LCD module, mounting, EMC question
Posted by jgmartinez at
EMI is a hard problem to solve and of course not cheap. There are some general indications that may be reduced to shielding and filtering. The display you have selected seems not to be specified any EMI performance, so we have to assume all EMI protection in part of your case design.
The casting were the display and electronics circuits are installed has to be EMI compliant, either metallic cast and conductive gaskets, etc. or plastic cast inside metalized for EMI compliance with. So the EMI problem may be reduced to display window and external device connections. For external device connections, filtering and cable shielding is the solution for conducted, radiated emission and susceptibility. The display window EMI problem may be solved by a EMI window. EMI window is very expensive but there are companies that supplies adhesive plastic foil EMI filter window that are cheap enough. I know about Chomerics but probably there are others.
The plastic foil is made of a very well transparent film built inside a conductive mesh that has to be grounded so the whole window is shielded by a transparent piece to light but block radiofrequency signals in both senses, emission and radiated. The success of this solution is concern to internal grounding and casting shielding.
Hope this helps.
Replies
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Hi JGMartinez,
Thanks for your very detailed reply with which I agree totally and I shall review the products you mention.
The real issue is that this is very much a standard (generic) LCD module and is available in many character formats but all using the same construction method. I have not found any advice on mounting with respect to emi. My view is the LCD glass clamp should be earthed (by the manufacturers design) as an earthed pad has been provided and indeed the metal clamp is in physical contact with it, just insulated by black paint from the gold plated pad!
I am not aware that I have an emi issue with this part and indeed do not know if this clamp assembly has any impact on the emi, indeed a plastic clamp may have been suitable.
Displays require a large opening in a panel and hence are emi/emc undesirable, but functionally essential. It is unfortunate if users are going to the expense of expensive emi window material when it is a bit of paint that may be amplifying the problem.
In summary my question is:
"has anyone reviewed these LCD modules for correct assembly/design regarding emi?"
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I am using a PC0802A Powertip LCD (2rows 8 characters) without backlight supplied by RS.
I have a question relating to its construction and use, particularly wrt to emc.
Three of the mounting holes are electrically connected and this trace is then via a solder jumper connected to a copper fill area, so are presumed to be at 0V potential.
The black LCD glass holder/connector clamp also has it fold over tabs onto pads, one of which connects to the same copper fill area. This black LCD glass holder is painted black and appears to be insulated from these circuit pads that the tabs are folder over onto!
Regarding EMC should the mounting holes be electrically connected to an 0V copper fill area on a main PCB, or if mounted in a metal case should they be mounted using conductive bolts?
Also the black LCD glass holder looks as if it should be electrically connected to the PCB, but is not, is this a design/manufacturing oversight?
I have scrapped the paint off the extremes of two tabs and they are electrically connected as expected, but definately not connected to the PCB pads.
Your comments appreciated.
Just to add that I did have a similar LCD a few years back and that had the black LCD glass holder/clamp touching the alluminium (anodised) front panel and it was noted that the characters were occassionally corrupted. Removing the display from the front panel cured the fault, so it was finally mounted using an insulated tape barrier, but I was unhappy with that solution.
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Hi Boss,
I used such LCD displays in the past, and never met any problem due to Electromagnetic Interferences.
For convenience, the displays were assembled on small PCBs with metallic screws and nuts, the copper foil acting as a screen (useful ?), without worrying about extra ground connections, except those foreseen by the connector itself, of course. The PCBs were fastened to the front panels with nylon bolts to avoid, now, unwanted ground loops through the case.
Note that the display may appear corrupted if pressure is applied to the screen, as when you gently press on the screen of a pocket calculator. An unforeseen torque, due to unequal fastening to a front panel, may also corrupt the display.
Kind regards,
B.B
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Hi B.B.,
Yes, I have used many of these myself but not on an item that will be CE marked and hence EMC tested. Having done several emc design courses I have just become more aware of good screening techniques and LCD modules are in my opinion not ideal, but am looking for the 'best' practice.
The point you mention on ground loops is good, but conflicts with screening integrity. As said I do not think the module design is ideal and interested in any comments on this aspect. Interestingly the module pcb has solder jumper links that would allow the metal LCD clamp or indeed the mounting holes to become isolated if the solder was removed.
