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May 03, 2017, 01:26 AM
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LED backlight replacement notes


In the very old days, every LCD display including every laptop had plain old, miniature fluorescent lights for backlighting, right down to the traditional fluorescent light glasswork & mercury. If you're lucky enough to have a piece of 20 year old vintage equipment, they'll need replacement. They slide into the LCD panel for easy replacement. Since this LCD panel is $300, it's a good idea to replace just the tubes.

New fluorescent tubes would be over $50 today, so an even better idea is to get an LED replacement kit from China for $8 delivered. The LEDs would be brighter. After 3 weeks, the LED replacement arrived.

They're nothing remarkable for an LED afficionado. The LEDs are packed as close as possible. The important note is the strips must be shortened for the display. The LEDs are wired in groups of 3 which have clearly visible traces joining each group of 3. They must be cut between the groups.

This driver board is a simple buck converter based on the DF6113. It takes 10-24V & outputs a fixed current. It amounts to a maximum voltage of 3V per LED. Input current at full power, for the full 24" strips is 1A at 10V.


Very important: When the strips are cut, the brightness increases because it's a constant current regulator. Also, the constant current varies based on the input voltage until it hits the maximum current. Then it becomes fixed for all input voltages.



The board is wired according to the "ENA+DIM inverted analog dimming" diagram. The datasheet says the DIM pin has a useful range of 0-3V to control the brightness. It goes up to 5V but maxes out at 3V. At 1V, the LEDs are off. Alternatively, the DIM pin can be connected to a fixed 5V & a voltage between the FB pin & ground can vary the brightness. Either way, the datasheet says DIM must not exceed 5V.



For testing, a 5V regulator & multiturn pot were whacked on. This board was mislabeled. The wire labeled ENA connected to the DIM pin & the wire labeled DIM connected to the FB pin. The maximum voltage was 24, not 30 as written. The red & black wires were also swapped. Rdim was 47k. Rfb was 4.7k.


The useful voltage range for the wire labeled ENA was more like 1.8-1.9, hopelessly fiddly to get it to work with shorter LED strips. This voltage range varied slightly, based on input voltage.


They were quite intense at full power, but the 20 year old LCD still blocks massive amounts of backlight compared to a modern IPS display. The lion kingdom has no LCD displays from 20 years ago, but they would appear quite dim to modern eyes. The LEDs must be pressed against the glass for maximum brightness. This display shows no unevenness from the LED spacing.
Last edited by Jack Crossfire; May 04, 2017 at 11:45 PM.
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May 09, 2017, 05:51 AM
Aka: Tom Jenkins
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Do you have a link? I've done some passive searching but haven't found anything. I managed to acquire a strip similar to the one you posted out of a dead note pad and wanted a couple more for a light table.
thanks
Tom
May 11, 2017, 11:18 PM
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Ebay searches for LCD backlight replacement


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