What could cause an LED to dim in just days?

I have a fixture which normallly takes a PAR16 50-watt halogen bulb, 120 V AC. The fixture is connected to a touch dimmer that has three settings of low, medium and high.

I put in a HR16 LED bulb, which took some searching to find. The bulb has its lowest brightness at the low setting and more brightness at the medium setting. The highest setting oddly does not affect the brightness.

The light has been on 24/7. In just a few days, I noticed the LED was growing dimmer. I have a light meter and was able to observe it for several more days. The LED is rapidly dimming, losing several foot-candlepower per hour!

The situation is reproducible on a second LED bulb from the same manufacturer and one from a different manufacturer.

The first two LED bulbs consist of several LEDs enclosed behind a glass cover. The glass cover started out clear, but in just hours began to fog up. I also noticed that it was actually hot to the touch. The fogging up effect appears to have permanently altered the glass cover.

The third bulb did not get hot at the top where the bulb is, but when I removed it, I noticed the base was very hot.

I tried the supplied halogen bulb, which works at different brightness levels for all three settings, and it does not dim at all.

What could the fixture be doing to selectively kill LEDs bulbs in what would probably be less than a week? Could the fixture somehow be oversupplying the LED with too much current?

I was told dropping brightness usually means the LED is overheating; I guess the LED on yours was getting heatsoaked. either it is not well connected to the heatsink or the heatsinking is just not designed well enough to be turned on for so long.

Can you provide a link to the exact bulbs you used?

Serious overdriving, poor heatsinking or low air circulation could definitely do it. I'm curious, did you notice any tint shifting?

The first and second bulbs are this one:

http://cgi.ebay.com/HR16-20-LED-Warm-White-110-120V-SPOTLIGHT-Bulb-4-Solar_W0QQitemZ4398682111QQcategoryZ20706QQssPageNameZWD1VQQrdZ1QQcmdZViewItem

The third bulb is the white version of

http://www.superbrightleds.com/specs/e27-w8.htm

I'm curious, did you notice any tint shifting?

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I don't think so. They both seem to keep the same kind of whiteness, but just grow dimmer and dimmer.

the thing was overheated to the max because i believe, those thing that you poke the bulb into is enclose. so even if your led has heat sink and heat sinking fins, it will nvr cool as no wind is there to "carry" away the heat.(wonder if they work in aircon rooms )

Over driving and overheating are LEDS worst enemies

I'm not exactly clear on something. What exactly does "overdriving" mean? I can certainly get an incandescent bulb in 60 watt, 75 watt and 100 watt varieties. I'm not the least worried about putting the 60 wattt bulb where I'd put the 100 watt one. Or for that matter, a 4 watt night light where I'd put a hair dryer, so what's preventing a circuit from blowing out the night light?

And of course, these LEDs are built with standard bases that fit right into this particular fixture. So where else are they going to be used where this wouldn't happen? Are you suggesting, for example, that LEDs are useful only in refrigerators?

Or is just the case that these particular LEDs are junk?

You can't use a dimmer on AC powered LED lights! They usually use a capacitor to limit the current to the LED array. When you alter the waveform (normal AC dimmers 'chop' the sinusoidal waveform in pieces) this method of current limiting fails and the LED array will fail due to very large peak currents.

Never use dimmers on LED replacement bulbs (and CFLs....)!!

What could cause an LED to dim in just days?

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Getting used to it. I swear all my lights get dimmer right before i buy a newer, better, and brighter one, its a phenomenon.......

I've read several places where the AC LED fixtures say don't use them on a dimmer.

What exactly does "overdriving" mean?

It means that the LEDS are pushed past their limits, Possibly by the driver circuit inside the bulb, some Flashlight manufatures do this to advertise a higher lumen output for their product, it may look good at first very bright? but for how long