LG 50UH500 LED Current Mod help needed

[jjanssen7]

I'm trying to lower the current going to the LEDS on the PS, however after searching for a service manual or schematic of the actual power supply I've come up with nothing.

I need to find a way to lower the LED current similar to what was done here:

https://www.badcaps.net/forum/showth...ht=current+mod

Here are the specs:

TV Model number Lg 50UH5500
LG Power supply model number? COV33697901
Skyworth power supply model number 168p-L5L01f-w0

I managed to find a schematic of a similar (but not the same 168p-L5L01f-00) Power Supply that drives the LEDS differently. It uses an 0z9903 current control chip (the one in the 50UH5500 is 0z9906, with the same number of pins).

Attached you'll find page 60 and 61 that refers to that 0z9903 chip, but beware, it's a translated version. The original information sheet for the "similar" PS is attached.

Through google translate I got this far, perhaps this is helpful.

https://translate.google.com/transla...%2F1533387.htm


Attached are pictures of the front and back of my board.

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Can any brighter minds identify which resistor(s) I need to modify?

[nomoresonys]

Re: LG 50UH500 LED Current Mod help needed

Tomtechtod's page, he has 3 or 4 current limit mods on the right: https://www.youtube.com/playlist?lis...gtd-hcUXG2KjSC

[jjanssen7]

Re: LG 50UH500 LED Current Mod help needed

Thanks, I watched all those videos first before my original post. He's very helpful, showing this mod is possible.

Typically he just increases resistor value but never actually explains how to identify the current resistor he changes. Helpful if you happen to have the same model as what he works on or his level of understanding of the circuitry.

In my case, if I understand it correctly, the LED current is controlled by a current IC which in turn has a current sensing signal I'm trying to modify, likely one of the SM resistors. Unfortunately, I think one of his examples has this system and he only mentions it in passing at the end of the video.

[Diah]

Re: LG 50UH500 LED Current Mod help needed

as i know LG plat forum LED TV can adjust the LED BL currents by firmware.by log in Service menu ( EZ_adjust )

[nomoresonys]

Re: LG 50UH500 LED Current Mod help needed

Can you access the service menu? maybe this: Press and hold <OK> buttons on remote as well as TV panel for a few seconds and the service menu will appear on your TV screen. For some TV models, you might need to press "Menu" buttons on remote and TV panel simultaneously for a few seconds to access the service menu.

[nomoresonys]

Re: LG 50UH500 LED Current Mod help needed

.

[jjanssen7]

Re: LG 50UH500 LED Current Mod help needed

I'll look into the service menu angle, but I'm still interested in a physical current limit rather than one through firmware.

[nomoresonys]

Re: LG 50UH500 LED Current Mod help needed

Some ohms law math: https://electronics.stackexchange.co...-the-same-time

[budm]

Re: LG 50UH500 LED Current Mod help needed

https://www.badcaps.net/forum/attach...7&d=1568625449
If you increase the value of the current sensing resistor (R177/178/179 for 0z9903) it will decrease the LED current, so you need to locate those 3 current resistors on your board version.
See OZ9906 spec sheet page 7, 8.

[jjanssen7]

Re: LG 50UH500 LED Current Mod help needed

Awesome... where did you find the datasheet? I think that's all I need and can investigate from there.

Looks like R8,R9,R10 off pins 15,16,17 are the ones I need to increase.

Looks like this Backlight controller IC has room for 6 channels and only uses three as there are 3 strings. They all connect through resisters to one single trace that eventually ends up at the ADIM pin.

Does it make sense that Pins 18,19,20 which are Led Current Sense for strings 4,5,6 (not used in the setting) are essentially unused?.

[budm]

Re: LG 50UH500 LED Current Mod help needed

You put down '0z9906' ZERO Z instead of OZ.
You only have 3 LED strings, not 6.
The current sensing resistor for each circuit is the one connected between Source pin and Ground pin of each power MOSFET which is connected for each Cathode return line of each LED string.
see page 1 where the ISEN pin of the IC connected to the Rsense that is connected to the Source pin of the MOSFET and GND.
I.E MOSFET Q404, you'll see R421, 422, 420 all connected in parallel, you can remove R420 (3R9) that should cause the LED current to drop, you will have to do that on all 3 circuits.
YOU 3 MOSFET's Q404, 405, 406.

[jjanssen7]

Re: LG 50UH500 LED Current Mod help needed

I get it now I think...

[budm]

Re: LG 50UH500 LED Current Mod help needed

The 3 resistors I mention is one of the 3 LED constant current control, you have 3 resistors connected in parallel, so when you remove on the total resistance will go up thus the current will go down, yes we are increasing the resistance of the Isense resistor by removing one of the parallel connected resistors so the constant current will be reduce based on the Vdrops on the current sensing resistor.
You do understand what the series and parallel connections are, correct? The spec sheet also provides the formula to calculate the current.

[nomoresonys]

Re: LG 50UH500 LED Current Mod help needed

.

[jjanssen7]

Re: LG 50UH500 LED Current Mod help needed

what do the R8,9,10 resistors off the Isen1,2,3 (pins 15,16,17) do? They are all connected to the ADIM pin 4.

[budm]

Re: LG 50UH500 LED Current Mod help needed

Look at spec sheet page 10 application circuit, you will see how it is hookup, then trace your board to see if it connected the same or close to what you have.
There should be resistor connected to I sense pin of the IC, the other end of the resistor is conNected to the Source pin of the MOSFET.
I cannot see the trace from pin 4 connected all the way to R8,9, 10. I see pin 4 connected to R430.
I see R411 (1K) connected to the Source pin of Q404, but I cannot see what the other end of R411 is connected to, it looks like it goes to R8.
https://www.badcaps.net/forum/attach...2&d=1568625586

[jjanssen7]

Re: LG 50UH500 LED Current Mod help needed

R411 is connected to the source pin of Q404 as you suggest. the other side of it is connected to isen4! isen 1,2,3 are connected to pin 3 VREF, not pin 4 ADIM, which I guess makes sense in a way I don't quite understand.

I'll remove one of 420 (3.9 ohm), 421 (4.7 ohm), 422 (4.7 ohm). Try one of the 4.7 ohm ones first?

[budm]

Re: LG 50UH500 LED Current Mod help needed

Just remove 3R9 (R419, 420, 423) for each LED driver, then you should notice that the backlights will be dimmer, just set the backlights to 50 first before the mod and see how bright the backlights are then do the mod and see how much dimmer the backlights will be.
https://www.digikey.com/en/resources...eries-resistor
4.7//4.7//3.9 = 1.4664 Ohms = 340 mA (per spec sheet calculation at max setting).
4.7//4.7 = 2.35 Ohms = 212 mA (per spec sheet calculation at max setting).
That should drop the current quite a bit.

[jjanssen7]

Re: LG 50UH500 LED Current Mod help needed

Removed (R419, 420, 423) and the current dropped nicely from 350 MA to 210. A fairly significant drop, but these TV's fail when 1 year old so they must have been seriously overdriven.

[nomoresonys]

Re: LG 50UH500 LED Current Mod help needed

Well done guys, I bet that resistor removal mod will add years of life to that tv, this would have been a good one to put on Youtube