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Rechargeable LED pairs - how do these work?

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Discussion in 'LEDs and Optoelectronics' started by wingnut, Jun 7, 2018.

  1. Jun 7, 2018 #1
    wingnut

    wingnut

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    double LED's.jpg Hi all




    I have been taking apart a rechargeable lantern which has panels with these pairs of LED's.

    Could someone tell me how they work?

    I have tried applying 5V through a resistor and without a resistor to the double strips but not a blink from the LED's.
     
    Last edited: Jun 7, 2018
  2. Jun 7, 2018 #2
    Harald Kapp

    Harald Kapp Moderator Moderator

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    The LEDs work as any other LED. If your photo, which is rather small by the way, shows front and back, one can assume the LEDs are in parallel. Therefore I'd expect around 3.5 V pass voltage (give or take a few hundred millivolts) for white LEDs. Current limiting could be internal to the LEDs. The easiest way to find the correct polarity is by checking the connection to the battery. The battery should also give you an indication of the correct voltage required.
     
  3. Jun 7, 2018 #3
    wingnut

    wingnut

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    Aug 9, 2012
    Thank you Harald.

    I am wondering whether the LED's are blown already. I get 1.6 Ohms across the tracks in both directions. There was some problem with the lamp which is the reason I took it apart in the first place. It does not light up with 5V in either direction across the tracks.
     
    Last edited by a moderator: Jun 29, 2018
  4. Jun 7, 2018 #4
    dave9

    dave9

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    It might help to start with a complete description of the lantern including model name, possibly links if it is still sold online, what type and voltage battery, better pictures and more - any and all data about it.

    5V might be a bad test... there are no 5V batteries so that is not the correct voltage. You could have 2 series of many in parallel which would make 5V too low, or you might not have two series of many in parallel. Details matter.
     
    davenn likes this.
  5. Jun 7, 2018 #5
    wingnut

    wingnut

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    Thank you Dave9

    Here is the lantern.

    https://www.bidorbuy.co.za/item/345...MIjNiX-abC2wIVCLXtCh0hDAC5EAQYASABEgIQDvD_BwE

    I cannot give a circuit because it has been disassembled. I presumed the LED's would work as per normal, but I suspect they are blown. What surprises me is that they give an almost complete short (no resistance) between the parallel connectors. I tried them with a resistor and 12V in both directions, and still nothing.

    The original battery was 4.5V lithium iron.
     
    Last edited: Jun 7, 2018
  6. Jun 7, 2018 #6
    dave9

    dave9

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    Okay, so it's running from 3 x AA = 4.5V at most, so shouldn't have any series of LEDs, all in parallel.

    If it was driving the LEDs too hot, it is definitely possible to blow them, but normally what would happen is those with the lowest (will vary slightly from one specimen to the next LED) forward voltage will blow first, so initially you would have one, then a second, and so on LEDs failing, not all at once.

    All at once suggests to me that it's more likely you have a cracked resistor solder joint or something as this kind of cheap (R115 = $8USD?) LED light often skimps on resistors barely rated for high enough wattage if that.

    As for why your testing isn't getting it to work, with low resistance,I can't see the test and can't see the circuit. There is no reason why the circuit can't be completely pictured without being assembled is there? That should have been the first thing done so that we can clearly see it.

    Is it possible that the 12VDC external power socket is damaged, that it has a switch mechanism in it to switch between internal battery and 12V external power, and it is shorting out?
     
    Last edited: Jun 7, 2018
  7. Jun 7, 2018 #7
    kellys_eye

    kellys_eye

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    Have a VERY close look at the LED die's (the yellow blob in the centre of the package) and see if you can see one with any physical defects - a small black dot perhaps?

    If you have one device shorted, none of them will work.
     
  8. Jun 8, 2018 #8
    dave9

    dave9

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    ^ A better picture would help but I am assuming these are the plastic encapsulated type LEDs rather than a stacked surface mount package where there could be a large leakage path. I mean if the only path is a bond wire that should fry open circuit if all current passes through it, and if it didn't it seems like it should do more than make a black spot, maybe melt the whole plastic encapsulation.
     
  9. Jun 8, 2018 #9
    wingnut

    wingnut

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    Aug 9, 2012
    LED's from recharger.jpg
    It was disassembled so the LED panels are separated and the rest of the circuitry does not interfere with the LED panels. There is no black blob in any LED.

    The LED's stopped working at least a year ago and it happened while I was charging it with a 12V charger, and switched the LED's on while it was charging if I remember correctly. I might have been cranking the hand cranker at the same time.

    As was noted, this was a cheap item, and the leads seemed poorly attached and on opening it, some leads seem almost separated, or the slightest tug and they come loose.

    The hand crank seems to work, and the 5V solar panel works fine, and I will try the rechargeable batteries in another solar device today when the sun shines, and see if these still work. But the LED's seem a lost cause.
     
    Last edited: Jun 8, 2018
  10. Jun 8, 2018 #10
    Audioguru

    Audioguru

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    The 12V adapter probably has a 4.5V output. If the LEDs are white then they probably use 3.0V and 1.5V is thrown away in a series current-limiting resistor.
     
  11. Jun 29, 2018 #11
    Externet

    Externet

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    Aug 24, 2009
    Are you sure ?

    (+)-----R------led------(-)
    (+)-----R------led------(-)
    (+)-----R------led------(-)
    (+)-----R------0Ω------(-)
    (+)-----R------led------(-)
    (+)-----R------led------(-)

    (+)-----R--------led--------led--------led-------0Ω-------led-------led------(-)

    Only here:

    (+)------R------led------(-) 'led' being many in parallel; if that is the case.
     
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