Is there anyway that one can tell with a multimeter if a device is a SCR or a Triac.? I am referring to high power devices ex large computers with only company(HP, Burroughs etc.) part numbers on them. Sorry for the dumb question.

 

An SCR can only conduct one direction (Cathode Negative, Anode Positive) when triggered and a Triac can conduct either direction when triggered.

 

Are the SCRs stud mount or Hockey Puck types? If Hockey Puck, they need to be clamped between heat sinks at a specified pressure in order to assure full current capacity. Some will not even conduct unless compressed.

 

The devices are stud mounted. Probably rated for about 20-25 amps (on a heatsink). With a multimeter, one probe on the gate, there is a reading to both anode and cathode. With the probes reversed there is nothing.
This reading is the same on another unit known to be a Triac and another (flat pack d106 scr) So the meter tells me the units are not shorted or open ciciut, but not what it is!

 

Do you have a current limiting power supply available? If so, set the current limit to about 500ma and connect Positive to the heavy lead, Negative to the stud and with a 100 ohm limiting resistor in series with it, momentarily touch the gate lead to the positive power terminal. Does the unit fire and latch? Now, reverse the power leads (+ to stud, - to heavy lead) and again, momentarily touch the current limited gate lead to the positive power lead. Does it fire and latch? If it only latches in one direction, it is an SCR and you have identified cathode and anode. If it fires in both directions, it is a triac. If you don't have a current limiting power supply you can use a 12v battery and an automotive light bulb for the test.

On smaller SCRs and Triacs, a multimeter may have enough current on the low ohm scale to fire and latch the device. Larger devices usually have higher minimum holding currents than the meter can supply, therfore an external power supply or battery with load is needed.

 

Hi Bill, yes I do have a current limiting power supply and will do the test as you suggest. Thank you. (In retrospect it is actually very simple. But like Warren Buffet says " the rear view mirror is always clearer than the windscreen")
Thank you again.

 

Hope it works out for you. I've been testing SCR's like that for years. About the only things it does not check for are the ability to respond and latch to a short trigger pulse and the possibility of PRV breakover..