Dissecting a Cree LED Incandecent Bulb Replacement

[mondrasek]

Sorry for this NON OU question. 

I purchased (6) Cree 60W (equivalent lumen output) light bulbs less than 8 months ago.  They are advertised as lasting up to 25 time as long as a conventional incandescent bulbs.  But one of the six failed last night.  So (guess what) I took it apart to try to learn what went wrong with it.  I had assumed that something in the built in Cree electronics between my (in the US) 120V 60hz mains and the LEDs had failed (similar to CFL bulb failures), but now I'm not sure.

After disassembly I plugged in the base of the Cree bulb that contains the electronics and it provided a steady 288V DC output.  That output would have gone directly to the LED module that appears to only have (20) surface mounted LEDs.

So my question is:  What in this Cree bulb stopped working properly?  The electronics (ie. does the 288V DC output mean there is a problem with the circuitry?), the LEDs, or possibly the connections in between?

Thanks for your consideration, and sorry again for being off topic for this forum.  It is just that I know there are many electronic experts that participate here.

M.

[TechStuf]

It's probably the number 7 Fetzer valve....

http://www.youtube.com/watch?v=Hjsfs49SRbc&t=1m41s

I kid....

Seriously though, these guys eat LEDs for breakfast:

http://www.ledforums.net/

http://www.candlepowerforums.com/vb/forumdisplay.php?91-LED

I'd bet you'd find the answer pretty quick at either forum.



TS

[TinselKoala]

Hmmm... if the 20 LED chips are in series, the voltage you are measuring might be appropriate to drive them, and the failure of a single LED chip, or a bad connection to the LEDs, would kill the whole light output.
Can you test the LEDs individually by applying power from a regulated DC supply to each chip in turn?

[mondrasek]

Thanks TS.  Interesting forums, but I didn't find anything quickly while searching on "Cree."  The majority of posts I saw appeared to be more about the "use" of LEDs rather than the inner workings of a bulb assy.

@TK, the LED assy is what they call the "Cree LED Filament Tower" in the video here:  http://www.youtube.com/watch?v=jnVw0doTbJ4  It appears to be made from some sort of flexible pcd type film that is laminated to an aluminum substrate which is bent into the decagonal "sleeve" that slides onto the main heat sink mast.  It is really a very nicely designed package, quality control issues aside.  No contacts are visible for any of the individual LEDs.  Only the entire assy's + and - poles that slide into clips going to the AC/DC convertor circuit are accessible as far as I can see.

If 288V DC is about right then I will assume the circuitry is sound and that a connection problem or LED defect caused an LED to blow?  At least I then can feel confident about the product's design and that my failure was a fluke.  But does that make sense after the bulb has been in daily use (several cycles per day) for over 8 months?

Now that I know how to disassemble the unit I can do less damage to the next victim if another expires.  If they do continue to fail I may end up with enough pieces to test by swapping components.  Until then, I have a neat little AC/DC converter from 120V AC to 288V DC if anyone wants one.

Is there any chance that a component failure on the converter could jack up the DC output to a stable value that blew an LED while not popping the cap?  I doubt that, but until I have another to compare the 288V with it is something I am curious about.

M.

[e2matrix]

No, 288 volts would be way too high for 20 LED's in series.    BTW I was an active member of one of the forums mentioned above for many years.   Unless that 288 volts drops a lot under load it's way too high.   LED's like these on average need around 4 volts.  So about 80 volts if in series.    Cree makes the LED's but most likely the electronics were done in China and may not be the best quality.   Did you see a bright flash before this went out - most likely with a bluish tint?   You can test the LED's with a meter or put around 4 volts across one but most likely they are toast along with the electronics.   
    Just guessing here but after looking at the picture I think this may have been setup as 5 groups of 4 LED's in series so it's more likely to have been fed by around 16 volts when it was working correctly.  The idea of putting all 20 led's in series would not be a very smart design since if one went out the whole light goes out whereas if one goes out with 5 groups of 4 then you only lose 20% brightness.