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A bad and a good LED in series

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    #1

    A bad and a good LED in series

    Hi members,

    One of my hobbies is taking apart faulty digital cameras to try to understand the problems, or even fix them. I have successfully repaired a number of mechanical issues but recently I came across an electronic issue that I hope to learn something about.

    A typical camera's LCD screen is illuminated by white LEDs. This particular Sony model I am looking at uses two surface mount "side view" LEDs to illuminate the edge of a clear plastic "glass", which is engraved with dots to reflect the light evenly out onto the "glass" surface. This particular LED circuitry is extremely simple: two LEDs in series. Somehow, it failed to light up.

    I took the panel apart and was able confirm that all the traces and contacts were continuous. When I tested each LED separately, to my surprise, they DID light up, although with different brightness. They just didn't turn on when a voltage is applied across both.

    I plan to replace the entire panel so I'm not going to make any repairs, all I wanted to know is, why would LEDs light up individually but not together? Is the dimmer (bad) LED affecting the circuit in a way that the good LED cannot light up, thus keeping the circuit open? Is there a technical explanation?

    Thanks and have a good day!

    Kevin
    Tags: None
    #2
    Re: A bad and a good LED in series

    There aren't really bad or good LED series, it just depends on what application they will be used in...

    Some engineers have done a big doo-doo though, that is, overpowering the LED! This is what happens over time; the LED starts to get dim even at 'full' current!

    See those white 5mm LED's rated for, like, 30mA? NOO! A safe - non life shortening - current of more like 15mA should be used.
    Muh-soggy-knee

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      #3
      Re: A bad and a good LED in series

      ...COMPLETELY EDITED...

      I think I am misunderstanding what you meant by "bad or good LED series" ... would you explain to me as I'm new to electronics? The situation is that two LEDs are in series (ie. not in parallel) and one LED seems to be causing the entire circuit to turn off, but individually each one lights up.
      Last edited by kktlam; 02-03-2013, 11:24 PM.

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        #4
        Re: A bad and a good LED in series

        simple.the bad(dim during test)one has a very high vf now.in series this now exceeds the supply voltage.

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          #5
          Re: A bad and a good LED in series

          Yes, the LED's have teensy imperfections in each, and over time these can manifest into a problem. My LED light bulb started flickering one day after one of the LED's failed, and had a very high Vf (that is why it was flickering). It was under warranty, and I just suspect it was a premature failure. Got a replacement and it has been going good for about 4 years!
          Muh-soggy-knee

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            #6
            Re: A bad and a good LED in series

            Or, a short on the camera's board where the LEDs are connected. A 3.7V or 2xAA camera will use a boost converter for those two LEDs, MLCC short would cause that fault.
            Please do not PM me with questions! Questions via PM will not be answered. Post on the forums instead!
            For service manual, schematic, boardview (board view), datasheet, cad - use our search.

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              #7
              Re: A bad and a good LED in series

              Originally posted by tom66 View Post
              Or, a short on the camera's board where the LEDs are connected. A 3.7V or 2xAA camera will use a boost converter for those two LEDs, MLCC short would cause that fault.
              You guys are good. Well, it was not a short - just a corroded trace, but indeed on the camera board. And the whole time I thought the LEDs were broken!

              Lesson learned: next time, check the obvious things first...

              Comment

                #8
                Re: A bad and a good LED in series

                Originally posted by kktlam View Post
                Lesson learned: next time, check the obvious things first...
                Yup. I wasted a bunch of time troubleshooting a FRS walkie talkie that had no sound. Tested the earphone jack switch contacts, then started scoping the audio output. Everything looked fine.

                Had I started at the end, I would have seen audio at the speaker.
                Yep, the speaker was bad.

                A SMART troubleshooter checks for signal at the end, then works backwards, usually half way at a time, to quicky isolate the problem. Not me .

                And a cordless drill charger problem. Spent a good 45 minutes testing, measuring disassembling. It was a bad battery. D'oh!
                36 Monitors, 3 TVs, 4 Laptops, 1 motherboard, 1 Printer, 1 iMac, 2 hard drive docks and one IP Phone repaired so far....

                Comment

                  #9
                  Re: A bad and a good LED in series

                  Originally posted by kktlam View Post
                  Well, it was not a short - just a corroded trace,
                  Corrosion can eat away at traces as you found. ModemHead documented a case on the Fluke 77 IV at

                  http://mrmodemhead.com/blog/fluke-77...per-corrosion/

                  causing the beeper and LED not work work.
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