@GregR posted:...
I decided to open one up and see what's inside. There are 3 components: bayonet base, 8mm "straw-hat" type LED, and a 300 Ohm resistor.
What's making these bulbs fail? They are installed in lamp posts running at a steady 10 VAC using a postwar ZW. Is it just my bad-luck? Is it a defective lot of bulbs? Is there an inherent problem such as reverse-voltage breakdown, overheating on pulsed AC, or an incorrectly sized resistor?
That you identified the resistor as being 300 Ohms tells me you probably have a meter? Your photo suggests the resistor is in good shape (not charred/toasted) but a simple meter test will confirm that.
I'd say the failure is because they did NOT install a 1-cent diode to protect the LED from reverse-voltage breakdown from applied AC voltage.
In this recent OGR post regarding a wedge-style plug-and-play LED replacement for an incandescent, they include the diode. I'd contact the manufacturer and refer them to this thread! In bulk the diode will cost them less than a penny.
Separately, and I realize you're running at "only" 10V AC, but a 300 Ohm resistor is suspicious. Your photo shows a 1/2 Watt LED. Let's say it's operated at half of that since it's AC...so 1/4 Watt. The math is a little more involved with AC operation but if operating at its rated 18V and dropping that down to ~3V as per a typical white LED...that means the resistor is absorbing several times the power of the LED and that looks like only a 1/4W resistor.
