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ke4pjw
12-18-2011, 03:10 AM
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)
12-18-2011, 03:36 AM
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
12-18-2011, 05:46 AM
Can you post the circuit?

F

ke4pjw
12-18-2011, 06:18 AM
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
12-18-2011, 06:30 AM
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
12-18-2011, 01:06 PM
It might be worth contacting ST Support about the problem, especially as other people are experiencing it.

frank freedman
12-19-2011, 04:43 AM
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

cobra6romeo
06-24-2013, 06:47 PM
I have seen this failure many times it happens when a overvoltage situation has occurred (usually lightning). It fails on one side or the other because the strike is polar one side of the TRIAC conducts without damage however the other is destroyed in the process. You can fix this by adding a bi-directional transorb from the high voltage side (not the load) to the gate. Choose your tranzorb so that it only turns on when higher than normal voltage is present. When the transorb turns on it will activate the gate and protect the TRIAC. The load in this case your christmas lights will get hit with the lightning. They may or may not survive but your controller will be ok. Incandescent lights will probally survive but leds will surely die.

Loopy Byteloose
06-25-2013, 08:31 PM
@cobra6romeo

Awesome insights into the problem. Great input. Please stick around and enjoy the Parallax Forums.