General > General Technical Chat
The "magic Smoke" LED light
amyk:
I'll take a cap dropper (in a metal bodied lamp --- nothing to burn, just don't touch it when on...) over that ridiculous overengineered mess.
To make matters worse they potted it which likely increased temperatures to the point of failure.
innkeeper:
i wonder if the potting material is flammable.
rdl:
--- Quote from: Red Squirrel on September 15, 2017, 11:27:39 pm ---They should maybe start using fire retardant filler in these things. If it does catch on fire it will just foam up and smother the fire.
--- End quote ---
Seeing how there's just the one spot that's burnt and the whole thing didn't go up in flame, it may be that it's a type of plastic that doesn't support combustion. Might be interesting to try and set fire to the plastic from the teardown sample.
tom66:
Have seen an interesting failure mode on the SMPS power supplies at my old workplace.
During testing, we had one power supply burn up after being subject to excessive load for 48 hours (below the current limit trip at high-line voltage of 253V AC). We couldn't figure out what happened but essentially the phenolic PCB and the MOSFET had got very very hot and burned up a huge section of the board. The MOSFET was shorted and the fuse was popped, but as far as we could tell, that wasn't the cause of the burning.
Later, we were able to replicate the failure. Under high temperature the power supply controller could overheat and fail, but when it failed its output didn't go to zero. It sat around 2-3V, which was provided by the Vcc charge circuit (the MOSFET was not switching at this point.) So the MOSFET was being continuously biased with low gate voltage when it was supposed to be off, so it was conducting current continuously. This wasn't enough current to blow the fuse, but was enough to make stuff very very hot and start burning stuff in the area. Eventually, the MOSFET would succumb to short-circuit failure due to overheating, and pop the fuse, but that wouldn't happen for some time. I wonder if something similar happened in your case.
innkeeper:
Interesting. I did notice a "Buss" branded device in series with the input, and assumed it was a fuse, but the markings were not that clear, i'll look it up at some point but.. was surprised that it didn't blow that fuse.
what your describing would explain that for sure.
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