Here is a wonderful article written by my good friend Jestine Yong.
Testing SCR (silicon controlled rectifier) can be done by using an analog multi meter or specialize tester (such as the Peak electronic atlas component analyzer) designed to check semiconductor devices easily. SCR can be found in many electronic circuits. Part numbers such as the FOR3G and MCR 100-6 were very common used in computer monitor. Some called SCR as thyristor but in actual fact the word thyristor should not be associated exclusively with the silicon controlled rectifier. It is in fact a general name given to all four layer PNPN devices including the commonly used SCR. The diac, the Triac, and the SCS are the other popular devices belonging to the family of thyristors.
SCR consists of three pin of Gate (G), Anode (A) and Cathode (C). In order to identify the pin out, one must find it from semiconductor data book such the famous Philips ECG master semiconductor replacement guide. The data book will list out the general specification of the SCR such as the volt and ampere. If you want to know more details about a particular SCR, you can always try to search from the internet. Usually the SCR manufacturers will provide the full datasheet for those who want it.
Once you know the pin outs of the G, A and C legs you can begin to test the SCR. If you have the Peak electronic atlas component analyzer tester, what you need to do is to connect the three small clips to each pin of the SCR (any part number will do). The tester will begin to analyze the SCR and prompt you with the display such as "Sensitive or low power thyristor" before it tells you the exact pin outs of G, A and C. After the first test, the tester will eventually show you the answer at the LCD display. Red is Gate, Green is Cathode and Blue is Anode. It is a simple process and you will know the answer in less than 10 seconds. If there is a problem in the SCR, the tester would not be able to show the results instead it shows a shorted reading.
If you don't have this tester for checking SCR, I'm showing you another easy way on how to test SCR fast. You need an analog meter set to X1 ohm. Place the red probe to the Cathode and black probe to the Anode pin. At this time the meter doesn't show any reading. Now gently move the black probe and touch the Gate pin (the black probe still touching the Anode pin) and you will notice the meter's pointer will kick as shown at the picture (low resistance).
Removing the black probe from the GATE pin (the black probe still touching the Anode pin) you would noticed that the resistance continues to be there (low resistance). This is due to the conduction of SCR as the meter battery is usually able to supply current more than the holding current. If at this stage you removed the black probe from the Anode pin and connect it back, the pointer will dropped back to infinity (high resistance). If the SCR could hold the resistance then the SCR is considered good. If it can't hold then the SCR is faulty.
Conclusion- Practice testing SCR more often to see how's the result like. Try some different part numbers and power SCR-and if the resistance don't hold using X1 ohm, you may try X10 ohm and etc.
Jestine Yong is a electronic repairer and a writer, for more information how you can test electronic components like a professional please visit his website by clicking here.
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