use spare led backlights from one tv to test the led driver of another tv?

[triplefour]

i came across an LG tv with a busted screen so i took everything apart down to the led strips. im thinking i should be able to use this to plug into other tv's that have the question: "is this LED driver problem, or LED strip problem?"
which is hard to answer it seems...without opening the panel which is a big deal.

upon closer inspection i realized that the strips are split in the L and R but each L and R are combined to make one long strip

the 4 pins coming from the power supplys LED driver are 2 pairs of + and - for the led's
each +- pair drives 3 of the strips in series

i guess i just want to use this as a tool for learning and diagnosing other TV's but im still foggy on a few things. or uh...a lot of things.

if i unplug one of the strips, the circuit is not completed and the rest of the strips only just flash for a moment and turns off..as to be expected...
but if there are 2 pairs of the +- and they are each driving 3 strips on a separate circuit, why do all the strips go out instead of just the 3 that were on that circuit?

something in the driver notices all the strips arent on, so shuts it all down, i guess?

if i just took a wire and bridged the circuit where i removed one of the LED strips, what would happen? would they all come on, but they would use more power than they were supposed to and then burn out?

like if it was meant to run 6 strips but i bridged all but one of them, that one led strip would receive all the power meant for 6 strips and then blow up?

and ultimately...how can i actually use this device to test other TV's LED strips? specifically this one for the sony bravia 55 inch:
https://www.badcaps.net/forum/showth...t=39670&page=4
but on that one i have 4 sets of +- pairs .... how would i go about rigging that up?

had this tv for a while now and i know the client is getting impatient but i REALLY want to avoid opening that big of a panel if i can. i almost did it the other day but i decided i should really try EVERYTHING else i can think of first

are LED strips basically all similar? or would mixing and matching led strips lead to trouble?

[Agent24]

Re: use spare led backlights from one tv to test the led driver of another tv?

They all use same voltage?

[triplefour]

Re: use spare led backlights from one tv to test the led driver of another tv?

not sure i understand your question. are you asking if all LED strips use the same voltage?
im not sure because i really dont understand LED strips and circuits yet...

so that's a good question. anyone have the answer?

on the sony bravia 55 inch there are 4 negative pins on one connector and 4 positive pins on another...so 4 +- pairs...if its anything like this one i have (the LG i stripped down to psu and backlights) then each of those +- pairs drives a certain number of LED strips in series. do the numbers have to be equal? im not so sure, because they are in series you could hook up odd or even numbers of strips... for example on this LG there are 3 strips per +- pair but i dont see why they couldnt have been 4 on one, and 2 on the other... but then the psu would have to give them
different voltages?

am i correct that the voltage coming out from the LED driver circuit cant be tested with my cheap multimeter?
just like the voltage coming from the inverter boards going to the back lights?

if you were asking if the voltage for the led driver in the sony is the same as the voltage in the LG, i dont know that either because im afraid to just try testing volts there

still just groping around in the dark but i will find the light switch eventually

[Agent24]

Re: use spare led backlights from one tv to test the led driver of another tv?

Exactly. I assume most use a fairly standard voltage (eg: 12v), if they do, probably you can swap them.. otherwise, maybe not (or you may burn them out)

Of course, since it's just LEDs, not something complex lile CCFL with strking voltage, surely any dummy load made of resistors etc would work to test the LED power supply? Then again, maybe they do have some trickery to their operation and I am wrong.

[budm]

Re: use spare led backlights from one tv to test the led driver of another tv?

Typical white LED Vf (Forward Voltage) of 3.1~4V (http://www.maximintegrated.com/en/ap...ex.mvp/id/3070)

So if you have a LED strip that has 10 LED in series, then the total Vf can be as high as 40V, to power it up you will need current limiter resistor to limit the current which means the power supply will have to be a little higher than 40V.
The problem with LED driver in the TV is that most of them have no label on the board to tell you what the drive output Voltage is, most of them are from 80 to over 100V, so if you connect the 10-LED strip to it, you will be in trouble. You need to know what the LED driver Voltage is before hooking up the LED strip.
Some LCD panel spec will provide the information.

