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I built a stick welder using two microwave transformers and a DC bridge rectifier. To control the heat, I created a triac current regulator attached to the AC input of the transformers. I have used this specific circuit for a long time with these transformers however I got an overheat blow out of my BTA41-600B triac last time (the triac physically caught fire). I ordered 4 new ones, and they seem to be instantly failing with the load terminals stuck open. If I allow full current to pass through the triac, it doesnt break, however when I try to regulate the current, it instantly stops regulation and stays shorted. Is there something wrong with my design? And if so, why are these the only triacs being affected?

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ Counterfeit parts are common. Make sure you are purchasing from a reputable distributor. \$\endgroup\$ – rdtsc Jul 18 '17 at 1:03
  • \$\begingroup\$ Can you provide a link to the source? \$\endgroup\$ – Russell McMahon Jul 18 '17 at 11:01
  • \$\begingroup\$ ebay.com/itm/4pcs-STMicroelectronics-BTA41-600B-Triac-600V-40A/… \$\endgroup\$ – jakerz Jul 19 '17 at 20:57
  • \$\begingroup\$ It doesn't seem that the seller is aware of any issues and offered a return. \$\endgroup\$ – jakerz Jul 19 '17 at 20:58
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    \$\begingroup\$ I would not have believed it if I had not seen it with my own eyes, but the following listing item title actually appears on a usual suspects ecommerce site: "BTA41-600B [absolutely original ic] fake a compensable special store diy electronic ic component" \$\endgroup\$ – Chris Stratton Jul 21 '17 at 19:06
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These were most likely counterfeit, I ordered a new batch directly from the company and they worked perfectly.

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