LED strip tester. Make or buy ?

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  • Dannyx
    CertifiedAxhole
    • Aug 2016
    • 3912
    • Romania

    #1

    LED strip tester. Make or buy ?

    Good day folks. So perhaps one of the most common issue we repairmen encounter is dead backlight LEDs, so naturally tools have been invented to make our life easier by speeding up the process of finding the dead ones, especially in large panels. This of course leads to the age-old question: how would one go about building one of these themselves ? These little cheapy, plastiky, nasty looking boxes are pretty expensive and although it would be handy to have one, it would sure be a lot more convenient and DIY to....well.....DIY

    There doesn't seem to much to them the way I see it at least. I imagine an adjustable constant current source, since that's how LEDs are normally driven in backlights (except in this case we have to adjust the current to drive various types of strips, since some may have more/less LEDs, requiring a different current).
    Perhaps adapting a pre-existing supply like a power brick or an actual TV PSU might be easier than trying to start from absolute scratch.

    How's THIS look ? Of course it's nothing fancy: plenty of people have discussed this before. The point is not so much building the tool itself, since I'm fine with testing LED by LED the way I'm doing now with a 5v adapter and limit resistor, but it would be a learning experience, plus it would still come in handy in practice since it would allow me to power up the whole strip at once. I'm aware that if there's a dead LED somewhere the strip wouldn't light up at all and I'd still have to test LED by LED to find it, but if the LED failed shorted, THAT'S when I'd instantly see it, since the rest of them would still come on.

    Any thoughts ? Perhaps someone has already made one or has suggestions Cheers and thanks.
    Wattevah...
  • budm
    Badcaps Legend
    • Feb 2010
    • 40746
    • USA

    #2
    Re: LED strip tester. Make or buy ?

    The one I bought is only $30, its output can go up to 300V for testing LED string with high Vf. I cannot make one for $30.
    https://www.ebay.com/itm/0-315V-TV-L.../201713742817?
    Last edited by budm; 01-30-2018, 02:24 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

    • dskall
      Badcaps Legend
      • Oct 2016
      • 2905
      • usa

      #3
      Re: LED strip tester. Make or buy ?

      Would be nice if they had one with adjustable current
      I assume no responsibility for any stupid suggestions I might post.

      Comment

      • Dannyx
        CertifiedAxhole
        • Aug 2016
        • 3912
        • Romania

        #4
        Re: LED strip tester. Make or buy ?

        Originally posted by budm
        The one I bought is only $30, its output can go up to 300V for testing LED string with high Vf. I cannot make one for $30.
        https://www.ebay.com/itm/0-315V-TV-L.../201713742817?
        That's not exactly cheap, that's why I was asking That's a good point: can you make something of comparable performance for the same or less money ? Probably not.
        Wattevah...

        Comment

        • gabiz_ro
          Senior Member
          • Mar 2011
          • 167
          • Romania

          #5
          Re: LED strip tester. Make or buy ?

          One separation transformer,or may use two identical back to back connected,bridge rectifier,LR8K4,or more via multi-position switch,volt amp meter 300v
          Result is much better than this one.

          You may also use some universal TV power supply to get 300V in place of transformer.
          You can also use more LR8K4 set to 10mA and test at 10-20-30-... mA

          But still problem may exist.
          On my build only 10 20 and 30mA encountered two problems until now.
          All led light up and seems OK all in series and one by one but still on TV doesn't work.
          One led was bad, tested with power source and resistor ,over 100mA led was off.
          Another one same result all was OK on tester but at high current led was blinking

          I plan to extend my tester to more than 100mA for better results.

          Comment

          • Dannyx
            CertifiedAxhole
            • Aug 2016
            • 3912
            • Romania

            #6
            Re: LED strip tester. Make or buy ?

            LR8K4 looks identical to an LM317, just with a greater input range. Current limit of only 20mA would mean having to parallel several of them to go higher than that. How much current does a typical LED strip take anyway ? I've yet to measure that, I admit, despite having repaired hundreds so far
            Wattevah...

            Comment

            • gabiz_ro
              Senior Member
              • Mar 2011
              • 167
              • Romania

              #7
              Re: LED strip tester. Make or buy ?

              How much current does a typical LED strip take anyway ?
              How much can


              Around 350mA most often encountered in many,so around 1W led.

              Comment

              • Dannyx
                CertifiedAxhole
                • Aug 2016
                • 3912
                • Romania

                #8
                Re: LED strip tester. Make or buy ?

                Also, with LEDs, current tends to increase as they get hotter, hence why they use CC rather than CV supplies I reckon.
                Wattevah...

                Comment

                • budm
                  Badcaps Legend
                  • Feb 2010
                  • 40746
                  • USA

                  #9
                  Re: LED strip tester. Make or buy ?

                  There are weak points with LED tester due to it does not putting out full drive current, but not different than using meter to check semiconductor device since the meter does not put out enough Voltage or current at the same level as the circuit.
                  LED tester is used as a quick test to know if you have open circuit or to find out if it has shorted LED by reading the Vf on that digital display.
                  When you tester on just one or two LEDs then you will have to make sure that the active element that controls the CC can handle the power dissipation.
                  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

                  • cobrartv
                    New Member
                    • Jan 2018
                    • 1
                    • POLAND

                    #10
                    Re: LED strip tester. Make or buy ?

