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Bad Bug Zapper
About 6 months ago, I purchased an electric bug zapper (“Stinger”, made by Kaz, Inc. in Southborough Massachusetts) for my shop.  Ugly flies buzzing around my head and biting my legs and arms while trying to work were driving me nuts so I thought this product would help knock down the fly population.  I figured I could leave it plugged in day and night and let it zap away.  It was a classic design with a high voltage screen grid inside a plastic case and an ultraviolet lamp, to attract the insects.  

After about 5 months of operation, just about a month ago, I was working in my shop when I heard a funny noise.  I saw out of the corner of my eye a big flash of light, coming from the bug zapper.  I looked over to see a big cloud of smoke and sparks flying several feet in all directions from the it.  By the time I ran over to unplug it, it was already dead.  I was very glad that it had failed when I was in the shop.  The sparks and flames were massive enough that they could have easily started a fire.

I opened up the Stinger to see the extent of the damage.  Based on the R and C numbers on the circuit board silkscreen, there were at least three resistors and a diode, charred beyond recognition. Two power transistors were also cracked and burned. Nearby wires and capacitors were also scorched.   I did see a fuse, at the 120vac input.  It measured an open circuit, so it did limit the arcing.  Still, it seemed to me that the arcing and sparking I saw went on for several seconds, so I wondered why the fuse had not blown sooner, before the catastrophic and violent breakdown occurred. 

It looked like the circuit was broken into two halves.  One half was dedicated as a ballast driver for the UV fluorescent lamp, using a number of high voltage capacitors to limit the lamp current.  The second half generated the high voltage for the bug zapping grid.  The failure occurred somewhere in the HV section.  However, the circuit board charring was so extensive that it was impossible to reconstruct the failure sequence.

“Stinger”  Bug Zapper

The device did carry a UL safety sticker.  But, how could this product get the UL safety listing and fail so violently?  About two decades ago, I pushed a number of products through UL.  Back then, UL required detailed accounting for each and every component.  They wanted failure trees and outcomes.  All parts had to be overrated, so none were pushed to their limit. Component spacing had to exceed their guidelines to minimize arcing. A non-conductive conformal coating was also something that UL liked, whenever the electronics was exposed to the outside.  No such coating was on this circuit board.  Has UL watered down their testing program or was this manufacturer cheating?  Perhaps once they got the UL sticker, they made changes to the circuit, making it cheaper but less safe.  It seemed to me that a product should be designed in such a way that the kind of large, almost explosion like action that I saw, would not be possible.

I wonder how many other products like this are in use around the country? How many house fires had been started by this sort of failure?

Burned Components Charred wires and capacitors

 

Circuit Boards with Conformal Coating


Third Quarter, 2012    

Page 1 Circuit
Diagnosis
Experimenter's
Corner
Good Idea
gone Badly
What the World
needs Now
Wily Widget


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