A bad and a good LED in series

Collapse
X
Collapse
 
  • Page of 1
  • Time
  • Show
Clear All
new posts
Previous template Next
  • kktlam
    Member
    • Jan 2013
    • 27
    • Canada
    #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
    • ben7
      Capaholic
      • Jan 2011
      • 4059
      • USA
      #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

        Comment

        • kktlam
          Member
          • Jan 2013
          • 27
          • Canada
          #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.

            Comment

            • kc8adu
              Super Moderator
              • Nov 2003
              • 8832
              • U.S.A!
              #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.

                Comment

                • ben7
                  Capaholic
                  • Jan 2011
                  • 4059
                  • USA
                  #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

                    Comment

                    • tom66
                      EVs Rule
                      • Apr 2011
                      • 32560
                      • UK
                      #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.

                        Comment

                        • kktlam
                          Member
                          • Jan 2013
                          • 27
                          • Canada
                          #7
                          Re: A bad and a good LED in series

                          Originally posted by tom66
                          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

                            • smason
                              Badcaps Legend
                              • Feb 2010
                              • 1652
                              • Canada
                              #8
                              Re: A bad and a good LED in series

                              Originally posted by kktlam
                              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

                                • retiredcaps
                                  Badcaps Legend
                                  • Apr 2010
                                  • 9271
                                  #9
                                  Re: A bad and a good LED in series

                                  Originally posted by kktlam
                                  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.
                                  --- begin sig file ---

                                  If you are new to this forum, we can help a lot more if you please post clear focused pictures (max resolution 2000x2000 and 2MB) of your boards using the manage attachments button so they are hosted here. Information and picture clarity compositions should look like this post.

                                  We respectfully ask that you make some time and effort to read some of the guides available for basic troubleshooting. After you have read through them, then ask clarification questions or report your findings.

                                  Please do not post inline and offsite as they slow down the loading of pages.

                                  --- end sig file ---

                                    Comment

                                    Previous template Next

                                    Related Topics

                                    Collapse
                                    • howardc64
                                      A1312 (27” iMac 2009-2011) A1407 (Thunderbolt Display) A1316 (Cinema Display) Display Black Screen Repair
                                      by howardc64
                                      Problem

                                      This is an LG edge LED lit LCD Display. The LEDs are on the bottom edge of the display. There are 2 bars (left and right) Each bar has many LEDs and a 6 pin connector. Each pin drive several LEDs thus is the highest current flow / heat junction. The weak lead free solder gradually fails with thermal expansion/contraction cycling and increases resistance. PSU will compensate up to a point, then when the current is too high, PSU just shut down the backlight causing a dark display. I have even seen one which the connector just fell off as solder points became completely detached....
                                      08-04-2024, 10:36 PM
                                    • Document Archive
                                      ASUS Pro Series D320MT-TR362D Pro Series i3-6100 Specification for Upgrade or Repair
                                      by Document Archive
                                      This specification for the ASUS Pro Series D320MT-TR362D can be useful for upgrading or repairing a desktop PC that is not working. As a community we are working through our specifications to add valuable data like the Pro Series D320MT-TR362D boardview and Pro Series D320MT-TR362D schematic. Our users have donated over 1 million documents which are being added to the site. This page will be updated soon with additional information. Alternatively you can request additional help from our users directly on the relevant badcaps forum. Please note that we offer no warranties that any specification,...
                                      09-12-2024, 03:28 PM
                                    • Document Archive
                                      ASUS Pro Series D310MT-0G32500650 Pro Series G3250 Specification for Upgrade or Repair
                                      by Document Archive
                                      This specification for the ASUS Pro Series D310MT-0G32500650 can be useful for upgrading or repairing a desktop PC that is not working. As a community we are working through our specifications to add valuable data like the Pro Series D310MT-0G32500650 boardview and Pro Series D310MT-0G32500650 schematic. Our users have donated over 1 million documents which are being added to the site. This page will be updated soon with additional information. Alternatively you can request additional help from our users directly on the relevant badcaps forum. Please note that we offer no warranties that any specification,...
                                      09-12-2024, 03:28 PM
                                    • Document Archive
                                      ASUS Pro Series D320MT-I564000234 Pro Series i5-6400 Specification for Upgrade or Repair
                                      by Document Archive
                                      This specification for the ASUS Pro Series D320MT-I564000234 can be useful for upgrading or repairing a desktop PC that is not working. As a community we are working through our specifications to add valuable data like the Pro Series D320MT-I564000234 boardview and Pro Series D320MT-I564000234 schematic. Our users have donated over 1 million documents which are being added to the site. This page will be updated soon with additional information. Alternatively you can request additional help from our users directly on the relevant badcaps forum. Please note that we offer no warranties that any specification,...
                                      09-12-2024, 03:28 PM
                                    • Document Archive
                                      ASUS Pro Series D320SF-I564000134 Pro Series i5-6400 Specification for Upgrade or Repair
                                      by Document Archive
                                      This specification for the ASUS Pro Series D320SF-I564000134 can be useful for upgrading or repairing a desktop PC that is not working. As a community we are working through our specifications to add valuable data like the Pro Series D320SF-I564000134 boardview and Pro Series D320SF-I564000134 schematic. Our users have donated over 1 million documents which are being added to the site. This page will be updated soon with additional information. Alternatively you can request additional help from our users directly on the relevant badcaps forum. Please note that we offer no warranties that any specification,...
                                      09-12-2024, 03:28 PM
                                    • Loading...
                                    • No more items.
                                    Working...