LED Backlight current mod.

[dskall]

I just repaired backlights on Vizio E500d-A0. I am now in the process of trying to modify led current. It is running about 410ma. I am trying to reduce it to between 320~350ma. This is trial and error for me as I have no math or engineering background. I would like to make mods to all Tv's when I repair the backlights. The led driver for this board is BD9285F IC300 and IC400. It gives formula to adjust current. Changing the isense resistors. The formula works out to a few ohms. I traced (I think) my isence resistors to R411 and R311 which are 1k resistors. So now am confused. I have changed some values with mixed results do to inconsistent readings with inductive ammeter. Any guidance would be appreciated.

Powerboard: FSP155-2PSZ01 https://www.shopjimmy.com/vizio-0500...t-inverter.htm

[Andrew F. Ali]

Re: LED Backlight current mod.

In the Service Mode on most TVs there is a selection to adjust the backlight. Trying to do it as you are doing will work to an extent but you would be wasting Watts and the Joules would result in heat. I wouldn't take that route though as for the most of it you would be guessing at approximate values and outcomes. This would be a tedious exercise also. Good Luck..

[budm]

Re: LED Backlight current mod.

I just repaired backlights on Vizio E500d-A0. I am now in the process of trying to modify led current. It is running about 410ma. I am trying to reduce it to between 320~350ma. This is trial and error for me as I have no math or engineering background. I would like to make mods to all Tv's when I repair the backlights. The led driver for this board is BD9285F IC300 and IC400. It gives formula to adjust current. Changing the isense resistors. The formula works out to a few ohms. I traced (I think) my isence resistors to R411 and R311 which are 1k resistors. So now am confused. I have changed some values with mixed results do to inconsistent readings with inductive ammeter. Any guidance would be appreciated.

Powerboard: FSP155-2PSZ01 https://www.shopjimmy.com/vizio-0500...t-inverter.htm

The I sense resistor is the one that is connected between the PWM control MOSFET and GND for the LED return current path, it should be very low Ohms usually <1 Ohm.

[dskall]

Re: LED Backlight current mod.

Ok thanks budm. I found 4 smd 1ohm's next to mosfet. I removed one which increased current. So will try adding one.

[dskall]

Re: LED Backlight current mod.

In the Service Mode on most TVs there is a selection to adjust the backlight. Trying to do it as you are doing will work to an extent but you would be wasting Watts and the Joules would result in heat. I wouldn't take that route though as for the most of it you would be guessing at approximate values and outcomes. This would be a tedious exercise also. Good Luck..

Haven't been able to enter service mode. Tried this
Turn The TV off.
Press and hold the {CH +} & {CH -} buttons on the TV.
Then press and release the {POWER} button on the TV.
Release the {CH +} & {CH -} buttons.
Now press the {MENU} button on the remote.
You should get your regular menu up, but with a "F" in the bottom right corner of the menu (This is for factory).
Press and hold the {MENU} button for few seconds.
The Service Menu will be displayed
No luck

[Andrew F. Ali]

Re: LED Backlight current mod.

Ok thanks budm. I found 4 smd 1ohm's next to mosfet. I removed one which increased current. So will try adding one.

OK. So we'll be adding this resistor in parallel, so the equivalent resistance is lower.

[budm]

Re: LED Backlight current mod.

You need to INCREASE the resistance so it will take less current flow through the LEDs and the circuit to develop the same Vdrops on the current sensing resistor as when the resistance is lower.

[dskall]

Re: LED Backlight current mod.

I tried the changing current sense resistor. It was four 1ohm .25ohm. Removed one so it was .33ohm but didn't work. Looked at datasheet again it said to change isense resistor which is connected to isence. They were four 12ohm. Changed to four 20ohm and now max. current is around 300ma and seems to be plenty bright as opposed to the 420ma they were being driven at. I feel better that the 350ma led's are not being overdriven since I want to sell this tv and not worry that it will get set to max.

[budm]

Re: LED Backlight current mod.

skip.

[budm]

Re: LED Backlight current mod.

I tried the changing current sense resistor. It was four 1ohm .25ohm. Removed one so it was .33ohm but didn't work. Looked at datasheet again it said to change isense resistor which is connected to isence. They were four 12ohm. Changed to four 20ohm and now max. current is around 300ma and seems to be plenty bright as opposed to the 420ma they were being driven at. I feel better that the 350ma led's are not being overdriven since I want to sell this tv and not worry that it will get set to max.

Are you sure they (Four 1 ohms) are not for the Boost converter MOSFET??
You have current sensing resistor for the Boost converter and current sensing resistor (this is the one that you want to change) for the PWM Controlled MOSFET for the LED current.
So when change to four 20 Ohms in parallel the Vdrops at 300mA will be = 5 ohms x 0.3 = 1.5V so the cirucit to main tain the same Vdrops on the original resistors (3 Ohms total resistance, it will need 500mA of current to produce the same 1.5 Vdrops.

[dskall]

Re: LED Backlight current mod.

As per page 13 of datasheet the resistor at bottom of gate and cs terminal are four 1ohm resistors. I pulled one to increase resistance but didn't work. According to page 17 to set current you have to change isence resistor which are four 12ohm resistors in parallel . I replaced with four 20ohm and it dropped max. current

[budm]

Re: LED Backlight current mod.

CS pin of the IC is for current sensing of the boost converter circuit not for the current control of the LED. CS goes to the Source pin of the MOSFET, the current sensing resistor for that MOSFET will not have effect on the LED current
Then you have Isense pin which is connected to the current sensing resistor (that is the one I have indicated that you need to increase the value to reduce the LED current) which is connected to the Source pin of the MOSFET that is used for maintaining the constant current through the LEDS.

[dskall]

Re: LED Backlight current mod.

I understand the circuit a little better now.

[augiedoggy]

Re: LED Backlight current mod.

In the Service Mode on most TVs there is a selection to adjust the backlight. Trying to do it as you are doing will work to an extent but you would be wasting Watts and the Joules would result in heat. I wouldn't take that route though as for the most of it you would be guessing at approximate values and outcomes. This would be a tedious exercise also. Good Luck..

I believe the issue is also the fact that when you first turn the set on the leds are at full brightness being overdriven for a few moments until the software adjusts it down. I saw a while back on youtube, another person adding some sort of resistors in the same fashion and he talked about this.

[dskall]

Re: LED Backlight current mod.

Well again just learning but if you set your sense resistor full brightness will be whatever max. is set on controller which now is around 300ma. Max for these leds is 350ma as opposed to 400ma which in my opinion is deliberate to limit life of tv.