Help with electroluminescent displays

[gregebert]

Has anyone here designed their own electroluminescent display driver ? Datasheet info is not very clear about the voltage required to drive these; from what I can gather, most applications use AC, around 400Hz (which was common in aircraft; not sure if it still is).

What I dont know is if the datasheet is specifying RMS, peak, or peak-to-peak values. Since these devices are basically a 'light-emitting-capacitor', they will get damaged/destroyed by over-voltage. My hunch is RMS.

Future project on the drawing board.....

[Dekatron42]

Have you checked out this website: http://www.155la3.ru/electroluminescent.htm it is in Russian but Google Translate is your friend! I tested a few of the thyristor and simple transformer designs and they work very well.

Unfortunately most of the EL-displays sold by Russian sellers seems to be factory rejects, or maybe they have aged very poorly, as many have black dots where the connecting wires are on the backside of the segments. Many also have a lot less shine compared to in the auction photos, maybe also age related. Used displays that are sold are usually very poor. Some sellers however have new and nice displays to sell.

Microchip (formerly Supertex) have a series of EL-drivers in the HV8XX-series where you can find some more design data, however most of those are made for fixed frequency drive and for large area EL-displays like backlight sheets or long wires. Driving a small transformer let me determine at what frequency and what voltage the display should be driven, then I could adjust brightness properly. I never built anything with the displays just experimented with them and decided not to build anything as they were so poor.

/Martin

[Zitt]

I have a couple of applications where I'd like to use EL panels... but my issue is the relatively short life of EL panels.

I'd love to use some OLED panels... but the issue there is cost.

Good luck; keep us posted.

[gregebert]

Unfortunately that's the same site I've been referencing; I wish it had a few more details.

I think I will need to sacrifice 1 unit and do some experimenting with it; the units I have are NOS, and I think I saw OTK stamps on them (currently they are boxed-up).

I'll probably do my own D/A --> HV amplifier design for the driver, which will let me choose the waveform. Right now I think a triangular wave will be the best option because it results in constant-current and for a given frequency/brightness it should require less voltage than a sinusoidal waveform. 

I know they have a rather short lifetime, so it will only be for night usage via PIR sensor; if I do it right I should get 30 years of usage as long as it doesn't run more than 3 minutes per night assuming a 500 hour lifespan. Hopefully the unit I sacrifice will give me some useful lifetime data.

[Richard Katezansky]

Hi,

Supertex (now owned by Microchip) made EL display drivers a number of years ago.  I was using one of their devices to drive an LCD back light. There were a series of application notes available at the time on EL basics and design of drivers.  Not sure if they are still available. The part numbers were in the HV8xx series.  I may have copies of some of the documents floating around if you can't find anything on line.

Richard  

[Dekatron42]

It's very easy to burn them and they burn up around the spot where the wire is soldered to the luminescent part, where it makes contact. Those I've bought have all had the OTK stamp but been very poor in light output and otherwise, some sellers have been good in acknowledging that the displays had been rejects, used or otherwise poor and shipped me replacement units (sometimes good ones sometimes worse).

There are some Russian books available on the Internet on these displays, unfortunately my Samsung 850 Pro 1TB SSD died earlier this year and I am not fully back from that crash, I just haven't had time to restore material that I don't use much but I've included one book that I had named properly on my drive so I could find it after some searching, maybe it will help if you translate it into English.

/Martin

[Mark Moulding]

They're actually really tolerant of over-voltage.  This is the same technology as electro-luminescent wire ("EL-wire"), for which I've made several different drivers and segment multiplexers.  You can get by with an off-the-shelf EL-wire driver; some are very cheap ($5 or so) and are available with supply voltages from 3V to 12V.  Most of these are simple resonant-transformer circuits, however, and require both a minimum and maximum load to work - and they'll smoke if your load isn't within the required range (either way).  I eventually gave up using them for my projects (animating large and small displays for Burning Man, and others), and ended up making my own supplies with a 555 oscillator, driver, and a filament transformer working backwards.  I started out using a bi-polar driver, but a simple open-collector driver seemed to work well enough for my needs.

The critical parameter is frequency - well, more critical than anything else, anyway.  The EL-wire (and also back-lights from old monochrome LCD displays) seems to work best (brightest) between 1-2 kHz.  Note that there are nightlights available that just plug into the the wall, and therefore work at 60 Hz, but not very well.  Most of the cheap supplies produce a spiky sort-of saw tooth wave, which of course has a lot of high-frequency components.  In my design, I used an RC filter to round off the corners a lot; if I'd really been doing a good job, I'd have cobbled up some sort of sine-wave oscillator, but as I mentioned above, what I had was good enough, and far better than the cheap units.

The brightness is directly proportional to the excitation voltage; for most of the numeric displays (and EL-wire, etc.) anything above 100 V P-P works, with 120-150 seeming to be optimal.  Note that as the brightness (due either to frequency or excitation voltage) increases, the lifespan decreases.  Some of my brighter displays only had a lifespan of a couple hundred hours (to 50% brightness) - suitable for a one-week project in the desert, but not for an heirloom clock.

To switch the high voltage, I used sensitive-gate TRIACs such as the MAC97A6, and referenced one end of the high-voltage supply to my logic ground.  That sounds a bit scary, but it worked just fine.

~~
Mark Moulding