[triplefour]

Re: use spare led backlights from one tv to test the led driver of another tv?

i will try looking again for information about the sony panel. thanks for the explanation!

maybe if i can find what the driving voltage is, then i can guess how many LED there are?

or basically if the driving voltage is about the same on the 2 boards then it should just work?

im thinking since the sony has twice as many +- pairs that maybe i can just use the LG psu to light up half of them?

[budm]

Re: use spare led backlights from one tv to test the led driver of another tv?

"maybe if i can find what the driving voltage is, then i can guess how many LED there are?" Yes.

[budm]

Re: use spare led backlights from one tv to test the led driver of another tv?

https://www.badcaps.net/forum/showth...195#post494195

How are these LED strip are connected? 6 strips total?

[triplefour]

Re: use spare led backlights from one tv to test the led driver of another tv?

yes in the LG that i have stripped down to the PSU, panel plate, and backlights there are 6 strips. each strip is actually 2 smaller strips that fit together to span the whole width of the screen.
but i think when they are connected like that u count them as one strip, right?
so yes 6 strips.
there are 4 pins going to the strip-board. labeled +-+-
one of the +- pairs runs 3 of the strips in series
the other +- pair runs the other 3

how can i tell the driving voltage coming from the psu? can i measure it myself? or do i have to just look it up?

[triplefour]

Re: use spare led backlights from one tv to test the led driver of another tv?

also is there a great difference between EDGE lit LED vs direct lit? i mean in voltages for driving.
i pulled out some edge lit backlights from another tv i parted out yesterday.

[Ltank]

Re: use spare led backlights from one tv to test the led driver of another tv?

Some LED LED sets use as much as 330 vdc at about 280ma.
I have tested known good panels with ohmmeter and the resistance was higher than 3 meg that my meter can read. I too tried to figure what resistance and wattage can be used to test power supplies

[budm]

Re: use spare led backlights from one tv to test the led driver of another tv?

330V @ 280mA = 1178 Ohms, 92 Watts if it is one string. So you can use 1200 Ohms resistor.

[triplefour]

Re: use spare led backlights from one tv to test the led driver of another tv?

the service manual for the sony Bravia 55 KDL-55EX640 doesnt say anything about the LED's other than that one line that they are edge lit! how can i tell what the driving voltage should be to be able to test the backlights?? the information has to be out there somewhere...
is it safe to test the voltage coming out of here while the tv is powered on and the backlights are on? (this is the LG panel that this thread is about originally)

[budm]

Re: use spare led backlights from one tv to test the led driver of another tv?

You can check the Voltage at those caps close by, they are for the power supply for the LED. If the backlights are working then you measure the Voltage at the LED+, LED -. It looks like it has two LED strings.
Since this https://www.badcaps.net/forum/attach...7&d=1413982161 is working, why don't you see how the LED strings are connected and count how many LED are in the stirngs. There are 54 LEDs total so see how they are connected, Is it 27 LED per LED+ LED-? You trace them out and also look at the ICs around that area of the LED connector to see which one is for the LED driver, that is how you learn, you have do the research.

[triplefour]

Re: use spare led backlights from one tv to test the led driver of another tv?

i did trace it... there are 2 +- pairs and they each drive 3 led strips in series

we call those 3 strips in series a string?

so yes it is 27 per string...27 per +- pair

so its safe to check those voltages? and amps too?

[budm]

Re: use spare led backlights from one tv to test the led driver of another tv?

27 LEDS x 3.1V (or 4V) = 83.7V (or 108V) so measure the Voltage at LED + LED -, the Voltage reading will not show up as calculated since the are may be PWM driven. So check the voltage to see what you get. Getting the IC spec sheet will give you more details.

[Alastair E]

Re: use spare led backlights from one tv to test the led driver of another tv?

How about using a 20-40W 120v bulb in place of the LED to see if the supply runs it....?

[budm]

Re: use spare led backlights from one tv to test the led driver of another tv?

40W cold resistance will have cold resistance around 36 Ohms (Hot reistance will be 360 Ohms) so the current sensing circuit will cause the power supply to go into shutdown mode due to current of of over 2A @ 83V at first power applied.

[Alastair E]

Re: use spare led backlights from one tv to test the led driver of another tv?

Always summit--Innit!

These damned things are getting too clever by half!

[triplefour]

Re: use spare led backlights from one tv to test the led driver of another tv?

is there a reisistor that starts out high resistance at room temperature and then goes low resistance as it heats up?