[sdiy] LEDs dying in LCD alphanumeric display

[J.D. McEachin]

On 6 Nov 2011, at 03:43, Adam Inglis wrote:

>Why would the LEDs that backlight my Proteus' 2x16 LCD display
>slowly die from left to right over the course of a couple of weeks?
>Seems like an odd failure mode.

It's remarkable how electronics manufactured at the same time can 
reach end-of-life within weeks or even days of each other.

It's odd that the LEDs would die, though, unless the unit has been 
left on for many years, or the LEDs were overdriven.


>  The LCD works and responds to
>contrast control.

The contrast control has nothing to do with the lighting.


>The 5 volt supply for the LED circuit is separate
>from the LCD supply, but they both measure up fine.
>
>It's one of these...
>http://www.densitron.com/displays/Alphanumeric_LCD.aspx

But which one - edge LED, SMD LED, or EL?  Big differences...


>Can you crack these open and replace the LEDs or do you replace the
>whole unit?

Maybe.  But I find it easier to just replace the unit.  Especially 
when the same model is available.

I've now replaced almost all the LCDs in my studio.  I started the 
project because my EL backlights were failing.  Then I decided that I 
hated the putrid yellowish-green LCDs that most studio gear sports, 
so I started changing out working LCDs so the colors matched the 
gear's paint scheme.  Call me crazy for spending the time & money; I 
laugh at myself about it sometimes.  :)  Here are some tips:

* Pretty much all character LCDs are compatible with the Hitachi 
HD44780 controller.  So far I haven't found any 80s gear that won't 
work with the CrystalFontz and Matrix Orbital LCDs I've been using.

* If the original display isn't available (or wanted), ALWAYS open 
the gear up and take some measurements before selecting a replacement LCD.

* LCDs w/ LED backlights are almost always thicker than the ones w/ 
ELs.  Usually this can be accommodated by changing out the standoffs 
or bending the mounting tabs, but sometimes requires more radical 
surgery.  The worst was my Prophet VS rack - I had to partially cut 
the pcb and let it flex back to get the .5mm clearance needed (it's 
ugly on the inside, but beautiful on the outside).

* Make sure the LCD is the right size for the window on the front panel.

* If the contrast circuit has a panel-mounted pot, it will probably 
work just fine.  If it's a fixed resistor, probably not.  If it's a 
CPU-controlled PWM circuit, forget it.  For the latter 2 cases, I've 
been drilling a hole in the back panel and mounting a potentiometer 
so the VO voltage (pin 3) can be adjusted.

* If there's a high voltage inverter for the old EL backlight, this 
is a great opportunity to remove it and get rid of a potential noise source.

* Make sure you get the current-limiting resistor for the LEDs 
right!  I usually calculate the value according to the datasheet, 
insert a trim pot in the circuit, and dial up the resistance until I 
can see the display is getting dimmer, then back off a little until 
it's back to full brightness.  There's usually no need to run at max 
current, and it will increase the LED life.

Hope this helps.  Good luck with your repair!

Jeffrey