testing a thyristor (SCR)


Thread Starter


Can anyone of you explain me how to find whether an scr (i.e thyristor which has a anode, cathode and a gate) is in good condition or not. This has to be found out by using a multimeter or any other simple tools. with thanks, veda india [email protected]

Alan Rimmington

The easy way to test an scr is to use a DC source such as a 9v battery, and a filament lamp. Connect negative from battery to cathode, connect positive to lamp, then other side of lamp to anode. At this stage the lamp should be off. Briefly connect anode to gate and the lamp should light. It will be necessary to disconnect the battery to turn the lamp off. The scr has failed if the lamp is on all the time, or will not come on when the gate is connected to the anode. With large scr it might be necessary to increase the voltage being used to provide a big enough cick to get it to work, also if the lamps current is less than the scr hold on current then the lamp will go out when the gate is disconnected from anode, hope this helps and is clear enough, if you want a simple diagram email me [email protected]

Kevin Hammond

One more thing to consider - Some large SCRs have to be "squeezed" to make them work. These are the type that are mounted between bus bars. All of the ones that I have seen have been disk shaped, about 5-6 cm in diameter and about 2 cm thick. Hope this helps. Kevin Hammond Electronics Engineer Norfolk Southern Corp. Charlotte, NC

Ricardo Ribeiro

Dear Mr. Veda, We did a very usefull device which consist of a transformer, diode bridge, a incadescent lamp and two Push-buttons . All this stuff was installed in a plastic box found in any "next door shop". Basically we get 110VAC through a transformer to something around 12 volts, rectify it and use this to feed anode and catode in series with the lamp and the NC Push-button. Wire a NO push-button to supply 12 vcc to the gate. Press the gate PB and the light will come on and stay this way until the NC Push-button is pressed. It has been working for 10 years for all the SCR's we have down here. Regards Ricardo Ribeiro Alcan Aluminio do Brasil Ltda

Colin Walker

Most semiconductor texts will give you information on Thyristor/SCR testing. The usual test uses a battery, lamp, resistor and switch, this usually works but not always! Are any of the operating parameters are close to the maximum device specifications? If so you may need a more elaborate test setup. What's the SCR used for? What's the device manufacturer's part number? Regards Colin

Ricardo Ribeiro

Mr. Hammond, You are absolute right! I forgot to say that all SCRs are tested with their heatsink. Regards, Alcan Aluminio do Brasil Ltda. Ricardo Ribeiro

Alfonso Padilla

What about reverse polarity testing? I've found after several years that some SCR's leak when reverse biased. Therefore I rather prefer to test them by using a 9-12 V secondary voltage transfomer connected in series with the lamp and the SCR. Then connect a SPST switch to cathode with the other end to a 500 to 1 kOhm resistor in series with a diode (1N4000 series), with diode cathode pointing to SCR gate. This branch triggers the gate when the SPST switch is closed. This circuit tests both forward and reverse operation of the SCR. Lamp glow should be of course about 40-50% since the SCR only conducts when the cathode is positive and the gate remains triggered . If you bridge the SCR's anode and cathode, the lamp should go brighter.
In the classroom, where CMOS static is not a problem), for a SCR or GTO, the Onmeter diode test is performed twice. With the gate open, the device reads hi-hi in both directions.
Then the gate is connected to the cathude (or anode) and the test is done again with a hi in one direction; a low in the other. For triacs, a lo-lo reading is normal.