how to test lcd led backlight?

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  • pnv
    Member
    • Apr 2013
    • 11
    • Romania

    #1

    how to test lcd led backlight?

    please post test methods you use
    thanks!
  • Hamie
    Badcaps Legend
    • Apr 2014
    • 1383
    • UK

    #2
    Re: how to test lcd led backlight?

    Jump with a low ohm resitor between ps-on and 5v stby, and 5vstby and bl-on (inverter on off)

    Comment

    • budm
      Badcaps Legend
      • Feb 2010
      • 40746
      • USA

      #3
      Re: how to test lcd led backlight?

      OK, are you asking how to test individual LED mounted on the circuit board that form LED string?
      You can use 3V battery with 100 Ohms resistor to limit the current for testing individual LED.
      Or you already verify that the LED driver circuits are OK but the LED still does not light up?
      Test LED string current draw using DCA.
      https://www.badcaps.net/forum/showpo...4&postcount=95

      Or used indicator:
      https://www.badcaps.net/forum/showpo...&postcount=107
      Last edited by budm; 03-27-2015, 11:55 AM.
      Never stop learning
      Basic LCD TV and Monitor troubleshooting guides.
      http://www.badcaps.net/forum/showthr...956#post305956

      Voltage Regulator (LDO) testing:
      http://www.badcaps.net/forum/showthr...999#post300999

      Inverter testing using old CFL:
      http://www.badcaps.net/forum/showthr...er+testing+cfl

      Tear down pictures : Hit the ">" Show Albums and stories" on the left side
      http://s807.photobucket.com/user/budm/library/

      TV Factory reset codes listing:
      http://www.badcaps.net/forum/showthread.php?t=24809

      Comment

      • pnv
        Member
        • Apr 2013
        • 11
        • Romania

        #4
        Re: how to test lcd led backlight?

        Yes, something like an adjustable voltage source with constant current

        Comment

        • pnv
          Member
          • Apr 2013
          • 11
          • Romania

          #5
          Re: how to test lcd led backlight?

          his would be a constant current source would be the voltage regulator.
          What do you think?
          Attached Files

          Comment

          • budm
            Badcaps Legend
            • Feb 2010
            • 40746
            • USA

            #6
            Re: how to test lcd led backlight?

            Originally posted by pnv
            Yes, something like an adjustable voltage source with constant current
            That will be correct if all the LED int he string are good, but if one goes open circuit then nothing will happen, but you need to know how many LEDs are in the string to determine what the power supply voltage needs to be, some string you will need more than 100VDC, Vf of each LED is between 3~3.6V.
            Last edited by budm; 03-27-2015, 12:36 PM.
            Never stop learning
            Basic LCD TV and Monitor troubleshooting guides.
            http://www.badcaps.net/forum/showthr...956#post305956

            Voltage Regulator (LDO) testing:
            http://www.badcaps.net/forum/showthr...999#post300999

            Inverter testing using old CFL:
            http://www.badcaps.net/forum/showthr...er+testing+cfl

            Tear down pictures : Hit the ">" Show Albums and stories" on the left side
            http://s807.photobucket.com/user/budm/library/

            TV Factory reset codes listing:
            http://www.badcaps.net/forum/showthread.php?t=24809

            Comment

            • pnv
              Member
              • Apr 2013
              • 11
              • Romania

              #7
              Re: how to test lcd led backlight?

              Please, if not asking too much to draw scheme Led string indicator to find out which string is dead.
              Last edited by pnv; 03-27-2015, 12:27 PM.

              Comment

              • Alastair E
                Chief Womble
                • Mar 2013
                • 1963
                • U.K.

                #8
                Re: how to test lcd led backlight?

                LED's are wired like christmas-tree lights....

                One dead--all out.

                You just have to test each one in turn. No schematics, just a meter.....
                TELEFIX

                How PLASMA SCREENS WORK, X-SUS and Y-SUS what they do--
                http://www.irf.com/technical-info/appnotes/an-1088.pdf
                PLEASE DO NOT EMAIL ME PRIVATELY FOR REPAIR ADVICE. QUESTIONS BELONG ON THE FORUM!

                Comment

                • pnv
                  Member
                  • Apr 2013
                  • 11
                  • Romania

                  #9
                  Re: how to test lcd led backlight?

                  I think it is exciting and dynamic method of insertion of an indicator Lord budm
                  Last edited by pnv; 03-27-2015, 12:48 PM.

                  Comment

                  • keeney123
                    Lauren
                    • Sep 2014
                    • 2536
                    • United States

                    #10
                    Re: how to test lcd led backlight?

                    It would be nice if these led strings had standards for each size of screen then one could build a test circuit just to plug in and see if the individual strings worked or not, but that would be to easy.

                    Comment

                    • budm
                      Badcaps Legend
                      • Feb 2010
                      • 40746
                      • USA

                      #11
                      Re: how to test lcd led backlight?

                      Originally posted by pnv
                      I think it is exciting and dynamic method of insertion of an indicator Lord budm
                      See picture, the diodes will get hot so do not run it too long.
                      Attached Files
                      Never stop learning
                      Basic LCD TV and Monitor troubleshooting guides.
                      http://www.badcaps.net/forum/showthr...956#post305956

                      Voltage Regulator (LDO) testing:
                      http://www.badcaps.net/forum/showthr...999#post300999

                      Inverter testing using old CFL:
                      http://www.badcaps.net/forum/showthr...er+testing+cfl

                      Tear down pictures : Hit the ">" Show Albums and stories" on the left side
                      http://s807.photobucket.com/user/budm/library/

                      TV Factory reset codes listing:
                      http://www.badcaps.net/forum/showthread.php?t=24809

                      Comment

                      • CapLeaker
                        Leaking Member
                        • Dec 2014
                        • 7971
                        • Canada

                        #12
                        Re: how to test lcd led backlight?

