LED identification (or equiv replacement) - Page 1
Forum section: electronics / repair

author: TheMG
author_rank: Frequent Contributor
timestamp: August 30, 2022, 11:02:14 pm
content: Got a TV given to me with a defective backlight. It's a HiSense 55Q7809.I want to do this repair as cheaply as possible so my plan is to just replace the defective LEDs (yes, I'm fully aware other LEDs may soon fail, but I don't care, one or two defective strips isn't very noticeable and the TV fortunately does not shut down the whole backlight when there's a few bad strips, besides this TV was on practically 24/7 whereas I am only going to use it maybe a couple hours a week). Each strip has two open-circuit LEDs (there are two series circuits on each LED strip in the backlight).Interestingly, they are blue LEDs (there's a white phosphor layer in the backlight diffuser).The LEDs have a forward voltage of about 6V, so each LED has two dies in series internally. Dimension is about 3mm X 3mm. It does mention "CREE" as the manufacturer on the LED strip PCBs.Unfortunately, I'm coming up empty on Digikey as far as LEDs meeting these requirements, especially ones with two dies. Ideas?

author: thm_w
author_rank: Super Contributor
timestamp: September 02, 2022, 09:24:17 pm
content: "As cheap as possible"https://www.aliexpress.com/item/32815512565.htmlhttps://www.bridgelux.com/sites/default/files/resource_media/DS297%20SMD%203030%201W%206V%20Gen3%20Data%20sheet%20Rev%20A%2020200320.pdfIt looks like most have the phosphor, I assume that wouldn't work.

author: TheMG
author_rank: Frequent Contributor
timestamp: September 03, 2022, 08:06:03 pm
content: Awesome, I think these will work, though shipping is likely to take a good amount of time from Aliexpress. The blue LEDs on offer do not have a phosphor coating.I tried an experiment and bridged all of the bad LEDs. The TV seems to be perfectly happy running like this, and as expected the current is normal as the LED driver is regulating the current so the lower total forward voltage doesn't seem to matter much.The good news is the few bad LEDs aren't noticeable on the image except on a solid background, so basically 95% of the time you can't tell, and the overall brightness is still more than sufficient.I'll run the TV like this, and in the mean time order the LEDs from Aliexpress to have on hand if more LEDs start to give out in the backlight in the future, at which point I'll probably replace the whole complement of LEDs.As for how the LEDs failed, the picture is the best I could do with my macro lens (I do not have a microscope), but there is an obvious crack in the potting/gel that protects the LED dies and bond wires. Looks like with long-term exposure to heat the coating hardened, shrunk, and cracked which probably severed the bond wire.

author: fzabkar
author_rank: Super Contributor
timestamp: September 03, 2022, 08:13:53 pm
content: Just curious, how many LEDs in each string, and what is the LED current per string?

author: TheMG
author_rank: Frequent Contributor
timestamp: September 04, 2022, 03:42:22 am
content: 7 LEDs per string (2-chip LEDs with Vf of 6V). Current measured about 260mA at max brightness setting though I can't confirm the accuracy of that measurement since the TV does employ local dimming and such, and the LEDs are definitely being PWM.