Determination of polarity of thyristor

Unidirectional thyristor is composed of semiconductor materials with three PN junctions.

A thyristor has three electrodes: anode (A), cathode (K), and control electrode (G). From the perspective of equivalent circuit, there is a PN junction between the anode (A) and the control electrode (G) with two opposite polarities connected in series, and a PN junction between the control electrode (G) and the cathode (K). Based on the unidirectional conductivity of the PN junction, select the appropriate resistance range for the pointer multimeter and test the forward and reverse resistance between the poles (the two resistance values measured by swapping the probes of the same two poles). For a normal thyristor, the forward and reverse resistance between G and K varies greatly; G. The difference in forward and reverse resistance between K and A is very small, and their resistance values are both very large. This test result is unique, and based on this uniqueness, the polarity of the thyristor can be determined. Use a multimeter R × 1K to measure the forward and reverse resistance between the thyristor electrodes, select the two electrodes with a large difference in forward and reverse resistance. In the measurement with the smaller resistance value, the black probe is connected to the control electrode (G), the red probe is connected to the cathode (K), and the remaining electrode is the anode (A). By determining the polarity of the thyristor, the quality of the thyristor can also be qualitatively judged. If the forward and reverse resistance between any two poles in the test is controlled by two types of thyristors: unidirectional thyristors and bidirectional thyristors, both have three electrodes. Unidirectional thyristors have a cathode (K), an anode (A), and a control electrode (G). A bidirectional thyristor is equivalent to two single-phase thyristors connected in reverse parallel. One unidirectional silicon anode is connected to the other cathode, and its lead end is called T2 pole. One unidirectional silicon cathode is connected to the other anode, and its lead end is called T2 pole. The rest is the control pole (G).

1. Discrimination between single and bidirectional thyristors: First, measure both poles. If the forward and reverse pointers do not move (R × 1 gear), it may be A, K or G, A pole (for unidirectional thyristors) or T2, T1 or T2, G pole (for bidirectional thyristors). If one of the measurements indicates tens to hundreds of ohms, it must be a unidirectional thyristor. And the red pen is connected to the K pole, the black pen is connected to the G pole, and the rest is the A pole. If both the forward and reverse test indications are tens to hundreds of ohms, it must be a bidirectional thyristor. Turn the knob to R × 1 or R × 10 and retest. If there is a slightly larger resistance value, connect the red pen to the G pole, connect the black pen to the T1 pole, and connect the remaining to the T2 pole.

2. Performance difference: Turn the knob to R × 1 gear. For 1-6A unidirectional thyristors, connect the red pen to the K pole and the black pen to both the G and A poles. While keeping the black pen in the A pole state, disconnect the G pole. The pointer should indicate several tens of ohms to one hundred ohms. At this point, the thyristor has been triggered and the triggering voltage (or current) is low. Then instantly disconnect the A pole and reconnect it. If the pointer should return to the ∞ position, it indicates that the thyristor is in good condition. Set the multimeter to 1K and test the forward and reverse resistance values of the three pins of the thyristor in pairs. Record the time with the largest difference in forward and reverse resistance. The black probe is connected to the control electrode, the red probe is connected to the cathode, and the other pin is the anode.