TRIAC failures

[ke4pjw]

It appears that I have had a couple of TRIACS partially fail in my Propeller controlled Christmas lights display this year. They are failing in a partial "on" mode. Has anyone ever seen something like that? I would have suspected that they would fail in a catastrophic manner and be "full on", however it appears that about 60 volts are present. I have verified that there is no moisture on the boards and that electrically everything is correct. When the TRIACS are triggered, they do turn "on" properly. It's very strange. Any comments from people familiar with TRIAC failure modes would be appreciated.

Thanks,
Terry

[Phil Pilgrim (PhiPi)]

Are you dead certain they're not getting triggered somehow -- e.g. at the peaks of the 60 Hz cycles, due to noise?

-Phil

[frank freedman]

Can you post the circuit?

F

[ke4pjw]

I am fairly certain. I removed power to the optoisolator and other circuity that drive the TRIAC. It was still triggered. I am seeing several threads about others that purchased TRIACS from ST last year that are failing in a similar way. Ug :( I didn't realize Mouser sold junk, but I suppose it's not their fault. They are the middle men. It's just a very strange failure mode.

[ke4pjw]

Can you post the circuit?

F87759

Here is the relevant portion of the circuit. I have 6 boards in operation just like this one.

[Leon]

It might be worth contacting ST Support about the problem, especially as other people are experiencing it.

[frank freedman]

I am fairly certain. I removed power to the optoisolator and other circuity that drive the TRIAC. It was still triggered. I am seeing several threads about others that purchased TRIACS from ST last year that are failing in a similar way. Ug :( I didn't realize Mouser sold junk, but I suppose it's not their fault. They are the middle men. It's just a very strange failure mode.

Be interesting to see what the waveform looks like on a scope, see if the triac is conducting on one half of the cycle when not triggered.

Frank