRIAC). They differ significantly in structure, operating principle, and control characteristics, making them suitable for different applications. This article will explore the differences between these two devices in detail.

I. What is a Unidirectional Thyristor?

A unidirectional thyristor, also called an SCR, stands for Silicon Controlled Rectifier. It is a three-terminal semiconductor device with electrodes named Anode (A), Cathode (K), and Gate (G). Structurally, it has a typical four-layer P‑N‑P‑N configuration.

The device only conducts when the anode is positively biased relative to the cathode and a trigger signal is applied to the gate. Once it starts conducting, it continues to do so until the main current falls below the holding current or the voltage reverses. Because it only conducts in one direction (from anode to cathode), it is called a unidirectional thyristor.

II. What is a Bidirectional Thyristor?

A bidirectional thyristor, or TRIAC (Triode for Alternating Current), also belongs to the thyristor family. Its main difference from an SCR is that it can conduct in both directions—it can conduct from main terminal T1 to T2 or from T2 to T1.

Structurally, it can be seen as two SCRs connected in inverse parallel, sharing a common gate. It has three terminals: MT1, MT2, and Gate (G).

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