About LEDs and LCDs

My lab power supply display slowly turned black during the last few month...:

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It is an old design and OK after all those years, but why does this process happen? I once had an LCD watch and when sitting in the sun outside all of the sudden it turned black.. was very hot, bought a new one. This thing has not been hot.

So, anyways had some other ones that did not fit, had a different driver, did not want to re-write the software, thing is way too good, so ebay: found a Geeetech Yellow backlight LCD 1602 16x2 Characters display that claimed to have the same driver chip, and same mechanical outline, start bid was 99 cents, so I did and got it, shipping 1 dollar. Installed it yesterday. Works perfectly, OR??? WOW such a bright backlight, blinding, switched it off again. Checked the datasheet, maybe it needed a series resistor ?? Nope, 5V on the LED pins, resistor is on the module. Still could not believe it, looked like a reading light, measured the current:

10 mA

Now you guys talk about LED aging, but it seems the newer ones are so good, those need attenuation. Added 270 Ohm in series, now it is OK. It is still bright in the evening, but no eye damage this way.

For new designs I only use OLED.

Reply to
Jan Panteltje
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Our front-panel LEDs keep getting brighter as the diodes improve. We've had to change resistors after customers complained. Blue LEDs especially.

Don't OLED pixels wear out? Like the old EL displays.

--
John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  
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Reply to
John Larkin

AFAIK OLEDs have bad lifetime

We use Everlight, but cannot get lifetime data, so probably moving to Osram or others that do provide data

LED lifetime can be optimized by running at the best efficiency point and driving it with PWM. Downside is higher drop resistance losses

Intensity differences between batches can be calibrated out with a more clever driver design

Cheers

Klaus

Reply to
Klaus Kragelund

On a sunny day (Mon, 09 Oct 2017 11:20:26 -0700) it happened John Larkin wrote in :

The blue OLEDs I have have now about 3 years of 24/7 use, no problem. Always have those on minimum brightness. Have not used color OLEDs yet. The OLEDs are getting cheaper each day, use less power than a LCD, have a better viewing angle, are just as bright, good readable in sunlight too. The small 128x64 ones I use have an I2C interface, simpler than those 8 or 4 bit bit LCD interfaces.

Reply to
Jan Panteltje

On a sunny day (Mon, 9 Oct 2017 11:42:24 -0700 (PDT)) it happened Klaus Kragelund wrote in :

Define 'bad lifetime'. I do not see any degradation after 3 years 24/7 with cheap ebay OLEDs. LCDs are always a problem, contrast, backlight needs to be adjustable, or switchable, bad viewing angle. You can run OLEDs at low brightness too, not sure if that extends life though.

Difference with LCD and backlight is that only the segments that are 'on' draw current, and then only as much as the selected brightness.

But sure there are applications that still need LCD, sunlight readable color monitors, but try finding a good cheap one. But for equipment used in house of normal lighting conditions OLED is the way to go.

There is an issue in color OLEDs with the 3 primary colors fading in a different way over time, to fix that I think LG uses white OLEDs with color filters for R. G, B, that way the white balance stays the same over time, but the filters take away light, so it is basically less efficient.

Reply to
Jan Panteltje

Just selected first random OLED display:

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10k hours lifetime

Just enough for 1 year

Our products must have minimum 100k hours lifetime

Might be that I just "found" a bad reference

Cheers

Klaus

Reply to
klaus.kragelund

The display module looks a lot like these I have in my bin (though I think they're 1 line):

No backlight, just silver reflective tape on the reverse; it can be pulled off and there are terminals on the PCB and room to slide a strip of EL material underneath.

Reply to
bitrex

On a sunny day (Mon, 9 Oct 2017 13:13:22 -0700 (PDT)) it happened snipped-for-privacy@gmail.com wrote in :

OK, this has been running 24/7 as my alarm clock now since about 01/07/2014

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in this project:
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It is on now, and charging at the same time. That is about 3 years or 24 * 365 *3 = 26,280 hours. It is running at lowest brightness, and that is bright enough. I do not see (but have not measured with a light meter) any degradation or other defects.

I guess luck is with me, as the lipo in it has now been recharged 365 * 3 = 1095 times, something that is not possible either ;-)

I have more of these displays in use, for example:

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very good readable in bright sunlight, even with the protection foil.

