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How to distinguish between bidirectional and unidirectional SCR
The thyristor is equivalent to a diode that can be controlled. When a certain voltage is applied to the control pole, the cathode and anode are turned on. The controllable silicon is divided into two kinds, which are three electrodes. The unidirectional thyristor has a cathode (K), an anode (A), and a control pole (G). The bidirectional thyristor is equivalent to two single thyristors in reverse parallel. That is, one of the one-way silicon anodes is connected to the other cathode side, its leading end is called T2 pole, one of the one-way silicon cathodes is connected to the other anode, its leading end is called T2 pole, and the rest is the control pole (G). 1. Discrimination of single and bidirectional thyristor: measure two poles first. If the positive and negative measurement pointers are not moved (R× 1 gear), it may be A, K or G, A pole (for unidirectional thyristor) or T2, T1 or T2, G pole (for bidirectional thyristor). If one of the measurement instructions is tens to hundreds of ohms, it must be a one-way 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 pole. If the positive and reverse measurement instructions are tens to hundreds of ohms, it must be a two-way thyristor. Then turn the knob to R× 1 or R× 10 gear for retest, one of which must have a slightly larger resistance value, then the slightly larger red pen is connected to G pole, the black pen is connected to T1 pole, and the rest is T2 pole.
Difference in performance: set the knob to R× 1 gear. regarding the 1 ~ 6A unidirectional SCR, the red pen is connected to the k pole, and the black pen is connected to the g and a poles at the same time. disconnect the g pole under the condition that the black pen does not leave the pole. the pointer should indicate tens of ohms to 100 ohms. at this time, the SCR has been triggered and the trigger voltage is low (or the trigger current is small). Then instantly disconnect the pole and then connect it again, and the pointer should return to the ∞ position, which indicates that the SCR is good.
Regarding the 1 ~ 6A bidirectional thyristor, the red pen is connected to the T1 pole, and the black pen is connected to the G and T2 poles at the same time. The G pole is disconnected on the premise that the black pen does not leave the T2 pole. The pointer should indicate tens to more than 100 ohms (depending on the magnitude of the thyristor current and the manufacturer). Then the two strokes are switched and the above steps are repeated. The pointer indication is slightly larger than the previous one by more than ten to tens of ohms, which indicates that the SCR is good and the trigger voltage (or current) is small. If you insist on turning on the pole or T2 pole and turn off the G pole, the pointer will immediately return to the ∞ position, indicating that the SCR trigger current is too large or damaged. According to the method of fig. 2, regarding the unidirectional thyristor, when the switch k is closed, the lamp should be on, and when the k lamp is turned off, it will still not be extinguished, otherwise the thyristor will be damaged.
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