Failed cheap LED lights, bad caps?

[jack black]

Here is a deal. I purchased a number of cheap LED lightbulbs on ebay at prices averaging near $1 (good sniping skills). I know, too good to be true. I used them for 3-4 months now (not continuous use of course) and while some are holding well, I had high mortality rate with one particular designs with a small regulated power supply inside.

They look like the one in the middle (4xLEDs spotlight, 4W total):


The failure mode is initial flashing when turned on, then they go very dim. I'm thinking power supply failure. Anyone had experience with bad caps in LED lights?

I will post some pictures later when I take them apart.

Thanks!

BTW, This picture is from another forum (mine have the power supply wrapped by heat shrink):

[jack black]

Re: Failed cheap LED lights, bad caps?

OK, additional info.
I took apart both good and bad lightbulbs. The good ones have power supply feeding about 12.5V to a series of 4 LEDs, about 3V per LED.
The bad ones with LEDs dimly lit have power supply running at 25-28V. This is weird and I did not expect such high reading. Furthermore, most of that voltage goes to one LED (failed one?) with other ones getting about 2V per LED.
I thought bad power supplies with failed caps produce 0V? Why such high voltage? Did it fry one of the LEDs?

BTW, if I posted in a wrong forum, please let me know.

EDIT: I just read about constant current design of LED power drivers. So the increase in voltage would be consistent with blown LED, right?

[Flyingdutchman]

Re: Failed cheap LED lights, bad caps?

So the increase in voltage would be consistent with blown LED, right?

Yes.

High power LEDs usually fail because of overheating. This can be due to improper assembly where the LED has not been correctly attached to its heat sink, but also to a flaw in the LED itself - most LEDs use the leadframe to conduct heat away from the die (chip) and if the die is not correctly bonded to the leadframe it will overheat.
LEDs are much more sensitive to overheating than say, power transistors. The latter may sustain die temperatures of 200 deg. C and survive, while LEDs will get serious reliability issues with die temperatures as low as 100 deg. C.

When LEDs fail because of overheating they usually develop excessive series resistance, hence the increase in voltage drop across the LED.

[jack black]

Re: Failed cheap LED lights, bad caps?

Thanks Flyingdutchman. That confirms my layman thoughts. I should be able to replace the defective LEDs.

[Flyingdutchman]

Re: Failed cheap LED lights, bad caps?

A few tips for replacing the LEDs:

1) remove all the old solder from the pads, using solder wick (a strand of copper braid treated with solder flux, available from most decent hobby electronics suppliers, position the end of the braid on the pad and gently place the tip of your iron on top of it, the braid will suck up the solder as it melts, cut off the solder saturated end and repeat as necessary)
2) make sure the new LED sits nice and flat on the pads
3) put a little dab of heatsink paste between the LED body and the circuit board
4) preferably use 60/40 solder rather than lead free - it flows better and melts at lower temperature
5) make sure the solder connection entirely covers the part of the lead that contacts the pad
6) take care not to overheat the LED - when properly done, making a good solder connection should not take more than 2 seconds

These tips may seem superfluous to some, but I've come across way too many examples of badly executed repairs...