Hack a DELL; LED backlight
- The inverter broke down.
- A wire got disconnected.
- The CCFL cracked.
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.
To 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 barThe 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.
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.
The 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.
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.
Next it is time to glue the LED bar in place.
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.
LED driverNext 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.
Outlined in blue is the PWM signal that controls the brightness of the backlight.
The PWM signal can be reused, the rest sadly not. As LED driver I used a LT1373 set to 35V.
- 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
ResultAnd finally some pictures of the result.
The end result is a little yellower than with a CCFL backlight.
03-'10 7 Segment Displays
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.and your daytime job is... working as a service engineer at dell?
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 Sunday 21 March 2010 21:12]
How in earth, do you know all these parts? That part in that particular color is used for this, that other part for that...
Can you make me an OLED screen for 50 bucks?
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 Sunday 21 March 2010 23:17]
O btw....what is your daytime job than anyway?
[Comment edited on Sunday 21 March 2010 23:50]
I was just wondering: are the 140 LEDS plus electronics cheaper than $30??
Really interested in pic's
white background color, yellow triangle, and some text...
Can u update the links, or upload the pix to other host... please????
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