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My TV has a damaged LED in its backlight. The LED had started flickering like 2 weeks ago and finally the whole backlight stopped working. Nothing obviously unusual on the LED driver board, the MOSFET and the sense resistors seem to be okay, no short, body diode working, no visible burn marks.

So I want to replace the damaged LED, but by which model? With a battery (8.2V) and a 400 Ohms resistor a single LED shows 2.69V @ 15 mA. There are 22 LEDs in total (two rows with 7 LEDs and one row with 8 LEDs), all in one series circuit. Each SMD-LED has approximately 3mm by 3mm body with <=1mm height and a circular radiative region.

I have no idea what could be the maximum rated current or the color temperature. Is there any chance of finding a right one? Are SMD LEDs of TV backlights somehow standard types?

enter image description here

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  • \$\begingroup\$ There is no way you can what to replace it with without knowing what part they used. Parameters like viewing angle and brightness won't be easily measured. \$\endgroup\$
    – Lundin
    Commented Dec 17, 2019 at 12:23
  • \$\begingroup\$ Of course I am not expecting to restore a high quality state. It is mainly my daughter who watches on that TV set and she is not so picky. Getting it to work again without failing again soon will be okay. \$\endgroup\$
    – oliver
    Commented Dec 17, 2019 at 12:29
  • \$\begingroup\$ Find a replacement TV from eBay. Even a non-working parts unit. Cannibalize an LED from that. Still, the color and brightness (due to age) may be slightly different. \$\endgroup\$
    – rdtsc
    Commented Dec 17, 2019 at 13:17
  • \$\begingroup\$ @rdtsc: good tip in principle, but since I am not doing repairs commercially, I would be accumulating electronics waste, which I would like to avoid \$\endgroup\$
    – oliver
    Commented Dec 17, 2019 at 13:39

1 Answer 1

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If the LEDs are all in series, then the driver is a constant-current source.

I assume you have a good idea of which LED is bad. Simply connect your ammeter across it and fire it up — this will tell you directly how much current the source is producing.

Color temperature will have to be judged by eye.

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  • \$\begingroup\$ I have already unsoldered the bad one, yes. The problem is, the TV is disassembled at the moment. And I am not so sure if the backlight will be lit (or probably I am gonna damage something) if the LCD itself is disconnected while trying to power the backlight on. \$\endgroup\$
    – oliver
    Commented Dec 17, 2019 at 12:27
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    \$\begingroup\$ Why would anything be damaged? But if you're really worried about it, put it back together. You can just solder a short across the bad LED's location and measure the current at the source. At this point, you don't have much to lose -- the TV is junk if you can't fix this, right? \$\endgroup\$
    – Dave Tweed
    Commented Dec 17, 2019 at 12:30
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    \$\begingroup\$ Yes, that's perfectly reasonable for backlight LEDs. They are typically 1W devices -- roughly 300 mA @ 3.3V. \$\endgroup\$
    – Dave Tweed
    Commented Dec 17, 2019 at 13:28
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    \$\begingroup\$ Buy another 6500k (or close) led rated for that current and it should work reasonably well. You could get a couple different color temperature diodes, but standard rec709 is 6500k white temp, so the other diodes will be close. Given the diffuser my guess is anything close to that won't look too bad. \$\endgroup\$
    – user1850479
    Commented Dec 17, 2019 at 15:50
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    \$\begingroup\$ I have bought 10 leds for 6 bucks on ebay, which happened to be designated as for TV backlight, and according to the (private) seller they are from LG like the panel I have been repairing (probably he was in the same situation and had to buy a lot more than needed). Current is in spec (400mA), the package is exactly the same size and ... tadaaa ... color temperature isn't perceptibly different from the original ones. \$\endgroup\$
    – oliver
    Commented Dec 18, 2019 at 14:44

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