?
Reply to
Jan Panteltje

On a sunny day (Mon, 9 Oct 2017 16:28:51 -0400) it happened bitrex wrote in :

Ah, yes, that is that other controller, I have a few of those in other projects, but with backlight.

This one I replaced uses the KS0066 + KS0065 LCD controller (2 chips).

Reply to
Jan Panteltje

This type of module (with one, two, or four lines) plus EL backlight underneath seems to have been the illuminated display of choice in a lot of pro audio equipment, synthesizers, etc. in the 1980s and early 1990s. On the plus side EL material degrades gradually; I'd estimate it has a "time to half brightness" of around 15 years run a couple hours a day.

On the downside EL material isn't that bright to begin with, and it seems usually the switching transistor or electrolytic caps in the blocking oscillator used to generate the HV go kaput before the backlight does.

Reply to
bitrex

Something has stopped it being a reliable liquid crystal. Applying DC bias for a long period will eventually degrade them as will enough heat to crack the envelope seal letting oxygen in. Old age gets them eventually but they are long lasting and potentially very low power.

LCD reversible thermometers seem to go bad after a decade or so. Quicker if they are exposed to sunlight or UV.

I have a toy rig with an old (first generation) 5mm red and a modern high brightness one in series and at 10mA the ancient one can just about be seen to be glowing in a normally lit room whilst the modern one is completely dazzling. The quantum efficiency has improved *THAT* much. Its a few years old now so the latest ones are likely even brighter.

You can make the old ones improve a lot by dunking in LN2 which stiffens up the crystal lattice but the thermal shock shortens their lifetime.

--
Regards, 
Martin Brown
Reply to
Martin Brown

On a sunny day (Tue, 10 Oct 2017 14:56:02 +0100) it happened Martin Brown wrote in :

Maybe that was it, mechanical failure. I noticed that pressing on it in the corner could sort of make it readable again, but then in a few weeks it detoriated more and more. It was never hot. I think it may have been mounted with some pressure from the front panel against it after my last mod to that supply a year or so ago, so I made sure it now has a few mm space.

Thanks, now that cooling is an experiment I need to try one day :-)

Reply to
Jan Panteltje

On a sunny day (Tue, 10 Oct 2017 08:06:38 -0400) it happened bitrex wrote in :

There is actually an other argument against LCD panels, and that is polarization. Try looking at one through a pair of Polaroid glasses, and then tilt you head. My LDC monitor is 45 degrees polarized it seems, and will then go black, the small multi-line LCDs 90 degrees.

That is also the reason airline pilots do not wear Polaroid sunglasses, in these days with glass-cockpits it would make reading instruments problematic if you look sideways etc.

So, OLED is is ;-)

Reply to
Jan Panteltje

On a sunny day (Tue, 10 Oct 2017 15:08:01 GMT) it happened Jan Panteltje wrote in :

Or not, my little OLED displays also go black at 90 degrees... Where does that leave us now :-)

Reply to
Jan Panteltje

They put polarisers on the OLEDs to cut down on reflected light to improve the "blackness" and hence contrast. Unlike an LCD, it would work without the polariser.

Allan

Reply to
Allan Herriman

I recall stepping into the lift lobby of a high-rise, and wondering why they had no directory to tell me what floor to go to. Then I took off my sunnies, and realized that the black rectangles on the wall weren't architectural features, but large TV screens mounted vertically, so my sunnies completely blocked the light from the screen. Strange experience.It made me think that screens should be made in various polarisations to suit the application.

Reply to
Clifford Heath

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Reply to
Lasse Langwadt Christensen

Den tirsdag den 10. oktober 2017 kl. 17.08.14 UTC+2 skrev Jan Panteltje:

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Reply to
Lasse Langwadt Christensen

On a sunny day (Wed, 11 Oct 2017 10:58:19 +1100) it happened Clifford Heath wrote in :

I have several pairs of sunglasses... only one polarized.

Wonder what happens when driving those Teslas with LCD dashboard.. Anybody driving Teslas here???

Reply to
Jan Panteltje

On a sunny day (Tue, 10 Oct 2017 17:12:39 -0700 (PDT)) it happened Lasse Langwadt Christensen wrote in :

It hurts seeings somebody cut up a good LCD panel... :-(

Reply to
Jan Panteltje

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