Manufacturers usually provide extensive data and charts regarding thyristor characteristics for user reference; however, independent testing is sometimes necessary. The following describes a method using a multimeter's ohmmeter function to identify the terminals and determine the condition of the device. The test can be performed as outlined below. For a functional thyristor, the resistance between the anode (A) and cathode (K) should be high; therefore, regardless of the multimeter probe polarity, the measured resistance between A and K should remain high. Additionally, a higher reverse resistance compared to forward resistance between the gate (G) and cathode (K) indicates good device performance.
1) Testing a Unidirectional Thyristor (SCR). Set the multimeter to the R×1Ω resistance range. Use the red and black probes to measure the forward and reverse resistance between any two terminals until you find a pair showing a reading of several tens of ohms. In this state, the terminal connected to the black probe is the gate (G), the terminal connected to the red probe is the cathode (K), and the remaining terminal is the anode (A). Next, connect the black probe to the identified anode (A) and keep the red probe connected to the cathode (K); the multimeter needle should remain stationary. Momentarily short-circuit the anode (A) and gate (G) using a jumper wire; the multimeter needle should deflect to the right, indicating a resistance of approximately 10 ohms. If the needle deflects when the black probe is connected to the anode (A) and the red probe to the cathode (K) [without the short-circuit step], it indicates that the SCR has suffered breakdown damage.
2) Testing a Bidirectional Thyristor (TRIAC). Set the multimeter to the R×1Ω resistance range and measure the forward and reverse resistance between any two terminals; two of the measurement sets should yield an "infinite" reading. If one set shows a reading of several tens of ohms, the two terminals connected to the red and black probes are the first anode (A1) and the gate (G), while the remaining terminal is the second anode (A2). After identifying terminals A1 and G, carefully measure the forward and reverse resistance between them; the terminal connected to the black probe during the measurement yielding the lower reading is the first anode (A1), and the terminal connected to the red probe is the gate (G). Connect the black probe to the identified second anode (A2) and the red probe to the first anode (A1); the multimeter needle should not deflect, indicating infinite resistance. Next, momentarily short-circuit terminals A2 and G to apply a positive trigger voltage to the gate; the resistance between A2 and A1 should drop to approximately 10 ohms. Then, remove the short-circuit connection; the multimeter reading should remain at approximately 10 ohms. Swap the probes: connect the red probe to A2 and the black probe to A1. Again, the multimeter needle should not deflect, indicating infinite resistance. Momentarily short-circuit A2 and G again to apply a negative trigger voltage to the gate; the resistance between A1 and A2 should also be approximately 10 ohms. Remove the short-circuit connection; the multimeter reading should remain unchanged at approximately 10 ohms. If the component behaves according to these patterns, the triac is undamaged, and the polarity of the three terminals has been correctly identified. When testing high-power thyristors, connect a 1.5V dry cell in series with the multimeter's black probe to increase the trigger voltage.
(3) Identifying Thyristor (SCR) Terminals Thyristor terminals can be identified using the following method: First, use the multimeter on the R×1k range to measure the resistance between the three terminals. The two terminals showing low resistance are the gate and the cathode, while the remaining terminal is the anode. Next, set the multimeter to the R×10k range. Hold the anode and the other terminal between your fingers, ensuring the two terminals do not touch. Connect the black probe to the anode and the red probe to the remaining terminal; if the meter needle swings to the right, the terminal connected to the red probe is the cathode; if it does not swing, that terminal is the gate. (8) Applications of thyristors 1) Rectification devices (converting AC to DC) Variable voltage power supplies, constant voltage/constant current sources, DC motor power supplies. 2) Inverter devices (converting DC to AC power) High frequency precision, stable output under varying loads; constant-frequency power supplies, uninterruptible power supplies (UPS), AC motor power supplies. 3) Frequency conversion devices (a combination of rectification and inversion) used for speed control of synchronous and asynchronous motors. 4) DC motor speed control (can replace DC generators) High efficiency, fast response, compact size, and light weight.
