Announcement

Collapse
No announcement yet.

OT: Dissection and partial resurrection of an LED lamp

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • OT: Dissection and partial resurrection of an LED lamp

    Around December I purchased four LED lamps from www.banggood.com, for just a few dollars each, and I put one in a bedroom ceiling fixture which is on most of the time. Occasionally I noticed a flicker which would usually stabilize after awhile, but last evening it got worse and then just barely glowed. I replaced it with a spare, which seems fine, and proceeded to take it apart to determine the cause of its demise and possible repair.

    This is the lamp (http://www.banggood.com/E27-LED-Bulb...-p-923266.html):


    Inside:


    The driver PCB assembly:


    Closer view:


    I applied a variable voltage to the LEDs through a 240 ohm resistor and half the LEDs lit brightly at about 27 VDC, and as I raised the voltage they flickered and then the other half lit up. I was able to use an ohmmeter to light up each LED individually and found one that was bad. There are two series strings of 9 in parallel. I made a short video of the results at various voltages:

    http://enginuitysystems.com/pix/LED_Lamp_1938.AVI

    More pictures follow...
    Last edited by PStechPaul; 05-07-2015, 03:38 AM.
    http://pauleschoen.com/pix/PM08_P76_P54.png
    Paul , P S Technology, Inc. and MrTibbs
    USA Maryland 21030

  • #2
    This shows half of the LEDs lit dimly at 22.3 VDC:



    I jumpered out the bad LED and one in the other string, and there is still some imbalance of brightness, but all are lit:



    At higher voltage, where I could get about 10 mA current, all lit brightly so I had to put a paper towel over them, but still very visible difference in brightness:



    This was at about 40 VDC and 20 mA:



    This was with the DC supply connected to the input, which is normally connected to the 120 VAC mains. I found that the circuit would turn on and maintain a fairly consistent current (brightness) at 38 VDC, and it would stay lit until I reduced the input to 16 VDC.
    http://pauleschoen.com/pix/PM08_P76_P54.png
    Paul , P S Technology, Inc. and MrTibbs
    USA Maryland 21030

    Comment


    • #3
      So I assume you are going to measure Vf for each device, and then swap whichever between strings to achieve an average balance between the strings.

      Comment


      • #4
        No, it is very difficult to unsolder and resolder the LEDs on the metal PCB which also functions as a heat sink. I might try to cut tracks and rewire with all (16 or 17) in series which would be about 50 volts, although I don't know the rating on the IC. Or I might add resistors (or diodes) to each string as needed to balance current. It's really not cost effective to spend hours fixing a $5 bulb, but it's a learning experience.
        http://pauleschoen.com/pix/PM08_P76_P54.png
        Paul , P S Technology, Inc. and MrTibbs
        USA Maryland 21030

        Comment


        • #5
          Well, I replaced that lamp with another of the same type, and it started to flicker and then blinked out. I replaced it with a CFL and took the cover off the recently deceased lamp. I found some dead bugs:



          I measured the LEDs with a DMM and found five of the 18 showed 50-800 mV drop on the diode test, while the others actually lit. I found that I could remove the DEDs (Darkness Emitting Diodes) by snapping them in half with a diagonal cutter and then pulling the metal pieces off the board while using a soldering iron. Then I made some fresh solder blobs on the pads and installed some smaller 1206 size white LEDs. These are probably 30 mA 100 mW maximum, whereas the lamp is 9W for 18 devices of 500 mW (180mA) each, and they are size 5630 (metric SMD size).



          The lamp works now, and is very bright. I didn't let it stay on very long. I might just try letting it stay on and see how long it lasts. The LEDs are fairly easy to replace, and the rest of the lamp seems well made, so I might get some real 500 mW LEDs and fix it right, along with the other one. New LEDs are about $0.20 each from Mouser, so about $3.60 (plus about an hour time) to fix:
          http://www.mouser.com/ProductDetail/...grYQJ4EA%3d%3d

          There are some US sources on eBay for about half that price:
          http://www.ebay.com/itm/100x-White-5630-5730-Single-Chip-Smd-Smt-Leds-0-5W-UltraBright-Light-USA-/161241663864

          And a lot less (about $0.02 ea) from China, but they are likely the same as those that failed in the bulbs in the first place:
          http://www.ebay.com/itm/100-pcs-SMD-...-/261422340819
          http://pauleschoen.com/pix/PM08_P76_P54.png
          Paul , P S Technology, Inc. and MrTibbs
          USA Maryland 21030

          Comment


          • #6
            Paul,
            I gotta ask you, as you might know.
            Do manufactures overdrive these LEDs on purpose to give them
            a finite life span? I mean, LEDs last an incredibly long time,
            so to actually sell people replacement bulbs, do they intentionally
            try and make them burn out in a certain number of such hours
            to have a market for replacement bulbs?
            I always wondered about this.

            -Doozer
            DZER

            Comment


            • #7
              Seems to me its always the support/driver components that fail. They like to use parts 'rated' for actual loads with no buffer. Then too, everything is designed for adequate cooling in free air, but typically used in some sort of fixture, either enclosed or at least causing stagnation. Same problem with most CFL's.

              Comment


              • #8
                I think the answer lies in the Chinese manufacturing philosophy that is based on an extreme minimum bottom line cost and factoring in the ramifications of failure. In this case, I think the LEDs themselves were overrated by their manufacturer and the company that made the bulbs just selected the lowest bidder. They probably accepted the specifications of 50,000 hour life and had no way to verify that, other than run some of the lamps for a few weeks (maybe 500 hours). I usually have the lamp on about 16 hours a day and each lasted about 20 weeks or 2200 hours. That's actually about the same as an equivalent 75 watt incandescent lamp, but less than 10% of the expected life.

