reusing LED tv backlights


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Old 05-02-19, 01:03 AM
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reusing LED tv backlights

Hi, I replaced all the backlights in my tv leaving me with about 52 used but still working LEDs (2 were faulty)
There are 6 strips of 5 leds and 6 strips of 4, then link each linking together to amount to 6 strips of 9leds. Voltage for all is 220v here's a link to the backlights:

https://www.aliexpress.com/item/12-P...847756348.html

The only other info about the lights is that each led has a voltage of 2.8

I watched a video on youtube of a guy running similar backlights with a 12v power supply but I'm confused with how exactly he did it.

https://www.youtube.com/watch?v=4O37rHsk-20

I have an old 19v laptop power supply and I want to know how to do something similar.
The amount of leds I can use is not really important, I just want it to be safe.
Any help is much appreciated
 
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Old 05-02-19, 11:28 AM
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Many (if not most) individual LEDs operate at 3-5 VDC. Despite what the website says, I doubt very seriously they operate directly from 220 VAC. They can be made to operate at different voltage by connecting some in series and some in parallel.
Wrong voltage and either they won't work or you burn them up. Unless you can determine the wiring of all the LEDs in a strip, your best bet is to meter the LED supply voltage in the TV. Otherwise, it's a bit of a crapshoot.
 
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Old 05-02-19, 11:40 AM
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Thank you for replying.
I've been reading about configurations of leds for a few days and apart from being able to solder that about sums up my abilities in this situation at the moment.
Sorry if it's a stupid question but why does 220v sound hard to believe?
Did you see the video of the guy powering the strips off a 12v power supply? In the comments I asked him for more details and he said he's running 16 leds on the same power supply.
 
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Old 05-02-19, 12:10 PM
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It would be 220v DC not AC.

The LED's are in series on those strips. Take one 5 LED strip and cut it from the rest of the group. Determine the (+) plus connection and the (-) minus connection. Connect the one strip to your 19vDC power supply. It may be very bright.

Do the same with two 4 LED strips. Leave two strips connected together and feed each end with your power supply. With 8 LED's in series they won't be so bright.
 
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Old 05-02-19, 12:33 PM
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Great thanks!

Just to be clear, are you saying my two best options are using 5 or 8 leds?
 
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Old 05-02-19, 12:45 PM
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You can experiment to see which combination looks the best. 8 on your power supply should be good. You may be able to run 3 or 4 sets of 8 on that power supply.
 
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Old 05-02-19, 01:49 PM
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So I've just tried a few strips.
It barely lights 8.
But 4 and 5 are very bright! However I can't tell which is brighter.

I left the 4 plugged in and after about 10 seconds they fried.
Now I'm afraid to try the 5.
 
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Old 05-02-19, 02:03 PM
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That's part of the fun.... experimenting.

The 4 would be brighter than the 5. 6 would probably be a correct quantity with no resistor. The LED's are on a board and connected with pc board foils. You will have to locate the foils and cut them between the LED's. That's what the guy in the video was doing.
 
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Old 05-02-19, 02:09 PM
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Okay it's good to hear you say that, I was just thinking that 6 should be the perfect amount.

I'm a bit limited space wise to 80cm/ 32" so I'm going to try and cut 2 off the end of the 8 to make 6 and solder the foils together.

Then the most difficult part for me to grasp is connecting say 3 sets of 6 parallelly (if that's a word)
 
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Old 05-02-19, 03:19 PM
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Yes..... you will be paralleling three sets of six.
That just means that all three +'s get connected together to power supply and all three -'s get connected together and then to the supply.

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Old 05-02-19, 03:30 PM
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Thanks so much!
So I took a strip with 5 leds and cut off the tip, then connected the end of a 4 led strip (with 1 led ) to give me 6, and it works!

Okay so I'll do that 2 more times to give me 3 strips. Then can I just solder some wires from any place that's positive on one strip to another positive place on the other?
 
