Repair LED spot light?

Discussion in 'General Electronics Chat' started by spinnaker, Dec 16, 2012.

  1. spinnaker

    Thread Starter AAC Fanatic!

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    Well I didn't hear back from anyone on this thread on replacing my LED spotlight.

    Wow I think that is the first time that has happened around here. :) Sos what I did was to purchase 4 of these.

    http://dx.com/p/gu5-3-3-5w-6500k-420-lumen-60-led-white-light-bulb-12v-131888
    I figure 2 for the light and 2 as backups.

    But I still have a question on temporarily repairing the current light so I figured I would post again with shorter post and change the subject line (can't be edited in the OP after a period apparently).

    Here is what I had in the other post on repair:

    The question is in reference to my Solar Light Timer project I posted. I was making some improvements and noticed that 6 LEDs turned up bad.

    The LEDs are in an array with 3 LEDs in series. The groups of 3 are in parallel of 36 rows for a total of 108 LEDs. The array is powered by a Buckpuck Plus to provide current regulation, all powered with 12vdc. It consumes < 500ma.

    The total LEDs that went bad are 4. Two per group of 3. They were located in the first 2 rows right where the power is connected to the light.

    Is this just coincidence that these LEDs went part or does their proximity to the supply wires have something to do with it?

    Now the best way to repair. I have some white LEDs on order. But I understand differences in forward voltage could prove problematic. Considering the configuration, will I have this issues? Or will it just be confined to the two rows? The repair only needs to be temporary, if I could get a few months more out of the light

    Huh, maybe this post is long as the other. :)
     
  2. SPQR

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    Nov 4, 2011
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    I note that your old light had six rows of 18 LEDs.
    Are all of the rows wired in the same way?
    With 1/6 rows bad, that would be a failure rate of 17% - do you see any evidence of "earlier failure" in any of the other rows?

    If you're like me, I'll be you've already taken apart the new ones.:D
    Are they wired similarly?
     
  3. spinnaker

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    Yes all wired the same. No previous failures after several years of use.. My question is can it just be repaired by changing leds? Will I only have issues with mismatched leds in those two rows?

    I don't yet have the new ones but they will be sealed I guess and contain their own current regulation.
     
  4. SPQR

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    Well, let's see.
    So, my first response to any question like this is "Of course you can fix it!":D
    Change the LEDs!

    But let's be more circumspect.

    The circuit is probably voltage regulated (from 110 down to X volts) so that won't be a variable.

    LEDs are current devices, so if I were the circuit, I would only be concerned about a new component drawing too much current.

    I'll assume you want white, T 1.75 and let's see if we can list a few high output LEDs
    Jameco - 30-100 mA
    Digikey - 30-500 mA
    Mouser - 120-700 mA

    So you could try to measure the current draw on one of your strips, divide by 18, then choose something specific.

    Or just choose a random white LED ~100mA and hope for the the best.
     
  5. ErnieM

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    I've reversed engineered several LED bulbs and they run between just a series resistor with each series string to some constant current device running everything. I have some in my kitchen also running off 12 V and had failures in both expensive Phillips units from the home center to cheapie Chinese units off EBay.

    What the failures had in common was heat. All the failures happened in lamps with a glass cover. When I let them sit in open air even the Chinese guys have lasted me over a year now in constant service.

    Have you changed the lamp housing recently? Sealed it in?

    Holes to let heat out will help.

    How effective repairing by replacing LEDs will depend on how they are driven. If each series string has it's own resistor you may not notice any change. If the series strings need to be matched since they all draw from one common current source then either it will not be as bright, or steal most of the current and die again shortly.
     
  6. spinnaker

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    As mentioned the whole array is regulated by a buck regulator.

    I did just order random LEds from ebay. I figure in those two row, I would sacrifice an led for each row and use a current limiting resistor for that row.
     
  7. spinnaker

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    Not possible it is for outdoor use. The compartment for the LEDs is fairly large. There is a large cavity behind the LEDs that formally housed the original battery and controller. I guess I could drill some holes in that but I doubt it will do any good as there is a reflection shield behind the lights. So if it is reflecting light it will also reflect heat.


    As said they are driven by a single buck regulator. How can I test to see if they are drawing too much? Could I temporarily place a resistor in series with them and measure the voltage across it? But then I would need to do that to an existing row. Or is there an easier way?
     
  8. ErnieM

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    I do not share your doubts. If air can get in and out then the heat will leave with it.

    You state there are numerous 3 LEDs in series strings. For 12V in that means 12/3=4V across each LED... or it means there is some other current regulation means inside the lamp.
     
  9. spinnaker

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    There is no current regulation om each lamp. They are standard LEDs. Is this a hint? Do I simply measure across each LED and be certain they are all 4V?
     
  10. ErnieM

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    It's not a hint, it's simple division. White LEDs on Digikey come with forward drops from 2.8V to 55V. If you have 3 LEDs in series, no other components between that and the 12V power source, then each is seeing 12/3=4 volts, and that is what you need to replace them.

    The hint I will give you is I've never seen a LED light without any internal limiting.
     
  11. spinnaker

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    The whole array is limited. Trust me. That is one reason why I am looking at replacing it. I bought 2 of those 12V 60 LED lamps from ebay to see how well they will work.


    So what is the best way to determine if each of my rows are drawing the same?
     
  12. ErnieM

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    Open traces to isolate each leg, insert ammeter.
     
  13. spinnaker

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    I said an easy way. :)

    I just went by brightness. Inserted a 470K resistor in the one new row that looked the brightest. Hopefully that will let me limp along while the replacement bulbs get here.

    Now I need to design a buck regulator and mosfet light switch to replace the buck regulator. That 5vdc out was a real nice feature of that buckpuck plus.
     
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