                    What do you think about this tester?
                    https://www.aliexpress.com/item/USB-...311.0.0.iuc0EM

                    Comment

                    • dskall
                      Badcaps Legend
                      • Oct 2016
                      • 2905
                      • usa

                      #11
                      Re: LED strip tester. Make or buy ?

                      Looks interesting
                      I assume no responsibility for any stupid suggestions I might post.

                      Comment

                      • budm
                        Badcaps Legend
                        • Feb 2010
                        • 40746
                        • USA

                        #12
                        Re: LED strip tester. Make or buy ?

                        If it can actually do 300mA then that will be a good tester, I will probably wait for the price to come down.
                        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

                        • Dannyx
                          CertifiedAxhole
                          • Aug 2016
                          • 3912
                          • Romania

                          #13
                          Re: LED strip tester. Make or buy ?

                          I just measured an LG I got on the bench right now out of curiosity and it's a 139mA CC supply apparently, as it's trying to maintain that even when I short across an entire strip with the meter in current measuring mode. It's got 3 strips with 7 LEDs each. Each LED has a drop of around 2.7v
                          Wattevah...

                          Comment

                          • budm
                            Badcaps Legend
                            • Feb 2010
                            • 40746
                            • USA

                            #14
                            Re: LED strip tester. Make or buy ?

                            Originally posted by Dannyx
                            I just measured an LG I got on the bench right now out of curiosity and it's a 139mA CC supply apparently, as it's trying to maintain that even when I short across an entire strip with the meter in current measuring mode. It's got 3 strips with 7 LEDs each. Each LED has a drop of around 2.7v
                            It has always been constant current circuit for driving the LED backlights, nothing new, 139mA is on the low side, most LG drives them around 400mA, adjust the backlights level and see, or put in VIVID mode, Vf will also goes up as more current is forced through the LED as you can see in the graph of the LED.
                            Last edited by budm; 02-01-2018, 10:19 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

                            • Dannyx
                              CertifiedAxhole
                              • Aug 2016
                              • 3912
                              • Romania

                              #15
                              Re: LED strip tester. Make or buy ?

                              Haven't really put my meter on LED strips before until now, despite having fixed quite a few, so it was mostly out of curiosity, not to report some interesting find.
                              Wattevah...

                              Comment

                              • dskall
                                Badcaps Legend
                                • Oct 2016
                                • 2905
                                • usa

                                #16
                                Re: LED strip tester. Make or buy ?

                                I am surprised it is 139ma. I would think they would be quite dim. But last forever
                                I assume no responsibility for any stupid suggestions I might post.

                                Comment

                                • budm
                                  Badcaps Legend
                                  • Feb 2010
                                  • 40746
                                  • USA

                                  #17
                                  Re: LED strip tester. Make or buy ?

                                  It must be small screen since it only has 21 LEDs.
                                  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

                                  • gabiz_ro
                                    Senior Member
                                    • Mar 2011
                                    • 167
                                    • Romania

                                    #18
                                    Re: LED strip tester. Make or buy ?

                                    Without input signal screen dim on some TV’s

                                    On all received for repair allways modified current on led driver to reduce current to extend life of led’s.
                                    Even so,led’s are pretty hot
                                    First time tested with my fingers,was too hot and I suppose was bad solder by me and heat can’t disipate.
                                    After few tries and same result I removed lens from another led and this one too was too hot.
                                    In some there is metal back plate somehow they keep a lower temp but in some cases led strips are mounted on plastic back plate (cheaper ones or in some Toshiba)
                                    That fails more often.

                                    Comment

                                    • Dannyx
                                      CertifiedAxhole
                                      • Aug 2016
                                      • 3912
                                      • Romania

                                      #19
                                      Re: LED strip tester. Make or buy ?

                                      Originally posted by gabiz_ro
                                      Without input signal screen dim on some TV’s

                                      On all received for repair allways modified current on led driver to reduce current to extend life of led’s.
                                      Even so,led’s are pretty hot
                                      First time tested with my fingers,was too hot and I suppose was bad solder by me and heat can’t disipate.
                                      After few tries and same result I removed lens from another led and this one too was too hot.
                                      In some there is metal back plate somehow they keep a lower temp but in some cases led strips are mounted on plastic back plate (cheaper ones or in some Toshiba)
                                      That fails more often.
                                      How do you normally do that ? By increasing the value of the sense resistor ? Haven't done it myself, but I imagine in a CC supply, you can modify either the "reference", let's call it or the feedback.
                                      Wattevah...

                                      Comment

                                      • gabiz_ro
                                        Senior Member
                                        • Mar 2011
                                        • 167
                                        • Romania

                                        #20
                                        Re: LED strip tester. Make or buy ?

                                        That depends on each one.
                                        Schematic and or datasheet of IC used and you can see what to adjust.
                                        Some have resistor in paralel for current sensing,remove one or two ar replace with another value.
                                        On crappy Toshiba,1333 series are four transistors there is need to disable one,removing from board,cut base trace.Result is 25% less current,but almost nothing signifiant on image.

                                        Comment

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