LCD displays and EMC

[Barry Lennox]

I have an application where I require 2 line by 8 character (minimum,
but more characters would be OK) LCD modules. The character hight
should be around 4-6 mm. They have to be "intelligent" in as much as
they can be driven directly by RS232. The end product will have a
total of six modules.

There's quite a number of these items on the market, but the end
product spec has to meet a fair amount of MIL-STD-461E for EMI/EMC.

Has anybody found LCD modules that are especially quiet and that
easily passed 461E or similar? I would much prefer not to use
conductive mesh or the like to reduce emissions and susceptibility.

Barry Lennox

 

[Pooh Bear]

Barry Lennox wrote:

I have an application where I require 2 line by 8 character (minimum,
but more characters would be OK) LCD modules. The character hight
should be around 4-6 mm. They have to be "intelligent" in as much as
they can be driven directly by RS232. The end product will have a
total of six modules.

There's quite a number of these items on the market, but the end
product spec has to meet a fair amount of MIL-STD-461E for EMI/EMC.

Has anybody found LCD modules that are especially quiet and that
easily passed 461E or similar? I would much prefer not to use
conductive mesh or the like to reduce emissions and susceptibility.

Possibly your biggest problem is the hole in the case needed to view a
display. That'll let in and out all manner of garbage. I doubt that the
display itself will be of any great concern.

Graham

 

The LCD display itself ought to be pretty inoffensive - the voltages
are low, the currents are very low, and there is no necessity for fast
edges.

The back-lighting can be a problem. Jim Williams of Linear Technology
has written a number of application notes of the subject - I've found
AN-81, AN-65 and AN-55 on the Linear Technology web-site - which refer
to generating kilovolts to drive cold cathode lamps at ten of kiloherz.

He does recommend a resonant inverter (an example of a Baxandall
class-B oscillator, thogh Jim Williams calls it a Royer inverter),
which minimises the switching transients.

----------
Bill Sloman, Nijmegen

 

[PeteS]

The hole noted by Graham will probably be your biggest issue, although
you don't say what material the box is to be made of.

If it's conductive, you can get EMI gaskets to fit between the LCD
display and the cutout in the box.

As to backlight inverters, they are always noisy ( I have one in a
design now, and it had to be shielded so the internal gubbins worked
correctly, let alone get through EMC

Cheers

PeteS

 

[Barry Lennox]

On 26 Aug 2005 02:23:16 -0700, "PeteS" <ps@fleetwoodmobile.com> wrote:

The hole noted by Graham will probably be your biggest issue, although
you don't say what material the box is to be made of.

If it's conductive, you can get EMI gaskets to fit between the LCD
display and the cutout in the box.

As to backlight inverters, they are always noisy ( I have one in a
design now, and it had to be shielded so the internal gubbins worked
correctly, let alone get through EMC

The box is 6061 alloy. and right now I'm looking at a simple LED
backlight. Efficiency or heat is not too much of an issue.

I'd prefer not to use EMI material for screening the displays. It's
pricey and obstructs vision to some extent. However, it can be kept
ready as a last resort. If it fails RE or RS via the viewing hole
(about 10 x 50 mm) I can just throw a sheet of it at the problem.

Thanks for the comments.

Barry Lennox