Hack a DELL; LED backlight

By xantus on zondag 21 maart 2010 17:45 - Comments (30)
Category: Weekend projects, Views: 24.093

After I accidentally dropped my DELL Vostro 1400 the screen immediately turned black. This usually means that;
  1. The inverter broke down.
  2. A wire got disconnected.
  3. The CCFL cracked.
Option a is most likely as the heavy flyback transformer of the SMPS tears itself clear of the PCB. But a quick inspection (sadly) shows that in my case it was option c, a cracked CCFL.

Cracked CCFL

A new CCFL (without inverter) would be €30+, which I find quite expensive for a small glass tube. So I decided to replace it by a LED backlight.

http://u1.ipernity.com/14/69/33/7606933.4555192a.240.jpgTo the left is a diagram showing a simplified cross section of a LCD. The left image shows the current CCFL setup. In this setup the CCFL emits light in all directions. Eventually (after bouncing of a few sides) the light will travel through the light guide (blue), bouncing of its side and illumination the screen (red) evenly. The reflection sheet (green) reflects any light lost in the light guide back to the screen.

The LED setup works in almost exactly the same way, only the CCFL is replaced by LEDS.

LED bar

The LCD screen is 14.1”, so it’s a theoretical 31.2cm width. The LEDs I will be using are 2.2mm long and 1.5mm width. The LEDs are arranged in strings of 10. In the end I will use 14 strings, so a total of 140 LEDs.
one LED string
To support the LEDs I cut off one side of IC tube. With some filing the LED will fit snuggly inside the plastic tube, and as a result will be ‘perfectly’ aligned.

LED alignmentThe small strings are soldered alternating plus to plus and minus to minus to form the large string.

After quite some time the bar is finished.Finished LED bar

A quick check showed that the screen lights up ‘nicely’. There is some extra light leakage at the bottom, due to misalignment of some LEDs, but the screen had a reasonable amount of leakage even when it was lit by a CCFL.Initial check

Next it is time to glue the LED bar in place. LED bar placed inside screen

A piece of copper foil is placed of the LEDs to direct the as much of the light into the screen as possible. And finally some pieces of foam are tapped to the back to press the deflector firmly against the light guide, because the loser the deflector the more light leakage you will get at the bottom of the screen.Finished screen

LED driver

Next it is time to connect the LEDs to the CCFL driver. Of course this can’t be done directly, so some modifications have to be made. The picture below shows the CCFL driver in its original form. In green is a MP1255 (WP1255?), this is either a SMbus controller or a tri-state buffer. In blue is a WP1016, this is a CCFL driver :). In red is the flyback. CCFL driver

Outlined in blue is the PWM signal that controls the brightness of the backlight. WP1016

The PWM signal can be reused, the rest sadly not. As LED driver I used a LT1373 set to 35V.LED driver
  • L1 47uH, 1.3A peak current
  • D1 SS16, ultra fast schottky diode
  • C1 25uF
  • C2 10nF
  • C4 4uF, low ESR capacitor
  • R1 470kΩ
  • R2 17k6Ω
  • R3 4k7Ω
LED driverAlso each string of 10 LEDs has a 330Ω resistor in serie to limit the current.


And finally some pictures of the result.
The end result is a little yellower than with a CCFL backlight.http://u1.ipernity.com/14/73/75/7607375.7261bda6.560.jpg

Volgende: 7 Segment Displays 03-'10 7 Segment Displays


By Tweakers user Gammro, zondag 21 maart 2010 17:53

Quite interesting how you fixed the problem yourself :)

By Tweakers user Jeanpaul145, zondag 21 maart 2010 18:26

This is something that is remarkably rare on a site dedicated to tweaking. Well done!

By Tweakers user Anoniem: 301785, zondag 21 maart 2010 18:28

nice job done ,really interesting reading this.Your a smart guy.,

By Tweakers user Hakker, zondag 21 maart 2010 18:39

well you can also say that the CCFL one is too blue which in fact is a common CCFL issue too.

