<back story>
A few months ago, we picked up a Xytronic IR610 quartz preheater at work. Recently, I populated a board and put it on it to soak at 100C. I stepped out of the room for a few mins (with someone else there to check on it), and a few mins later, they came to me and said there was a problem. Well, the board was toast, burnt to a crisp, with the now-black epoxy having bubbled out from within the board...
It didn’t take me any time at all to determine what happened: the preheater, despite being set to 100C, was heating at full blast (800W) continuously. </back story>
I opened the controller (which is basically a lamp dimmer with digital controls; there’s no temperature feedback) and quickly my suspicions were confirmed: the triac had failed short. No visible damage on it or any other component. I ordered a replacement triac, installed it, and the problem was fixed. I turned it back off to redo the wiring properly (to add ferrules, zip ties, etc), did that, and then turned it on again, only to discover the fault was back. The triac had failed again.
Clearly, there’s a bigger problem here. Can anybody give me some hints on to what to look for? The triac is mounted onto a heatsink which itself is bolted onto the steel housing, and it did not appear to get hot (neither before nor after failure). The triac is a type (BTA12-600CW) with an insulated tab, so no unintentional shorts to ground. The 800W of the heater is under 4A at 230V, so well below the 12A rating of the triac. I don’t think ESD damage is to blame, since the room is a fully ESD-safe lab, and I took all the usual precautions (not to mention that the original triac failed while within the device). What are things that cause triac failure?
I’m hoping there’s something simpler I can fix so we don’t have to ship the thing away for repair.
Thanks everyone!