The internationally recognized name for a thyristor (SCR) is Thyristor. It operates under high voltage and high current conditions and has advantages such as high voltage withstand capability, large capacitance, and small size. It is a high-power switching semiconductor device widely used in power and electronic circuits. 1. Characteristics of Thyristors Thyristors are divided into unidirectional and bidirectional thyristors. A unidirectional thyristor has three leads: anode A, cathode K, and gate G. A bidirectional thyristor has three leads: first anode A1 (T1), second anode A2 (T2), and gate G. A unidirectional thyristor can only be triggered to conduct when a positive voltage is applied between the anode A and cathode K, and simultaneously a required positive trigger voltage is applied between the gate G and cathode. At this time, A and K are in a low-resistance conducting state, and the voltage drop between anode A and cathode K is approximately 1V. After a unidirectional thyristor is turned on, even if the controller G loses its trigger voltage, the thyristor will remain in a low-resistance conducting state as long as a positive voltage is maintained between the anode A and the cathode K. Only when the voltage at anode A is removed or the polarity of the voltage between anode A and cathode K changes (AC zero crossing) will the thyristor transition from a low-resistance conducting state to a high-resistance cutoff state. Once the unidirectional thyristor is cut off, even if a positive voltage is reapplied between anode A and cathode K, a new positive trigger voltage must be applied between the controller G and cathode K for it to conduct again. The on and off states of a unidirectional thyristor are equivalent to the closed and open states of a switch, and it can be used to make a contactless switch. For a bidirectional thyristor, regardless of whether the applied voltage polarity is positive or negative, as long as a trigger voltage of different polarities is applied between the controller G and the first anode A1, it will be triggered to conduct in a low-resistance state. At this time, the voltage drop between A1 and A2 is approximately 1V. Once a bidirectional thyristor is turned on, it will continue to conduct even without a trigger voltage. The bidirectional thyristor will only turn off when the current at the first anode (A1) and the second anode (A2) decreases to less than the holding current, or when the polarity of the voltage between A1 and A2 changes and there is no trigger voltage. In this case, it will only turn on again when a trigger voltage is applied. 2. Testing a unidirectional thyristor. Use a multimeter set to the R*1Ω resistance range. Use the red and black probes to measure the forward and reverse resistance between any two pins until you find a pair of pins with a reading of tens of ohms. The pin with the black probe is the control electrode (G), the pin with the red probe is the cathode (K), and the other pin is the anode (A). Connect the black probe to the identified anode (A) and the red probe to the cathode (K). The multimeter pointer should not move. Briefly short-circuit anode (A) and control electrode (G) with a short wire. The multimeter pointer should deflect to the right, and the resistance reading should be around 10 ohms. If the multimeter needle deflects when anode A is connected to the black probe and cathode K is connected to the red probe, it indicates that the unidirectional thyristor is damaged. 3. Testing a bidirectional thyristor. Using the multimeter on the R*1Ω range, measure the forward and reverse resistance between any two pins using both the red and black probes. Two sets of readings should be infinite. If one set is tens of ohms, the two pins connected to the red and black probes in that set are the first anode A1 and the control electrode G, and the other unconnected pin is the second anode A2. After identifying A1 and G, carefully measure the forward and reverse resistance between A1 and G. The pin connected to the black probe in the measurement with the relatively smaller reading is the first anode A1, and the pin connected to the red probe is the control electrode G. Connect the black probe to the identified second anode A2 and the red probe to the first anode A1. At this point, the multimeter needle should not deflect, and the resistance should be infinite. Next, momentarily short-circuit terminals A2 and G with a jumper wire to apply a positive trigger voltage to terminal G. The resistance between A2 and A1 should be approximately 10 ohms. Then disconnect the jumper wire between A2 and G; the multimeter reading should remain around 10 ohms. Swap the red and black probes, connecting the red probe to the second anode A2 and the black probe to the first anode A1. Again, the multimeter pointer should not deflect, and the resistance should be infinite. Next, momentarily short-circuit terminals A2 and G again with a jumper wire to apply a negative trigger voltage to terminal G; the resistance between A1 and A2 should also be approximately 10 ohms. Then disconnect the jumper wire between A2 and G; the multimeter reading should remain unchanged at approximately 10 ohms. If these patterns are observed, the tested bidirectional thyristor is not damaged, and the polarity of the three pins has been correctly determined. When testing higher-power thyristors, a 1.5V dry cell battery should be connected in series with the black probe of the multimeter to increase the trigger voltage. 4. Thyristor (Silicon Controlled Rectifier) Pin Identification The pin identification of a thyristor can be done as follows: First, use a multimeter on the R*1K range to measure the resistance between the three pins. The two pins with the lower resistance are the control electrode and cathode, respectively, and the remaining pin is the anode. Next, set the multimeter to the R*10K range. Hold the anode and the other pin with your fingers, without letting them touch. Connect the black probe to the anode and the red probe to the remaining pin. If the meter needle swings to the right, the pin connected to the red probe is the cathode; if it does not swing, it is the control electrode.
