How to Test Thyristors and Identify SCR vs TRIAC？
Accurately checking whether a thyristor is in good condition and clearly telling the difference between unidirectional and bidirectional types is a basic yet critical skill in power electronics applications and repair work. Using the right methods not only helps you diagnose faults quickly but also ensures the reliability of circuit designs. This article systematically explains the core steps for testing thyristors and identifying their types, based on the general operating principles of power semiconductor devices, with an emphasis on clarity and practical usability.
Catalog
II. Methods for testing thyristors
III. Differences between unidirectional and bidirectional thyristors
I. What is a thyristor?
A thyristor is the Chinese name for the Silicon Controlled Rectifier (SCR) and belongs to the family of power semiconductor devices known as thyristors. Structurally, it uses a four-layer PNPN semiconductor structure and allows high-current, high-voltage loads to be switched on through a trigger signal applied to the Gate terminal. Based on current conduction direction, thyristors can be divided into unidirectional thyristors and bidirectional thyristors, where the former conducts only under forward bias when a trigger signal is applied, while the latter can be triggered to conduct in both directions of an AC waveform. Bidirectional thyristors are commonly known as TRIACs (Triode for Alternating Current), and their internal structure is equivalent to two unidirectional thyristors connected in reverse parallel, allowing control over both the positive and negative half cycles of AC power. A key characteristic of SCRs is their latching behavior: once triggered and the current reaches the holding level, the device remains on even if the Gate signal is removed, until the current falls below the holding current or the voltage polarity reverses. TRIACs have a similar latching characteristic, but their triggering behavior is more complex, as they can accept both positive and negative Gate pulses.
II. Methods for testing thyristors
A physical visual inspection is the first step in any test process and should start with checking the device package for obvious mechanical damage such as cracks, burn marks, or loose leads, as these issues often indicate electrical failure. A good thyristor should have an intact package with clear markings that match the specifications.
Static V-I characteristic testing is a common method for making an initial judgment of a thyristor's condition by using a digital multimeter (DMM) or ohmmeter to check the PN junctions and resistance between terminals. For a unidirectional thyristor (SCR), measuring between the Anode and Cathode without triggering the Gate should show a high-resistance state, while the resistance between the Gate and other terminals should exhibit normal PN junction behavior or high resistance; a clear short-circuit reading usually indicates internal damage. For a bidirectional thyristor (TRIAC), when not triggered, measurements between the two main terminals MT1 and MT2 should show high resistance in either polarity, and the PN junctions between the Gate and MT1 or MT2 should appear normal, otherwise the device may be faulty. These tests can be done using the resistance or diode mode on a DMM, but it is important to note that low-voltage static tests cannot fully represent behavior under high-voltage operating conditions.
Dynamic V-I characteristic testing more closely reflects real operating conditions and involves applying controlled voltage and trigger signals to the thyristor using a curve tracer or dedicated test setup, then observing whether the V-I curves under different voltages and trigger conditions match standard characteristics. Dynamic testing allows measurement of key parameters such as trigger voltage, trigger time, holding current, and turn-off behavior, making it highly effective for determining whether the device is operating within its rated specifications, especially in practical design and production quality control.
Temperature testing and thermal stability testing are used to evaluate a thyristor's reliability under different temperature conditions and its stability during long-term operation under load. Key indicators of thermal performance include changes in trigger voltage, latching behavior, and thermal drift at elevated temperatures. By placing the device on a controlled heating platform and gradually increasing the temperature while monitoring electrical characteristics, it is possible to assess thermal stability and identify potential thermal breakdown risks. These tests are commonly used in R&D and high-reliability application validation.
III. Differences between unidirectional and bidirectional thyristors
Unidirectional and bidirectional thyristors differ fundamentally in both structure and function.
A unidirectional thyristor is built from four layers of semiconductor material and has a clearly defined conduction direction, allowing current to flow only from the anode to the cathode. It effectively acts as a controlled DC switch and is mainly used in DC circuits or for half-wave control in AC circuits. In schematic symbols, it is typically shown as a triangle arrow pointing toward the cathode line.
A bidirectional thyristor, on the other hand, can be physically understood as two unidirectional thyristors integrated in a reverse-parallel configuration, with no fixed anode or cathode, only two main terminals, usually labeled T1 and T2, along with a Gate. Its defining feature is that regardless of the voltage polarity between the two main terminals, as long as the Gate receives a trigger pulse of the appropriate polarity, the device can conduct in either direction. Because of this, bidirectional thyristors are well suited for full-cycle AC control applications such as light dimming, AC motor speed control, and solid-state AC switching, and they often simplify circuit design. Their schematic symbol is typically drawn as two opposing triangle arrows connected in reverse parallel.
IV. Conclusion
Evaluating the condition of a thyristor requires more than just a visual check; it also involves combining static and dynamic electrical tests with temperature and thermal stability assessments to form a complete picture of device performance. For unidirectional thyristors, which are core components in DC or single-direction AC control, testing focuses on forward triggering and conduction behavior, while bidirectional thyristors must be tested and triggered in both directions. Understanding the fundamental structural and operating differences between unidirectional and bidirectional thyristors helps engineers choose the right device and apply the correct testing strategy in real-world applications, ultimately improving system reliability and product quality.
