LCD backlight, LED type

[Buriedcode]

Hi,

I have a thing for LCD's, and use them often in various projects.  Recently I noticed a load of cheap 16x2 standard character LCDs on ebay and decided to order a couple.  They look great for plain STN, with the common "yellow-green" backlight along with the green filter thats used to make the grey-ish pixels look black without resorting to FSTN. They aren't transflective - but because of the green filter are quite readable in normal lighting.

For years this "yellow-green" backlight colour has been a standard, very common ~570nm. Since these LED's were never really that efficient, many LCD's used backlights that were arrays of SMD chips requiring up to 250mA (~4.2V) for a small 16x2 character LCD.  For blue/white LCD's with a white backlight, because white LEDs were more efficient, they can get away with a single LED lighting up the entire panel, requiring only ~20mA (~3.8V ish).

Now, these are different, they have a single green LED lighting the backlight, and it looks like its 570nm "yellow-green", with a Vf of ~2.83V when running at 20mA.  I'm not an expert on LED's but I cannot seem to find a SMD/TH single LED of the same colour that has that Vf.  To complicate things in recent years theres a lot of hybrid LEDs that use different phosphors to produce wideband colours - essentially like white LED's just with different phosphors.  So, I used a UV source near the backlight to check for any fluorescence - but nothing, so it looks like its a single die.  The backlight is really very bright, amazing for 20mA.

The LED die doesn't look like it was inserted into the backlight lightguide, or glued in, it looks like it was actually in the mold so I can't say its just a 3mm LED thats been used.

This is purely my own curiousity here, I googled the various types of LED (AlGaP, InGaN, AlGaInP etc..) but the ones with high brightness and a large (>2.5V) Vf were all <550 or 525nm, which looks completely different, and the ones that matched the wavelength (570) were all <2.1V and low brightness.  Any other common types that I'm missing?

[AndyBeez]

Google "side looking" or "side view" LEDs, which are often fitted to these back reflective LCD modules.

[Buriedcode]

Google "side looking" or "side view" LEDs, which are often fitted to these back reflective LCD modules.

Do you mean the SMD right angle ones?  It looks like pretty much whats inside a 3mm LED, except completely molded into the lightguide - often they just use a 3mm LED put into a groove in the lightguide, sometimes they epoxy it in for better optical coupling, but this doesn't appear to be that.  Here's the LCD:

https://www.ebay.co.uk/itm/225502677810

An example of the backlight contstruction (this is white, but looks identical): https://thepihut.com/products/white-led-backlight-module-small-12mm-x-40mm

I have made a few backlights in my time - just as above, by epoxying a clear-case 3mm LED into a lightguide/lightpipe.  I'm just more comfused the by forward voltage of the LED used here, and the impressive efficiency.  Its a pity I don't have a way to measure its output spectrum

[Buriedcode]

Ok,

So, I probably should just let this go, but I sacrificed one (I bought a few), desoldered it from the board, and peeled back some of the reflective backing to get a look at the die.

Using a UV LED (well, 400nm, close enough) shone onto the die (before I tried it form the back, which wouldn't have shone onto the die cavity) it causes a slight red/pink glow on the die, which doesn't happen with any of the other green LEDs I have (thought I'd check just incase some die materials fluoresce), so it looks like it manages to be so efficient because it's a phosphor LED. 

This explains the high Vf, but is it a green LED with a red/pink phosphor? (not sure if green is short enough to excite any phosphors), or is it a blue LED that has most of the blue blocked out?

I have a few of these phosphor LEDs I got free from Rapid a while back, but I haven't seen one this colour before.

[Buriedcode]

At the risk of turning this thread into a blog post - seems these displays are pretty unique.  They're VATN, which explains their great viewing angles and the fact that they are almost completely transparent - apart from a greenish filter, which means they can use the white diffuser on the backlight as a back reflector, allowing it to be viewable in daylight without a transflective layer backing.

It also is more efficient at transmitting light when pixels are off, so the backlight doesn't have to be as bright, since its not losing 80% of it to the display.

So, its just a much more modern character display with better display technologies, and a highly efficient backlight - thats much better for battery operated devices - even when operating the backlight at 15% (Av 3mA) its plenty bright in low light conditions.

[Buriedcode]

Sorry to ressurect this thread, but after using one of these displays in a recent project, I was reminded of it and I thought I'd ask if anyone has any more idea on sourcing these kinds of LED's.

The ones in the backlight are 3x2mm square, yellow-green emission, but with a small phosphor dot in the middle of the die.  Their efficiency is so impressive I'm wondering about using them for low power seven segment displays.  Even at 25% duty <1mA they are more than bright enough.

I'm really after search terms.  "Phosphor LED" doesn't seem to bring up much apart from research papers.  The closest I've seen is "pastel" LEDs, that rapidonline have.  These have the same Vf (clearly blue dies) and are very efficient, with a range of non standard colours, but none of them seem to fit the right colour.

After seeing this https://hackaday.com/2022/02/16/weird-phosphor-conversion-leds-found-in-cheap-led-string/  I'm more than convinced they are quite common, but I'm coming up blank sourcing individual LEDs.

[tooki]

I feel your pain. It’s so frustrating that phosphor-converted LEDs are inexpensive, efficient, look great, and can be made in practically any color, yet (with the exception of white) are essentially impossible to buy. For years, I’ve been trying to find true visible-spectrum purple LEDs. I know they exist because I have a string of Christmas lights in that color. But if you search for them, invariably what are sold as “purple” or “violet” LEDs are actually the ~395nm near-UV LEDs, or occasionally the phosphor-converted pink ones (which I love, but they aren’t purple!!).

Digikey has a handful of pastel colored phosphor-converted LEDs, but the purple one of those was not a saturated purple. I think they have a few phosphor-converted high-power LEDs, too.