The polarity and quality of the SCR can be judged with a pointer multimeter or a digital multimeter. Yunnan Changhui Instrument Manufacturing Co., Ltd. separately introduced the use of these two multimeters in the process of measuring the polarity and quality of the SCR.

1. Use a pointer multimeter to check the polarity and quality of the SCR

According to the principle of PN junction, the resistance between the three poles of the thyristor can be measured by the ohmic block “R×10” or “R×100” block to judge whether it is good or bad. There is a PN junction between the control electrode G and the cathode K of the thyristor. Under normal circumstances, its forward resistance is between tens of ohms to hundreds of ohms, and the reverse resistance is generally larger than the forward resistance. Sometimes the measured reverse resistance of the control pole is small, which does not necessarily mean that the control pole has poor characteristics. It mainly depends on whether it meets the characteristics of the PN junction.

2. Use a digital multimeter to check the polarity and quality of the SCR

Judge the electrode digital multimeter of the thyristor to the diode block, connect the red test lead to one electrode, and the black test lead to contact the other two electrodes respectively. If one of them shows that the voltage is a few tenths of a volt, then the red test lead is connected to the control electrode G, the black test lead is connected to the cathode K, and the rest is the anode A. If it shows overflow both times, it means that the red test lead is not connected to the control electrode, and the electrode needs to be replaced and retested.

To test the triggering ability of the thyristor, the digital multimeter is set to the PNP block. At this time, the two E holes on the hFE socket are positively charged, and the C hole is negatively charged, and the voltage is 2.8V. The three electrodes of the thyristor are led out by a wire, the anode A and cathode K lead are inserted into holes E and C respectively, and the control electrode G is suspended. At this time, the thyristor is turned off, the anode current is zero, and 000 will be displayed.

Insert the control pole G into the other E hole. The displayed value will increase rapidly from 000 until the overflow symbol is displayed, and then immediately change to 000, and then change from 000 to overflow again, and so on. This method can be used to determine whether the triggering of the thyristor is reliable. However, the test time should be shortened as much as possible due to the relatively large current in such a test. If necessary, a protection resistor of several hundred ohms can be connected in series on the anode of the SCR.

If the NPN block is used, the anode A of the thyristor should be connected to hole C, and the cathode K to hole E to ensure that the applied forward voltage is. When checking the triggering ability, do not insert the control electrode into the B hole, because the voltage of the B hole is low, and the SCR cannot be turned on.