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Old 05-03-19, 12:03 PM
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Hi again,

So today I wired my 3 sets of 6 leds in parallel.
When I plugged it in no leds lit up and I heard a crack/pop.

I took it apart again and each individual strip now doesn't work either.

I tested each led and they're all fine except for three. Exactly the last one at the end of each strip at the positive end.
I'm sure there's a reason why these 3 specific leds blew but I don't know it, any ideas?
 
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Old 05-03-19, 12:12 PM
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It was like this, does that look okay?

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Last edited by PJmax; 05-04-19 at 10:24 PM. Reason: resized pictures
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Old 05-03-19, 02:13 PM
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It looks like there is a two pin plug at the end of each strip...... that should mean that the + and - connect at one end. Typically one of those pins would connect to the nearest LED while the other pin would connect to the last pin. You need to trace the foils on the board to see how they are series connected.

Diagram as an example......
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Old 05-03-19, 02:20 PM
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Yes it's exactly like the diagram, the + connects to the first and the - connects to the last.
What did I do wrong?
 
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Old 05-03-19, 03:11 PM
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Maybe it should be like this?

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Last edited by PJmax; 05-04-19 at 10:27 PM. Reason: resized picture
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Old 05-03-19, 03:40 PM
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Yes..... that would seem to be correct. Try one first.
 
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Old 05-04-19, 01:41 AM
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Next problem haha.

I need to replace the last led on each strip but because the last one is wired differently to the others I'm out of spare "last leds".

Here are some pictures and how I believe they work.

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Last edited by PJmax; 05-04-19 at 10:30 PM. Reason: resized pictures
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Old 05-04-19, 01:47 AM
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So I think if I do this I will have joined the + and - through the last led.
Is that correct?

Okay I can't attach any more pictures.
 
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Old 05-04-19, 10:31 PM
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I'd use an ohmmeter to trace the circuit. Makes a positive confirmation.
I resized your pictures and you have plenty of space.
 
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Old 05-04-19, 11:52 PM
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I put 2 sets of 6 in parallel and it works great! Thanks a million for all your help!!
 
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Old 05-05-19, 10:46 AM
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I've been reading and trying to find out the forward current on these lights just for interests sake.

My 19v power supply has 3.16A written on it.
If I have 2 strips in parallel then that means 1580mA per strip, so 263mA per led? That sounds wrong.

Most information online says these leds run 20-80mA but that would mean I could run from 7 to 26 strips in parallel. That also sounds wrong haha.
 
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Old 05-05-19, 11:08 AM
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Side note: As LEDs get hot, the more current they draw. The more current they draw the hotter they get. If the current is not regulated this cycle can continue until they burn themselves out. Many LED lights will have current limiting drivers to prevent this from happening.
 
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Old 05-05-19, 11:16 AM
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The power supply provides an approximately constant 19V independent of the load current up to the maximum allowable load current of 3.16A. The current drawn by an LED depends on (and increases with) the amount of voltage applied to it. With a 19V supply and 6 LEDs in series, each LED is supplied with about 3.17V and this establishes the current that is drawn. The 20mA to 80mA that you mentioned is a maximum recommended operating current for the LED.
If you have 6 LEDs in series on a strip then the same current flows through every LED in the chain. So if each LED is drawing 80mA then the strip of 6 LEDs also draws 80mA.
 
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Old 05-05-19, 11:18 AM
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Yes I'm aware thanks. In my first post I noted that I just want it to be safe

I'm just wondering now if it's possible or even safer to add more strips without pushing the limits.
I've read that if/when one led burns out that the forward current will be divided by one less, putting more current through each led, so maybe starting with more is actually safer??
 
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Old 05-05-19, 11:22 AM
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Okay interesting, so how many strips can this power supply run?
Bear in mind that I don't have any resistors.
 
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Old 05-11-19, 07:00 AM
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Does anyone know how to calculate a safe amount?
The limit would be 39 strips but obviously I won't do that.
Is it safe to add a few more ?
 
 

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