I think I might carefully remove the LCD clamp, scrape off the rouge paint and reassemble, but I do not have any suitable way of testing!
The corruption I saw was definately data line related with wrong characters displayed and not LCD screen pressure. In fact I forced the fault to occur by gently electrically connecting the metal clamp to the metal front panel (probably causing an earth loop that you mention).
By the way my current design is a non-screened plastic case (due to emc version being a 'special'!), but I shall move to an emc screened version for the final products.
Thanks for your comments.
Boss.
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Hi Boss
About screening techniques, my opinion is that the more ground planes surround a module, the more efficient the EMI's suppression effect will be. And as the LCD is already EMC compliant... Without the circuit under the eyes, I don't really understand what you search. Sorry.
About ground wiring, I always design my sets with a star-like distribution as ground plane, not a web-like! This may occur when screening devices are connected to the electronic ground, and simultaneously to the case. I also ensure that analogue grounds are separated from digital grounds, to prevent interferences between return paths... Screening/shielding of what “exist” at the ends of these star's arms is a quite different matter. I consider shielding/screening a little bit like creating a "opaque cocoon" protecting what it contains against interferences, but without touching what is inside. Everything the cocoon catches is sent to the electrical earth, not to the electronic ground.
If the corruption of characters is data line related, I should look if sequences between stable data’s, r/w, rs and enable signals are correct. Unfortunately, no information about timing diagrams is available on the net. I can't help more.
Something else, maybe. The internal clock driving the LCD display may interfere with some other clock elsewhere in your design. Are your different power supply lines correctly decoupled? A small 0.1µF across the pins should help, if nothing sufficient is foreseen on the PCB itself. Another trick: use twisted pairs for such lines, it prevents (hum and) unwanted HF (switching) signals pick-up. Easy to get. Ask your wife to sit down in one corner of the kitchen, firmly holding the ends of two wires in one hand. Fasten the other two ends in the mandrel of your drill, go to the opposite corner, tighten the wires, pull the trigger, and it's done! :-)
Good luck,
B.B.
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Hi BB, again many thanks for your detailed reply. I agree fully and follow the same methods. The interferrence I had on the old project was resolved once I put a scope on the signal lines...HORROR. Noise absolutely everywhere!! The problem was immediately identified to a microstepping stepper motor drive. Although I had taken my usual precautions and it was a nicely enclosed in the manufacturers screened module bolted to a good earth plane, that was not good enough! The final cure was ferrites on the motor cable and indeed some internal ferites to ensure the motor noise went back to the source and nowhere else.
The real point I was trying to make with the original post is "Do LCD manfactures design their modules with enough thought to emi?" and I pointed out some possible issues that I thought might be wrong or could be improved.
I love you statement:
"Screening/shielding of what “exist” at the ends of these star's arms is a quite different matter. I consider shielding/screening a little bit like creating a "opaque cocoon" protecting what it contains against interferences, but without touching what is inside. Everything the cocoon catches is sent to the electrical earth, not to the electronic ground."
This really does sum up the issue. The LCD modules obviously require a large hole for visibility and that is the worst possible scenario for emi (except by using the transparent materials you mentioned). However the LCD 'cocoon' ( I really like that descriptive word), could this be improved over what is currently provided before these extra methods are employed? If say the painted LCD clamp was actually conductive and designed to fit against the internal front panel with say a conductive gasket?
The situation could be further improved by having a conductive sheet behind the LCD glass connected to the LCD bracket thus reducing the gapping hole on the front panel down to some slits the length of the LCD glass edge, (still a long slit though, but may be an improvement).
Finally the LCD mounting bracket could be electrically isolated from the LCD circuitry by unsoldering the solder links already there, hence seperating Ov from the earth.
So these are my thoughts on the current construction of the module I used.
On a different point the Schroff EMC cases appear to be incredibly well thought out using conductive coating (nickel?) on the aluminium, copper gasgets to cut in to panels on assembly, conductive emi compressible gaskets on other faces, earthing studs, so different from some of the older enclosure I used.
But still have to cut holes in them for my LCD's!
Again, many thanks BB for your comments.
Boss