How to find specs for LEDs without a circuit diagram

Thread Starter

yourminky

Joined May 31, 2019
33
What are the typical forward voltage and current for high power individual LEDs used inside TVs? Is there any way to find out this information doing reverse engineering techniques when there are no circuit diagrams available? Thanks.
 

bidrohini

Joined Jul 29, 2022
93
If you're talking about the pixels of the LED TV, I don't think we can find out the specs. Because, we'll never know which brand uses which model or package of LEDs.
 

crutschow

Joined Mar 14, 2008
30,765
What would you do with that information?
If you have electrical access to the LEDs than you can do some measurements to roughly determine their ratings.
 

MrSalts

Joined Apr 2, 2020
2,350
What are the typical forward voltage and current for high power individual LEDs used inside TVs? Is there any way to find out this information doing reverse engineering techniques when there are no circuit diagrams available? Thanks.
Start by assuming a forward voltage of about 3.0v and selecting a resistor limiting current to about 20mA. Once connected, measure the actual Vf with your meter. Vf actual may be much higher and that means you won't have 20mA. Replace with a smaller value resistor to get the 20mA again and repeat Vf. Vf tends to increase a bit with increased current so you'll have to play a bit. That's Vf at 20mA.
Then you can hope and pray that you have enough LEDs across the backlight array to experiment a bit. Just keep adding current (slowly with smaller and smaller value resistors until yu get a bright light that doesn't suddenly change color (more bluish or more red). Once you hit that point, you've gon too far. Then try again and stop at a current level about 20% lower current and hold at that level for some hours to see if the color shifts considerably. At some point, you have to feel comfortable with your guess.

In general, backlight LEDs from 10-years ago we're about 120mA and now the newer, more efficient LEDs will be closer to 60mA (if you can find a back-lit display- most newer displays are edge-lit with 150mA LEDs (3v Vf)).

good luck.
 

LesJones

Joined Jan 8, 2017
3,805
Can you post a picture of the LEDs ? (From my understanding if it was one LED per pixel for a 1920 x 1080 resolution TV it would require over two million LEDs.)

Les.
 

MrSalts

Joined Apr 2, 2020
2,350
Can you post a picture of the LEDs ? (From my understanding if it was one LED per pixel for a 1920 x 1080 resolution TV it would require over two million LEDs.)

Les.
LCDs do not make any light, they must be illuminated from behind. Up until about 2012, most LCD displays were backlight with Fluorscent tubes. Then they started to get edge light by fluorescent tubes for thinner displays, then backlight by an array of white LEDs and, most recently, edge light by a row of LEDs for smaller lap-top displays. You should really try taking some alert and seeing how the technology has evolved. Some of the older fluorscent tube edge-lit monitors had surprisingly thick cover lenses with engineered texture and white translucent ink to homogenize and outcouple the light evenly behind the LCD panel.

QLED TVs are made by the likes of Samsung and OLED TVs by Sony and LG. These have the one LED per pixel but the "LEDs" are in a die with multiple LED pixels on each chip - there is no way to remove these and illuminate them as a hobbyist.

The OP is surely talking about the edge light or backlight LED arrays for an LCD screen. If not, he will be sad about the answers we'lol have to give him.
 

Thread Starter

yourminky

Joined May 31, 2019
33
Thanks for your comments. I am talking about backlight LEDs for 4k TVs. I haven't opened the TV yet so I do not have pictures. I am doing as much research as possible before opening the TV. Only the middle part of the curved TV is bright and rest is very dim, which is a typical burnt backlight LED issue. I got this TV for free and do not want to buy a replacement LED strip from eBay, I just want to experiment with it and see if I can fix it with the spare high power LEDs I have.
 