                The driver circuitry is probably good. I used the driver PCB from a broken CFL to repair a fluorescent desk lamp a couple years ago, and it is now well into the second replacement bulb.

                Banggood.com seems to promote quality and they have a fairly good warranty policy, but I think in most cases they require the defective merchandise be returned to them. Their shipping to the US is free, but I would imagine it would cost me at least the $6 cost of the bulb to send it back to China.

                They do have a customer feedback mechanism and I have posted several, including on this bulb. I had thought the first was a "fluke" but with the second failure it seems to be a clear case of improper specifications and substandard components. I have bought quite a few things from them and I think I will write a new review and request at least a partial refund. They must approve all reviews, but I think they will accept an honest assessment of the problem, and they should publish it. I would also accept replacement lamps of a different model, and I actually purchased four of them, with two given as gifts.
                http://pauleschoen.com/pix/PM08_P76_P54.png
                Paul , P S Technology, Inc. and MrTibbs
                USA Maryland 21030

                Comment


                • #9
                  I wonder if it's also LED lamps getting in on the act?
                  http://topdocumentaryfilms.com/light-bulb-conspiracy/
                  Mark

                  Comment


                  • #10
                    Hi Paul:

                    Are they wired as:

                    two series strings, of 9 in parallel

                    or

                    two series strings of 9, in parallel

                    ?

                    I assume the latter....

                    Comment


                    • #11
                      Originally posted by boslab View Post
                      I wonder if it's also LED lamps getting in on the act?
                      http://topdocumentaryfilms.com/light-bulb-conspiracy/
                      Mark
                      Are you sh!ttin me? Anyone reporting a similar problem with Cree bulbs? What good is a guarantee on a cheap crap off-shore item where it cost more to ship back the defective item for replacement than it's worth to begin with?

                      Comment


                      • #12
                        But perhaps they ship more of them if they fail, Paul's got the right idea fixing them, they never counted on that!, perhaps we can all benifit in the long run as I for one wouldn't have a clue how to repair something made to fail,
                        I'm reading with interest how this works
                        Mark

                        Comment


                        • #13
                          Originally posted by boslab View Post
                          But perhaps they ship more of them if they fail, Paul's got the right idea fixing them, they never counted on that!, perhaps we can all benifit in the long run as I for one wouldn't have a clue how to repair something made to fail,
                          I'm reading with interest how this works
                          Mark
                          Yes, we should all encourage them to flood us with more cheap crap, designed with a short service life and then waste our time fixing something that was poorly designed to start with. Makes perfect sense.

                          Comment


                          • #14
                            I have zero issues with my Cree, Philips or Osram, and I have some of the early generation.

                            Not all semiconductors/leds meet "spec". Do the "out of spec" get dumped? na... not if someone wants to buy them. They are graded and sold off to the lower tier manfs.

                            Comment


                            • #15
                              I decided to let the lamp run until it failed. It had already run about 2 minutes. About 1 minute into the second test it blinked briefly. I felt the fixture and it was still cool, but when I touched the plastic globe, it was slightly warm. Then at about 5 minutes it flickered a lot and finally went out. After cooling a bit, one bank of LEDs flickered while the other was dead. I found that three of the five replacement LEDs had failed to a low resistance state, and one of the original LEDs had also failed.

                              It may be possible to run accelerated life tests by overdriving the LEDs, but I don't know what the function might look like. I would suspect that each doubling of current might give ten times less life, so for the 30 mA 100 mW LEDs, rated 50,000 hours, 2x might be 5,000, 3x for 500, 4x for 50, and 5x for 5 hours. The current is about 180mA, or 5x, and the life was only about 6 minutes, or 0.1 hours. Of course, the replacement LEDs were also cheap Chinese devices, and may have started out as 1000 hour MTBF, in which case 5x overload would result in 1000 * 10e-4 or 0.1 hour as experienced.

                              I do plan to order some OSRAM 500mW LEDs in various color temperatures and use them in the two lamps I have. With proper SMT equipment it probably takes maybe 20 minutes to R&R the LEDs, and I should then have two good lamps that may last 20,000-50,000 hours, which is 3-8 years at 67% duty cycle. I also have some smaller LED lamps that work marginally and perhaps can be rejuvenated in a similar manner. I also got them on eBay and they seemed to have rather low light output and poor (too blue) color temperature, but I don't think any of them actually failed.

                              Does anyone have experience with more costly name brand LED lamps? HD has the Feit brand of similar size lamps for $50/6, and the reviews are good. They are rated 25,000 hours, or 23 years based on 3 hours a day. And they are also dimmable, which mine are not:
                              http://www.homedepot.com/p/Feit-Elec...6-24/206036849

                              Those are actually 3.75" diameter while mine are 3.00", so they would not fit in my desk lamp fixture. But I found 60 watt equivalent Cree lamps at HD in a "normal" bulb size for only $5 each. But it had several poor reviews:
                              http://www.homedepot.com/p/Cree-60W-...U100/205597078
                              Last edited by PStechPaul; 09-19-2015, 06:43 PM. Reason: HD Cree lamps
                              http://pauleschoen.com/pix/PM08_P76_P54.png
                              Paul , P S Technology, Inc. and MrTibbs
                              USA Maryland 21030

                              Comment

                              Working...
                              X