LED Driver board Question

[JonathanAnon]

Hi there,

I have been given a store display panel to fix, The output of the B20N15D MOSFETs are fed to the output connector through parallel two resistors, which 2R00 (2ohm) and one R330 (0.33ohm), giving effective resistance of 0.28ohm. Both of these are open and need to be replaced. I replaced both temporarily with a 2R2 resistor and the TV works fine.

I could understand why you would have two resistors of the same value (so as to increase the power you can carry), but I just don't understand why you would have two resistors in parallel with such differing values. Surely most of the current just passes through the R330 resistor and almost nothing passes through the 2R00 resistor.



Is there something I'm missing? thanks

[yokoono]

Re: LED Driver board Question

For fine adjustment of the voltage on the bars and implicitly of the current flowing through the LEDs. The values of the resistors differ depending on the type of LEDs, their typical current and the number of LEDs.
Low value of resistors / higher voltage on LEDs.

[JonathanAnon]

Re: LED Driver board Question

Yes, I understand. But the bit I'm curious about is why there are two different resistor values. The circuit will see the effective resistance of the two resistors in parallel.

I have two options here:

1. Buy exact replacements for the two resistors in the circuit.
2. Use slightly higher resistance to reduce the current through the circuit.

[yokoono]

Re: LED Driver board Question

I also found 5 resistors in parallel for thermal dissipation.

[dick_barton]

Re: LED Driver board Question

It's not unusual to have a mix of resistors in parallel to give you the exact value you want.

[JonathanAnon]

Re: LED Driver board Question

Interesting Dick, I didn't know that they need to be that finely "tuned".

So here's my plan. The resistance that would have been there originally would have been 0.28 ohms (2ohm and 0.33 in parallel). Now, I have a load of 2R2 SMD resistors, so I'm gonna put five of them in the parallel on the available pads which gives effective resistance of 0.44 ohms. This will reduce the current, which is what I want, and will also mean I don't have to order any small bits in the post. I will checked afterwards to make sure that there is enough current in the LEDs so that the screen has adequate backlight.

Anybody have any issue with this?

[yokoono]

Re: LED Driver board Question

On the TVs I repair, I reduce the current through the LEDs to a correct illumination of the panel. In this way I protect the LEDs when the user increases, from the menu, the voltage to the maximum. This prolongs the life of the backlight.

[budm]

Re: LED Driver board Question

Can I see straight shots pictures of the whole top and bottom side of the board?
They may be being as current sensor.

[JonathanAnon]

Re: LED Driver board Question

My apologies, I had the output cable turned upside down. I just noticed A and C on the output connector, which I presume is Anode and Cathode of the LEDs - so those resistors connect the cathode side of the LEDs to ground.

There is nothing on the back side of the board. So I think the purpose IS current limiting, not current sensing. I dont want to take up any more of your time, guys, so I think I'll just get the same resistors as what was in it. I will improvise on the next one!

[budm]

Re: LED Driver board Question

They are current sensing resistors for maintaining constant current through the LED's.
You can see the VIAS on the CC cooper plane that goes back to the control circuit.
They set the maximum current by adjusting the values of the sensing resistors.
So for the resistor may be burnt out due to some shorted out LED's.

[JonathanAnon]

Re: LED Driver board Question

I see it. It must be pretty forgiving, as I have increased the resistance at that junction from 0.29 ohm to 2.2 ohm and still the backlight comes on and stays on. I think I'll just replace those resistors with exactly what was there previously.

thanks.

[Diah]

Re: LED Driver board Question

you have 5 rank only 2 used.. it mean the same power boards used by other TV size.. here are the Numbers of LED play the roll. use one Res as you calculated will work smoothly.

but the question always remain why they are OPEN... need to inspect the LEDs first

[budm]

Re: LED Driver board Question

I see it. It must be pretty forgiving, as I have increased the resistance at that junction from 0.29 ohm to 2.2 ohm and still the backlight comes on and stays on. I think I'll just replace those resistors with exactly what was there previously.

thanks.

The value of the current sensing resistance will determine the maximum regulated LED drive current will be when the backlights is set to max, the higher the resistance the lower the max current will be.
BTW, what is the IC U1 P/N?

[JonathanAnon]

Re: LED Driver board Question

you have 5 rank only 2 used.. it mean the same power boards used by other TV size.. here are the Numbers of LED play the roll. use one Res as you calculated will work smoothly. but the question always remain why they are OPEN... need to inspect the LEDs first

Yeah I am concerned by this also, but I have tested the backlight LEDs with a tester and they work fine, and even with just that 2R2 resistor in place they all turn on fine. Because I dont know what caused them to open, I would like to reduce the current a little if possible. And I will leave on test a long time giving back.

use one Res as you calculated will work smoothly

You mean my option of using the 5 2R2s in parallel giving 0.44ohms?

[JonathanAnon]

Re: LED Driver board Question



From the pics I currently that's the best one Bud. I'm not sure if you can work it out from that. I'll take another pic later.

[budm]

Re: LED Driver board Question

From the pics I currently that's the best one Bud. I'm not sure if you can work it out from that. I'll take another pic later.

I cannot really read the number.
D2330 on the first line?
Second line I cannot read.
You can turn up the backlights level then measure the Voltage drops on that resistor to calculate the LED current.
The LED strings look to be connected in parallel.
What is the panel number any way?

[JonathanAnon]

Re: LED Driver board Question

innolux v500hj1-pe8 is the panel. Just be aware that the LEDs that they sell for this one are NOT the same as what is in this panel. The LEDs in my version of the screen are all at the bottom of the screen, there are two LED strips and each has four parallel sets of 26 LEDs. The back of the panel says 720ma / 78-89V

It is a store display unit where people can walk up to it and look for websites and they can run demonstration videos on. If I was to describe it, it looks like a 6 foot mobile phone. My seven year old nephew asked me what it was, and I told him that a giant had asked me to fix his iphone.

[Diah]

Re: LED Driver board Question

Just be aware that the LEDs that they sell for this one are NOT the same as what is in this panel. The LEDs in my version of the screen are all at the bottom of the screen, there are two LED strips and each has four parallel sets of 26 LEDs.

so you mean the BL are Edge one not Direct LED ??
Edge= LEDs with out lens at the bottom the Type could be single or Double LED unit.
Direct= LEDs with Lenses direct behind the screen. as in the net show of this Display LEDs Type 3035 6V.

[JonathanAnon]

Re: LED Driver board Question

so you mean the BL are Edge one not Direct LED ??
Edge= LEDs with out lens at the bottom the Type could be single or Double LED unit. Direct= LEDs with Lenses direct behind the screen. as in the net show of this Display LEDs Type 3035 6V.

Exactly, and I was looking for the terms that described them thanks.

You will notice that if you search for v500hj1-pe8 LEDs, you will find something like the link below.

https://spares2repair.co.uk/sound-vi...ts/v500hj1-pe8



And these are not the LEDs in my version. My version has edge LEDs similar to this:

[JonathanAnon]

Re: LED Driver board Question

Closer shot of the chip.