How to diagnose whether a TV LED is bad (without using an LED tester)?

[Coldblackice]

If a multimeter in diode mode isn't sufficient enough to diagnose which LEDs may be bad on a TV, how can this be measured (without having to buy an LED tester tool)?

Or rather, perhaps a better way to ask is: how can one replicate the specific testing an LED tester is able to do without having an LED tester tool? And particularly when a multimeter in diode mode shows that every TV LED lights up just fine + every LED has the same forward/reverse voltages.

In another thread, neilc6 mentioned that a multimeter in diode mode isn't sufficient enough to determine, as it won't put out enough current and it there's some issue regarding "delay" that it won't be able to see. I'm wondering if these things can be seen using just standard/routine repair tools, like multimeter, oscilloscope, logic analyzer, etc.

[budm]

Re: How to diagnose whether a TV LED is bad (without using an LED tester)?

As Nelic indicated, when using the meter which only puts out very low current or even with the typical LED tester which puts out may be around 30mA or so may show the LED as good but when you force 200 ~ 300mA as in real test then the bad LED may fail or bad connector with bad contact resistance may show the failure. Some of the LED tester now can putout 200 ~ 300mA now. Basically you need to drive the LED string with about the same amount as the LED driver is putting out to get the true test.

[diif]

Re: How to diagnose whether a TV LED is bad (without using an LED tester)?

It would be a funny repair bench to have an oscilloscope and a logic analyser but no power supply.

[SLK001]

Re: How to diagnose whether a TV LED is bad (without using an LED tester)?

Wire two AA batteries (or any 1.5V battery) in series with leads. There are usually bare test points on LED strips near all devices. Just touch the two leads to the test points to see if it lights up.

Most TV LEDs are 3V devices, but there are 6V devices also. If your make-shift tester doesn't light up any devices, increase the batteries to four in series. You can look at the device with a magnifying glass and usually tell that there are two devices in series in the diode, thus making it a 6V device.

[Coldblackice]

Re: How to diagnose whether a TV LED is bad (without using an LED tester)?

As Nelic indicated, when using the meter which only puts out very low current or even with the typical LED tester which puts out may be around 30mA or so may show the LED as good but when you force 200 ~ 300mA as in real test then the bad LED may fail or bad connector with bad contact resistance may show the failure. Some of the LED tester now can putout 200 ~ 300mA now. Basically you need to drive the LED string with about the same amount as the LED driver is putting out to get the true test.

Wire two AA batteries (or any 1.5V battery) in series with leads. There are usually bare test points on LED strips near all devices. Just touch the two leads to the test points to see if it lights up.

Most TV LEDs are 3V devices, but there are 6V devices also. If your make-shift tester doesn't light up any devices, increase the batteries to four in series. You can look at the device with a magnifying glass and usually tell that there are two devices in series in the diode, thus making it a 6V device.

Thanks guys. My hesitation was there seems to be a general warning against willy-nilly sourcing unregulated power to test an LED, at least for "specialized" TV LEDs, versus powering through the LED's constant current driver -- the (seeming) risk, as far as I understand, being inadvertently burning or damaging the LED because it's not powered through its current-regulated driver or at least a regulated power supply like an LED tester. Some of the comments here, for example. Is this the case? Will I potentially damage any of these LEDs by using two AA's, since it's not current-regulated?

I've also been hesitant because I can't find a datasheet for these LEDs, either (LG Innotek). I can find other datasheets for LG Innoteks, but can't find one with any of the seeming part numbers on these strips. Any suggestions on where I can find the datasheet?

It would be a funny repair bench to have an oscilloscope and a logic analyser but no power supply.

Indeed and agreed! I haven't been able to find a good/best value power supply that doesn't also cost the same as the scope. I've tempted to buy a $70-80 one off Amazon, but there's always a few reviews that scare me away. In the interim, I crudely repurpose wall warts and laptop power supplies to bridge the gap. But yes, I very much need a bench power supply.

[diif]

Re: How to diagnose whether a TV LED is bad (without using an LED tester)?

A clever idea using batteries. Draws around 200mA.

[thetvguy2000]

Re: How to diagnose whether a TV LED is bad (without using an LED tester)?

https://www.youtube.com/channel/UCMJ...DJhmU6vmtWya0Q

Hi guys check out the videos on my channel showing how to make a DIY led tester out of 3 9v batterys

also i have a video showing how to diagnose a bad backlight in any led tv using only a torch

thanks

[ndriana]

Re: How to diagnose whether a TV LED is bad (without using an LED tester)?

I'm not sure about regular power supply but I have a desktop led lamp. I used its 16V power supply to test each strip of 6 leds of a 40 inch rca tv. They give dim lights. I guess it's dim because it should be around 6x3 = 18V > 16V. But still I managed to find out that one strip does not light up like the rest.
If I had a regular 18V charger, I would not be afraid to try it.

When the strips are joined together and put back on the tv, all the leds, including those on the supposed to be bad strip, blink once. (I need to turn off the light at night to see it though.)

The voltage from the power board then drops down from 133V to 83V. I really want to know what's wrong in that strip with a multimeter. Why is there a huge drop of voltage across it when it blinks. I measured it and it's not the same as the drop across the other strips.
As far as I know, a strip is just made of resistors and leds. Is it possible that the resistor died but not entirely, that its resistance became very huge?

[SLK001]

Re: How to diagnose whether a TV LED is bad (without using an LED tester)?

I've never seen a strip with resistors on it.

The PS ramps up current until a certain value is obtained (up to its maximum voltage capability). Good strips and strips with bad LEDs will have different voltages for that current value. The "blink once" phase is the PS testing the LEDs for proper operation. If the LED test fails (ie, to pass the test, 150 mA at 125-135V has to occur), PS won't provide the current to ANY strip.

If you're seeing all the LEDs light during the test, some may be weak and drawing too much current. You'll have to find these weak ones with individual tests. Use my technique in post #4 above.

[ndriana]

Re: How to diagnose whether a TV LED is bad (without using an LED tester)?

Thank you for the reply.
I dug some more information about the led and on its datasheet, they gave certain temperature for reflow and for 10seconds. I tried a lower temperature for about 4 seconds and now that led really died.
I decided to shorten it to check and the tv works but adding the darker spot I know that it is giving work to the rest of the leds.
Do 3528 leds all have the same characteristics? There is an old cracked screen tv for parts I left in the basement. It's pretty light and cheap but it's backlight might be good.

[ndriana]

Re: How to diagnose whether a TV LED is bad (without using an LED tester)?

I'll answer my own question. No 3528 may not have the same characteristics. Only the size is the same but they can go from 2V to 3.3V, may be more, I didn't check. The current can be from 20mA to 200mA. Again, I didn't investigate more.
So, yeah, we'll really need to find out which led they used, not only the size.

[budm]

Re: How to diagnose whether a TV LED is bad (without using an LED tester)?

It should be rated at 1W to be used for backlights application, you also need to match the pads polarity, and the color temperature, cool white is typical. Example:
https://www.ebay.com/i/202391692646?
www.ebay.com/i/323435186185?
I use LATWT470RELZK (3V) for LG repair, but these days I replace all strips instead.