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December 9th, 2018:

Lazy LED Bulbs

Some months after moving into this house at the end of 2015, we went on a sweep and replaced nearly all of the incandescent bulbs with 2700K LED units. The drop in power usage was obvious from our monthly bills. However, I’ve had a whole lot of bulb failures within those three years. Some bulbs, in fact, didn’t even last out their first year. So much for 25,000 hours of service.

I’ve done teardowns on four or five dead bulbs, and found both dead power supplies and dead LED wafers. One of the power supplies was intermittent: Tap on it with the plastic handle of a screwdriver, and it will suddenly light its wafer again. Tap on it some more, and eventually the light will go out. The solder joints looked fine under my digital microscope. I even reflowed a few of them, but the unit’s behavior did not change.

However, the oddest failure mode we’ve experienced is this: bulbs that take longer and longer to illuminate after you flip the switch.

We have four ceiling can fixtures in our kitchen. During our LED sweep, we replaced the 75W incandescent floods with 75WEQ LED floods. A few months ago, I noticed that one of the bulbs took five or ten minutes to come on after flipping the wall switch. Once lit, the tardy bulb shone at identical light levels as the immediate bulbs did. Before it lit, it remained completely dark. (I.e., it spent no time at partial brightness.)

Well, as the months rolled on, the tardy bulb grew tardier and tardier. When it was up to about half an hour delay, a second bulb in the group of four started coming on late. A month or two after that, a third bulb in the group began delayed illumination. By that time, the first bulb would take almost two hours to light up. However, in every single case, all four bulbs eventually came up to full illumination from full dark.

I admit that once the second bulb started acting up, I put off replacing them to see what would happen. Yesterday I got tired of it, and replaced all four (even the one that still lit up immediately) with identical EcoSmart 75WEQ floods from Home Depot. The new floods produce five more lumens than the old ones but only draw 11.5 watts. (The originals drew 15 watts.)

I’m trying to figure out what sort of electrical failure would cause this. When time allows, I’m going to remove the plastic envelope from the original malefactor and take a close look. All of the bulbs I’ve cut open have used switching power supplies built into their bases. There is another kind of LED power supply: a capacitive voltage drop/rectifier system. (Wiki article here. More discussion here.) If the bulb uses a capacitive dropper, the capacitor is probably electrolytic. Electrolytics dry out over time (though it generally takes more than three years) and I’m wondering if poor-quality capacitors are at the heart of the problem. (Bad caps have caused trouble before. And again.) It’s not a time-constant thing, and in truth I don’t know what it might be, but doing a little probing will be fun.

And if any of my EE regulars know or have other (less wild-ass) guesses, I’d sure love to hear them.