Strange problem with LED light

Discussion in 'The Projects Forum' started by Uskok, Feb 21, 2016.

  1. Uskok

    Thread Starter Member

    Aug 19, 2015
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    Hello everybody.
    Few days ago i dropped my led lamp.
    And now stuggling to get it work

    The lamp has 19 LEDs in rounded shape. And only the middle one works.
    So i assume that series has been broken somewere. But i cant find out where.
    Thing is that i examined every diode and every connection, and all seems to be fine.

    I hope someone will help me solve this. Thank you
     
  2. tranzz4md

    Member

    Apr 10, 2015
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    A bit difficult to see, but the soldered connection of the leads looks poorly executed. An LED could be internally damaged as well. You might use a lead to bypass each individual LED one at a time to see the result. Its also difficult for me to see the pic well enough to sketch out the circuit as built, but I'm guessing that a multi-mode switch is used to actuate the center LED separately from the rest of them. This would lead me to suspect malfunction of the switch.
     
  3. Uskok

    Thread Starter Member

    Aug 19, 2015
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    Thank you for reply. Well the lamp doesnt have modes. Its just on and off. I allready did some jumping with lead but only one more diode turns on, not the whole circuit. Tomorrow i will post some clean pics on daylight and try more to examine the board
     
  4. absf

    Senior Member

    Dec 29, 2010
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    Cant see very clearly. But that could a dry joint in the red circle...

    image (1).jpeg
    Allen
     
  5. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Those bare copper strands on the white wire seem to be shorting the pcb tracks?
     
  6. Uskok

    Thread Starter Member

    Aug 19, 2015
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    Dont worry about that joints. Thats just resistor i changed just to see if anything happens. But nothing.
    Here is better pic.
     
  7. Uskok

    Thread Starter Member

    Aug 19, 2015
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    When i did the test i make sure that they dont touch pcb. I take care of thoose simple details.
     
  8. absf

    Senior Member

    Dec 29, 2010
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    You just changed the resistor but did you solder them properly? Is the resistor the same value as the original one?

    Allen
     
  9. Tonyr1084

    Active Member

    Sep 24, 2015
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    When it comes to inspecting solder joints, it's best to have the part in hand and examine it under plenty of light and magnification. Simply looking at a photograph makes it almost impossible to determine if what I'm seeing is bad solder or a shadow. What I would suggest is you simply apply a little fresh flux to each joint and reflow the solder. Be sure not to dwell too long or use an iron set too hot, as excess heat can cause the LED to burn out.

    Now: You say it was working and you dropped it. Now only the one light works. Thinking about the electrical circuit, I can't figure out how one LED would work if many LED's are in series. In a series circuit, if one goes out the whole series goes out. Your lamp may be wired in "Series / Parallel" which means there are several parallel circuits of series lamps. I can draw a picture, and I'll post it shortly.

    OK, here's my best guess at what might be going on:

    LED Flashlight.jpg
     
    Last edited: Feb 22, 2016
  10. Uskok

    Thread Starter Member

    Aug 19, 2015
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    You are right. Its so strange tat only middle one works. I tought that circuit starts from the middle one and try to figure out which one is the next LED but its just doesnt make sens. The whole inner ring of 6 LEDs is next u can tell. So i think the best solution is to reflow the joints.
     
  11. Tonyr1084

    Active Member

    Sep 24, 2015
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    I've done my best trying to decipher the circuit traces. It even appears there may be a jumper besides the resistor. I just can't tell.

    Here's how I would proceed: Power the circuit. Test for voltage across every LED. You may find no voltage on many of them. One of them may be "NOT LIT" but have voltage. I'd suspect it's possible that may be where the fault lies.

    Those wires: Did you add them or are they part of the original circuit? Also, wha value resistor and what is the supply voltage?

    Honestly, I think the only way you're going to find the culprit is through tracing every step of the way. But I suspect you probably have broken the element that connects the PN junction to the leads. Break one and the rest in that string will go out.

    I have to ask: Is it worth fixing? Or would it be a whole lot easier to simply replace the lamp? And what kind of lamp is it? A flash light? A desk lamp?
     
  12. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    This may seem to be a silly question but, have you checked the supply voltage?

    It appears that the LEDs may be in series groups which are then connected in parallel but with a common resistor. With this type of (poor) arrangement, as the supply voltage falls, some LEDs will not light which gives the impression that they have failed.
     
  13. Uskok

    Thread Starter Member

    Aug 19, 2015
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    Okay for now i reflowed all the joints with no luck. Ill post pic how it look now.
    There is jumper as you said and the ressistor os 150 ohm.
    Supply voltage is fine. And supplying 4 volts. 3V to each LED. Now ill check voltage on each od the LED againg. And let you know results.
    As concern of wires I added them quicli as extension to original circuit wires.
     
  14. Uskok

    Thread Starter Member

    Aug 19, 2015
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    You can se in pic below. If i jump betwen LEDs with wire the next LED in a row start light. Also if i short circuit one of the LEDs next one starts to work.
     
  15. absf

    Senior Member

    Dec 29, 2010
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    From the pict below there are 9 parallel of 2 LED each. The middle LED has a resistor just for itself.
    image.jpg
     
  16. Tonyr1084

    Active Member

    Sep 24, 2015
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    This helps. OK, you're saying there are nine sets of "Two Series" LED's. A little different from my first drawing where I had three sets of six.

    This is making even less sense. If one diode were out then two of them would not light. For 18 of them to be out you'd have to have at least 9 burned out LED's.

    Want my advice? Scrap it. Too many of the LED's have gone out for whatever reason and short of replacing each and every last one of them, you're not going to repair it. I think it's time to visit Ebay or Amazon. The time and resources you're putting into this is way over the cost of replacing it.
     
    Last edited: Feb 22, 2016
  17. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    What is your supply voltage? if you have two white LEDs in series you will need at least 6V. If your supply is only 4V then the observations you have made are consistent with this being the case.

    My guess is that the LEDs are wired this way so that as the battery discharges all but one LED (the one in the middle) extinguish, therefore, you won't be left completely in the dark.
     
    Last edited: Feb 22, 2016
    absf, ebeowulf17 and JohnInTX like this.
  18. Tonyr1084

    Active Member

    Sep 24, 2015
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    OK, here's what I came up with: It's junk. Buy new. And if you like, here's your picture with schematic. For all but one diode to be out - there's no common point that would define that.

    LED Schematic.png
     
    absf likes this.
  19. ebeowulf17

    Active Member

    Aug 12, 2014
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    Except for the supply voltage. Pretty sure that blocco a spirale got it figured out. Sounds right to me anyway.
     
  20. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    It looks to me as though at the point where you connected your +ve supply wire (originally white, now grey, just to confuse us!), a chunk of the pcb track is missing or oxidation/solder-resist is preventing solder from wetting the track. The joint at that point may be high resistance and hence dropping the voltage to all LEDs except the central one. Try attaching the +ve wire instead to, e.g., the pcb patch at the anode of D17 (as shown in the post #18 pic).
     
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