MrSalts

Joined Apr 2, 2020
2,350
Thanks for your comments. I am talking about backlight LEDs for 4k TVs. I haven't opened the TV yet so I do not have pictures. I am doing as much research as possible before opening the TV. Only the middle part of the curved TV is bright and rest is very dim, which is a typical burnt backlight LED issue. I got this TV for free and do not want to buy a replacement LED strip from eBay, I just want to experiment with it and see if I can fix it with the spare high power LEDs I have.
LEDs for a back light panel are typically "binned" before they are selected for use. In this case, binning, means they are each powered up to illuminate a white test wall, and a photograph is taken of the light they make on the test panel. The photo is processed and the LED is put into a bin with other LEDs that makes the same color. Then panels are made out of all the same color LEDs. It sounds time consuming and it originally was but there are now high speed robotic binning systems that can do more than 10k LEDs per hour.

I have an old laptop that needed a replacement display. The .Lenovo display price was $275 and a cheap, non-Lenovo replacement on eBay was $68. Well, there was a mis-binned backlight in the cheap display that looks like someone dropped coffee there (warmer color LED). Moral of the story, your replacement backlights may look weird with a checkerboard array of light and dark, tan to bluish.

Good luck.
 

LesJones

Joined Jan 8, 2017
3,805
I have recently replaced the backlights on two 32" TVs. One set cost £16.00 (2 strips of 11 LEDs) and the other £20.00 (3 strips of 6 LEDs) I think that was a reasonable price to get a set of matched LEDs. (The LEDs are surface mount on aluminium substrate so they would be difficult to replace.)

Les.
 

bassbindevil

Joined Jan 23, 2014
554
Sometimes the drive current is printed on the power supply board. It was 430 mA for the TV that fell from a precarious mount in a friend's bedroom. (Who needs expensive flat-screen mounts when there are dollar store bungee cords?) IIRC the power board also included a voltage that worked out to 3.6v per LED, but when I put 430 mA through a salvaged LED it measured 3.02 volts. It's possible that newer TVs have SMD LEDs with multiple LEDs in series hiding inside, like I've seen in LED light bulbs. Try probing a good LED with a power supply (with current limiting or a series resistor) and measure the voltage.
 

Thread Starter

yourminky

Joined May 31, 2019
33
Sometimes the drive current is printed on the power supply board. It was 430 mA for the TV that fell from a precarious mount in a friend's bedroom. (Who needs expensive flat-screen mounts when there are dollar store bungee cords?) IIRC the power board also included a voltage that worked out to 3.6v per LED, but when I put 430 mA through a salvaged LED it measured 3.02 volts. It's possible that newer TVs have SMD LEDs with multiple LEDs in series hiding inside, like I've seen in LED light bulbs. Try probing a good LED with a power supply (with current limiting or a series resistor) and measure the voltage.
Thanks, I'll try that.
 

Thread Starter

yourminky

Joined May 31, 2019
33
I have recently replaced the backlights on two 32" TVs. One set cost £16.00 (2 strips of 11 LEDs) and the other £20.00 (3 strips of 6 LEDs) I think that was a reasonable price to get a set of matched LEDs. (The LEDs are surface mount on aluminium substrate so they would be difficult to replace.)

Les.
It is just a challenge project for me. Trying to fix it with whatever I have.
 

Thread Starter

yourminky

Joined May 31, 2019
33
LEDs for a back light panel are typically "binned" before they are selected for use. In this case, binning, means they are each powered up to illuminate a white test wall, and a photograph is taken of the light they make on the test panel. The photo is processed and the LED is put into a bin with other LEDs that makes the same color. Then panels are made out of all the same color LEDs. It sounds time consuming and it originally was but there are now high speed robotic binning systems that can do more than 10k LEDs per hour.

I have an old laptop that needed a replacement display. The .Lenovo display price was $275 and a cheap, non-Lenovo replacement on eBay was $68. Well, there was a mis-binned backlight in the cheap display that looks like someone dropped coffee there (warmer color LED). Moral of the story, your replacement backlights may look weird with a checkerboard array of light and dark, tan to bluish.

Good luck.
Thanks for the tip.
 
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