                        Originally posted by budm
                        See picture, the diodes will get hot so do not run it too long.
                        Now that is a neat testing device I hafta make! Thanks budm!

                        Comment

                        • keeney123
                          Lauren
                          • Sep 2014
                          • 2536
                          • United States

                          #13
                          Re: how to test lcd led backlight?

                          Bud M don't laugh at me, but I was hoping to unplug all the LED strings and plug them into a separate power source that would eliminate any feed back circuit from turning the the LED driver chip off.

                          Comment

                          • budm
                            Badcaps Legend
                            • Feb 2010
                            • 40746
                            • USA

                            #14
                            Re: how to test lcd led backlight?

                            If you want to use external constant current power source to drive the LED string then the strings are no longer connected to the LED driver board) so there is need to have any power to the TV at all since you are just testing LEDs.
                            The harder part is to figure out how the strings are connected together and which wire is the Anode and which wire is the Cathode if the panel is not open up to see the LED string, or you can trace the connector on the driver board to see how it is done.
                            Never stop learning
                            Basic LCD TV and Monitor troubleshooting guides.
                            http://www.badcaps.net/forum/showthr...956#post305956

                            Voltage Regulator (LDO) testing:
                            http://www.badcaps.net/forum/showthr...999#post300999

                            Inverter testing using old CFL:
                            http://www.badcaps.net/forum/showthr...er+testing+cfl

                            Tear down pictures : Hit the ">" Show Albums and stories" on the left side
                            http://s807.photobucket.com/user/budm/library/

                            TV Factory reset codes listing:
                            http://www.badcaps.net/forum/showthread.php?t=24809

                            Comment

                            • keeney123
                              Lauren
                              • Sep 2014
                              • 2536
                              • United States

                              #15
                              Re: how to test lcd led backlight?

                              Yes that is where standards would work.

                              Comment

                              • Peter Chiu
                                Banned
                                • Dec 2012
                                • 145
                                • USA

                                #16
                                Re: how to test lcd led backlight?

                                ? Auto-recovery LED screen ? Resettable Fuse ? Auto-bypass LED?

                                ST has LBP01 part can bypass any defected LEDs

                                Benefits
                                ■ Keep LED strings on in case of LED open mode failure
                                ■ Reduced maintenance cost
                                ■ Increase lifetime of the lighting system

                                https://cdn.badcaps-static.com/pdfs/...34696944c0.pdf

                                But, new LED TV used 2000 LEDs to make more brighter LED screen. I predict ( Auto-bypass LEDs will be use to build Auto recovery LED screen) in the future !
                                (I mean, LED will build in bypass circuit in the chip which just two more diodes. )

                                Tips: So next time don't put a cooper wire cross the defect LED put a diode.This allows the right amount current to flow through the other LEDs.
                                Last edited by Peter Chiu; 03-28-2015, 12:39 PM.

                                Comment

                                • Peter Chiu
                                  Banned
                                  • Dec 2012
                                  • 145
                                  • USA

                                  #17
                                  Re: how to test lcd led backlight?

                                  Constant Current Driver for LED Backlights Tester

                                  https://cdn.badcaps-static.com/pdfs/...eeae3f24ee.pdf

                                  Comment

                                  • tom66
                                    EVs Rule
                                    • Apr 2011
                                    • 32560
                                    • UK

                                    #18
                                    Re: how to test lcd led backlight?

                                    Originally posted by Peter Chiu
                                    ? Auto-recovery LED screen ? Resettable Fuse ? Auto-bypass LED?

                                    ST has LBP01 part can bypass any defected LEDs

                                    Benefits
                                    ■ Keep LED strings on in case of LED open mode failure
                                    ■ Reduced maintenance cost
                                    ■ Increase lifetime of the lighting system

                                    https://cdn.badcaps-static.com/pdfs/...34696944c0.pdf

                                    But, new LED TV used 2000 LEDs to make more brighter LED screen. I predict ( Auto-bypass LEDs will be use to build Auto recovery LED screen) in the future !
                                    (I mean, LED will build in bypass circuit in the chip which just two more diodes. )

                                    Tips: So next time don't put a cooper wire cross the defect LED put a diode.This allows the right amount current to flow through the other LEDs.
                                    Problem is, who's paying for this? Manufacturers won't care too much unless it lowers cost of production or reduces in-warranty failures.
                                    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

                                    • Peter Chiu
                                      Banned
                                      • Dec 2012
                                      • 145
                                      • USA

                                      #19
                                      Re: how to test lcd led backlight?

                                      LED is a diode too, when manufacture do the masking, it wont cost them not too much to do it.
                                      Basically just a P-N junction.
                                      Attached Files
                                      Last edited by Peter Chiu; 03-28-2015, 02:38 PM.

                                      Comment

                                      • tom66
                                        EVs Rule
                                        • Apr 2011
                                        • 32560
                                        • UK

                                        #20
                                        Re: how to test lcd led backlight?

                                        Not as simple as that. Introduces yield issues, LEDs are made from different types of semiconductor (GaAs is quite common), GaAs doesn't make good transistors.

                                        One possible solution would be a zener diode across the LED, the zener would conduct, heat up then anti-fuse shorted if the LED went open.
                                        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

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