Dell S2715Ht LED backlight

[leggot]

Re: Dell S2715Ht LED backlight

It appears as if all the LEDs turn on except for the one at the end of the strip. Will attaching a jumper wire around this LED allow the circuit still function as normal?

It is odd that it is the LED at the end, is this an indication that it burned out from over voltage or failure in another part of the circuit?

[budm]

Re: Dell S2715Ht LED backlight

If the other LED in that string are on but not that one, it means it is shorted out so it trips the over current shutdown.

[leggot]

Re: Dell S2715Ht LED backlight

Sorry, are the LEDs are in series or parallel? From your previous statement, I would assume parallel, thus not requiring a jumper; simply remove the broken LED from the circuit.

[Davi.p]

Re: Dell S2715Ht LED backlight

"that position would just have a jumper wire on it."
Is the monitor yours? If not it's not a well done repair because if one exceeds britghtness you destroy other leds.
60v/15leds=4v per led, so find out a 3v battery and make 2 probes with needles or something cheap in house, then test leds connected to green wire. If you success in led replacement don't forget to check power compo on board for soldering crack.

[budm]

Re: Dell S2715Ht LED backlight

Sorry, are the LEDs are in series or parallel? From your previous statement, I would assume parallel, thus not requiring a jumper; simply remove the broken LED from the circuit.

There are 4 set of LED strings (each string has 15 LEDs connected in series, the Anode of each string is connected to the same Boosted Voltage source.
The Cathode return line is connected 4 return lines on the LED driver IC, as you can see from the spec, it is made to drive 4 strings of LED.. also noted:LED short circuit or open string protection.
See picture:
https://www.badcaps.net/forum/attach...6&d=1485137703

FEATURES
• Boost converter controller for up to four strings of high power LEDs
• Integrated transistors for LED current balance control circuit
• Supports both analog and direct PWM dimming control
• Time-shift PWM phase dimming control for low audible noise
• Built-in converter for PWM signal to analog dimming control conversion
• Wide range dimming control
• Supports single power train, multiple-IC parallel operation
• Power MOSFET over-current protection
• LED short circuit and open string protection
• LED return to ground short circuit protection
• Thermal protection

[leggot]

Re: Dell S2715Ht LED backlight

Hi,

I am sorry, but I am still rather confused. I think I understand how the circuit works and thanks for finding all the documentation and being patient with me.

What I don't know is what to try next. I have already identified the specific LED that is broken, it doesn't light up when a small voltage is applied to it. Should I attempt to find a replacement LED or try something else?

Thanks

[budm]

Re: Dell S2715Ht LED backlight

Since you found bad part (LED), then it is needed to be replaced.

[Davi.p]

Re: Dell S2715Ht LED backlight

I never replaced leds but i think you have to measure the led then go to some sites, example ebay, aliexpress, then search the words + the measure like so:
if the led is 70x30mm = "backlight led 7030", good luck..

[leggot]

Re: Dell S2715Ht LED backlight

Thanks for the help. I think some of the previous calculations may be off by a bit, the high is 60volts and low is 20volts, thus the difference is 40 volts.

40/15 = 2.67 volts per led, removing one led from the circuit would yield 40/14 = 2.86 volts per led.

This site seems to reference the lamp voltage as 46.5, thus 46.5/15 = 3 volts. http://www.panelook.com/LM270WF7-SSA...ter_23622.html

Since the voltage per each in a string of 14 leds is less than the datasheet's expected voltage, it seems like just removing the led from the circuit would work and adding in a resistor to create the extra voltage drop, perhaps I am missing something. Otherwise, it seems as if I should be looking for a 3v smd led with the same size specs.

On a different note, I think the backlight brightness is driven by the voltage drop across the leds, thus a variance in voltage of nominal amount shouldn't burn out the leds, rather just make them brighter or dimmer.

[budm]

Re: Dell S2715Ht LED backlight

The circuit needs 60VDC supply, you then will have Vdrops on the LED string of 15 LEDs in series, and then you will also have Vdrops on the internal PWM Control MOSFET, and the current sensing resistor inside the LED driver IC.
You will get 20V when the boost converter is not on, that is the input Voltage to the boost converter, when BL-ON is present then the Voltage is boosted up to 60V (without load), the LED string does not have 60V across the string due to Vdrops on the MOSFET and the current sensing resistor. Vdrops on the string should be about 45V. Remember that we are dealing with constant current source.

[Davi.p]

Re: Dell S2715Ht LED backlight

"40/15 = 2.67 volts per led, removing one led from the circuit would yield 40/14 = 2.86 volts per led. " Yes, seems right, only is not easy to me to understand where are dissipated the other 20v in the circuit..
"This site seems to reference the lamp voltage as 46.5, thus 46.5/15 = 3 volts."
This because your monitor is probably not set to higher brightness.
yes, if you place a resistor is OK in any way (not sure, since there's not light it can dissipate too much and cook lcd foils..), if you do a jump you have to remeber to never set the higher brightness level, you will destroy the segment. It's also possible to tune the boost stage by changing the feedback resis. but it's easier and better replace the led, i thought, Bye.

[leggot]

Re: Dell S2715Ht LED backlight

Hi all,

I figured out how to solve the problem - I found a comparable LED (3V and 0.5W) and soldered it into the green wire circuit - to provide the voltage drop necessary. I removed the broken LED from the strip and jumped the two positions. The monitor works - despite missing one LED at the end of the strip.

Thanks again

[leggot]

Re: Dell S2715Ht LED backlight

Also, I seem to have mixed up the films that go between the backlight and the panel. There are two semi transparent films and one that is somewhat reflective (like a mirror).

Currently the two transparent ones are closer to the LEDS and the reflective one is near the panel. The light looks distributed from only one angle. Does anyone know what order these films should be in?

Thanks