how to test lcd led backlight?

[pnv]

please post test methods you use
thanks!

[Hamie]

Re: how to test lcd led backlight?

Jump with a low ohm resitor between ps-on and 5v stby, and 5vstby and bl-on (inverter on off)

[budm]

Re: how to test lcd led backlight?

OK, are you asking how to test individual LED mounted on the circuit board that form LED string?
You can use 3V battery with 100 Ohms resistor to limit the current for testing individual LED.
Or you already verify that the LED driver circuits are OK but the LED still does not light up?
Test LED string current draw using DCA.
https://www.badcaps.net/forum/showpo...4&postcount=95

Or used indicator:
https://www.badcaps.net/forum/showpo...&postcount=107

[pnv]

Re: how to test lcd led backlight?

Yes, something like an adjustable voltage source with constant current

[pnv]

Re: how to test lcd led backlight?

his would be a constant current source would be the voltage regulator.
What do you think?

[budm]

Re: how to test lcd led backlight?

Yes, something like an adjustable voltage source with constant current

That will be correct if all the LED int he string are good, but if one goes open circuit then nothing will happen, but you need to know how many LEDs are in the string to determine what the power supply voltage needs to be, some string you will need more than 100VDC, Vf of each LED is between 3~3.6V.

[pnv]

Re: how to test lcd led backlight?

Please, if not asking too much to draw scheme Led string indicator to find out which string is dead.

[Alastair E]

Re: how to test lcd led backlight?

LED's are wired like christmas-tree lights....

One dead--all out.

You just have to test each one in turn. No schematics, just a meter.....

[pnv]

Re: how to test lcd led backlight?

I think it is exciting and dynamic method of insertion of an indicator Lord budm

[keeney123]

Re: how to test lcd led backlight?

It would be nice if these led strings had standards for each size of screen then one could build a test circuit just to plug in and see if the individual strings worked or not, but that would be to easy.

[budm]

Re: how to test lcd led backlight?

I think it is exciting and dynamic method of insertion of an indicator Lord budm

See picture, the diodes will get hot so do not run it too long.

[CapLeaker]

Re: how to test lcd led backlight?

See picture, the diodes will get hot so do not run it too long.

Now that is a neat testing device I hafta make! Thanks budm!

[keeney123]

Re: how to test lcd led backlight?

Bud M don't laugh at me, but I was hoping to unplug all the LED strings and plug them into a separate power source that would eliminate any feed back circuit from turning the the LED driver chip off.

[budm]

Re: how to test lcd led backlight?

If you want to use external constant current power source to drive the LED string then the strings are no longer connected to the LED driver board) so there is need to have any power to the TV at all since you are just testing LEDs.
The harder part is to figure out how the strings are connected together and which wire is the Anode and which wire is the Cathode if the panel is not open up to see the LED string, or you can trace the connector on the driver board to see how it is done.

[keeney123]

Re: how to test lcd led backlight?

Yes that is where standards would work.

[Peter Chiu]

Re: how to test lcd led backlight?

? Auto-recovery LED screen ? Resettable Fuse ? Auto-bypass LED?

ST has LBP01 part can bypass any defected LEDs

Benefits
■ Keep LED strings on in case of LED open mode failure
■ Reduced maintenance cost
■ Increase lifetime of the lighting system

https://cdn.badcaps-static.com/pdfs/...34696944c0.pdf

But, new LED TV used 2000 LEDs to make more brighter LED screen. I predict ( Auto-bypass LEDs will be use to build Auto recovery LED screen) in the future !
(I mean, LED will build in bypass circuit in the chip which just two more diodes. )

Tips: So next time don't put a cooper wire cross the defect LED put a diode.This allows the right amount current to flow through the other LEDs.

[Peter Chiu]

Re: how to test lcd led backlight?

Constant Current Driver for LED Backlights Tester

https://cdn.badcaps-static.com/pdfs/...eeae3f24ee.pdf

[tom66]

Re: how to test lcd led backlight?

? Auto-recovery LED screen ? Resettable Fuse ? Auto-bypass LED?

ST has LBP01 part can bypass any defected LEDs

Benefits
■ Keep LED strings on in case of LED open mode failure
■ Reduced maintenance cost
■ Increase lifetime of the lighting system

https://cdn.badcaps-static.com/pdfs/...34696944c0.pdf

But, new LED TV used 2000 LEDs to make more brighter LED screen. I predict ( Auto-bypass LEDs will be use to build Auto recovery LED screen) in the future !
(I mean, LED will build in bypass circuit in the chip which just two more diodes. )

Tips: So next time don't put a cooper wire cross the defect LED put a diode.This allows the right amount current to flow through the other LEDs.

Problem is, who's paying for this? Manufacturers won't care too much unless it lowers cost of production or reduces in-warranty failures.

[Peter Chiu]

Re: how to test lcd led backlight?

LED is a diode too, when manufacture do the masking, it wont cost them not too much to do it.
Basically just a P-N junction.

[tom66]

Re: how to test lcd led backlight?

Not as simple as that. Introduces yield issues, LEDs are made from different types of semiconductor (GaAs is quite common), GaAs doesn't make good transistors.

One possible solution would be a zener diode across the LED, the zener would conduct, heat up then anti-fuse shorted if the LED went open.