By Tweakers user cosmo_roel, zondag 21 maart 2010 19:02

and your daytime job is... working as a service engineer at dell?

By Tweakers user r00kw0rst, zondag 21 maart 2010 19:23

Cool, just like new :)

By Tweakers user BastiaanCM, zondag 21 maart 2010 19:35

And for once a complete/finished blog rather then a "topic". Well done !

By Tweakers user bonus, zondag 21 maart 2010 20:34

Nice work :0 I like that !!!

By Tweakers user woekele, zondag 21 maart 2010 20:37

and your daytime job is... working as a service engineer at dell?
to be honnest, I think the average service engineer is nog capable of this. They are generally only capable of replacing standard/pre-made parts.

By Tweakers user naftebakje, zondag 21 maart 2010 21:10

In fact, to get a good whitepoint, white leds alone are not sufficiënt. You've either have got to use RGB leds (too expensive) or add some blue leds to the whites.
This is not the "fault" of the CCFL, but the LCD: since it is made to be CCFL-backlighted, the color filters on each subfilter are matched to the spectrum of the backlight.

If you've done this trick with a LED LCD the image wouldn't be that yellowish since this LCD would be matched to the spectrum of white leds.

[Comment edited on zondag 21 maart 2010 21:12]

By Tweakers user RangedNeedles, zondag 21 maart 2010 21:39

Damn, nice job!

How in earth, do you know all these parts? That part in that particular color is used for this, that other part for that...

Respect! ;)

By Tweakers user Soldaatje, zondag 21 maart 2010 22:03

Hey, psst!

Can you make me an OLED screen for 50 bucks?


By Tweakers user et36s, zondag 21 maart 2010 22:15

Nice work dude I would love to see more of this kind of posts :D

By Tweakers user Mentalist, zondag 21 maart 2010 23:17

Did you use 2800k (warm white) LEDs? That would explain the yellow-looking result. It looks like it, since the LEDs are yellow when they are off, something I have seen before with 2800k LEDs.

I'm guessing cool white LEDs are better for a backlight. I am still planning on fixing/replacing the backlight in an old (classic) Thinkpad with LEDs when I get the time. I hope soon.

[Comment edited on zondag 21 maart 2010 23:17]

By Tweakers user temp00, zondag 21 maart 2010 23:46

In the past, I've done some CCFL and inverter exploration repair stuff :) myself too (mostly with what we call 'natte vinger werk' :) ), but deciding not to pay 30euro's (which is already not that much amount of money :) ), and just replacing the CCFL with a custom LED backlight is the way to go! Very nice job dude :)

O btw....what is your daytime job than anyway?

[Comment edited on zondag 21 maart 2010 23:50]

By Tweakers user DapperDen, maandag 22 maart 2010 09:06

Very nice job!

I was just wondering: are the 140 LEDS plus electronics cheaper than $30??

By Tweakers user Gleighton, maandag 22 maart 2010 11:45

Please host your pictures elsewhere, it gives 404 errors now.

By Tweakers user Barleone, maandag 22 maart 2010 12:17

Please host your pictures elsewhere, it gives 404 errors now.

By Tweakers user Loekie, maandag 22 maart 2010 13:24

It still looks broken, 404 everywhere!

By Tweakers user afraca, maandag 22 maart 2010 15:49

[sarcasm mode][original mode] 404's![/] [/]

Really interested in pic's :)

By Tweakers user Kees-Jan, maandag 22 maart 2010 20:18

screen looks OK to me:

white background color, yellow triangle, and some text...


By oldie__, zondag 30 januari 2011 16:25

Seems a good project, but the links to the pictures are all broken.
Can u update the links, or upload the pix to other host... please????

By keb, vrijdag 4 februari 2011 02:35

I would like to do the same for my laptop, unfortunately I cannot see your pictures, it gives me a 404 link ...

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