Crashing LCD

[George]

Hope everyone had a good Christmas and new yrs, unfortunately I'm bringing a problem that plagued me from last year into this. I built a little jig for testing energy from small piezo igniters. It cycles them several thousand times and records the data. The basic system is I'm wanting to record the energy levels in the piezo device and see how it falls off over time. However much above 250uJ from the piezo and the LCD will crap itself either going blank, missing a line, moving half a line up etc. The Piezo discharges itself through a schottky bridge rectifier into a cap, then I just measure the voltage on the cap with the ADC through a 10K resistor. It works fine until I get a piezo thats a little gruntier than the rest and that will just wipe out my LCD until power down and back up again. The power to the LCD is filtered through a resistor and a bunch of caps I think a couple of 33pF, 1nF, 100nF, 1uF and 47uF. But it seems to make little if any difference.

One thing thats interesting is the screen will still go blank if there is no positive from the cap hooked up, hows the best way to quench out any nasties coming through the line? Ferrite beads? Small inductors?

For the moment it has me beat, any thoughts would be greatly appreciated.

Many thanks

[Jerson]

I have seen such a problem very recently with one of my products. The problem seems to be(at least till now) that the LCD bezel tabs (the ones that hold the LCD together with its PCB) were in close proximity(around 0.5mm closest point) of the relays that were switching on/off some motors in my case. Just like you experienced, I would get the same behaviour even when the motor was not powered by closing the electrical circuit. Just one wire would drive the lcd mad. What I did was to bend away the offending tabs on the LCD so as to increase the air gap beyond 2mm. Now, the problem has not totally died away, but the frequency of occurence has gone down manifold.

My suggestion is that you look for any possibilities that the circuit is getting coupled due to HV breakdown of air. Try to increase distances between the HV and LV sections to whatever may be practical. You might want to consider occasionally re-initializing the LCD if it still troubles you.

[Ioannis]

Is the bezel earthed?

Ioannis

[Jerson]

The bezel is of metal and has 6 small projections behind the PCB that are twisted to hold the bezel & LCD together. Earthed? No. Those pads are totally isolated, but have a copper fill ground a few mils away from them.

[Ioannis]

If you can solder a cable and earth this bezel, I am sure it will help a lot. Now it acts like an antenna and picks the spikes from the relay and your load.

Ioannis

[Jerson]

Thanks Ioannis,

My problem was solved as noted in my first post. I mentioned all this detail just so it maybe of help to George in his problem. The issue could be pickup (EMI).

Regards

[Ioannis]

...Now, the problem has not totally died away...

I supposed by that phrase that problem is still present. Even if it happens sporadically,earthing may help.


Ioannis

[Jerson]

Ok, I understand. I hope to know for sure once the boards are on the field in real-life conditions. Thanks for the tip. I appreciate it.

[Acetronics2]

Hi, George and Jerson

Heard of something similar on a modelling forum ...

http://www.rcgroups.com/forums/showpost.php?p=19570295&postcount=80

Alain

[Ioannis]

Jerson: No problem. That's why we are here after all.

Alain: They usually put a black tape on these COG lcds obviously for this reason. Electronics are sensitive to light energy.

By the way, I am looking for such COG LCDs either 2x8 or 2x16 prefferably with backlight. Do you have any sources for such devices? Cheap I mean...

Ioannis

[George]

Thanks for the input so far, unfortunately the Bezel is ali, have put solder on all the pads under the bezel tabs, soldered a separate earth cable then pulled the tabs onto the solidified solder so they scraped the solder and in theory should make good contact. Still no luck tho the LCD does the same. I'm guessing its more the power coming down the line than radiated power, I'll try chuck some ferrite beads on n see what happens.

[Jerson]

George, actually a small handphone video of the problem will go a long way in understanding your problem.
Ioannis : Don't know if this will help you, I got some COG graphic lcds from them pretty cheap and easy to use except the connector is very fine pitch. You need to be good at soldering fine pitch smds.
http://www.aliexpress.com/product-gs/397103872-Free-Shipping-1-8-inch-128x64-graphic-LCD-Display-in-Blue-LED-wholesalers.html

[Ioannis]

Thanks Jerson for the link.

Ioannis

[George]

Finally think I have it resolved, I tried a lot of things, ferrite beads, rf inductors, ceramic caps and all failed. The thing that seems to work is I got 5m of sheilded cable and ran the signal through that, appears like the combined inductance and capacitance of the sheild soaks it up. Bit rough - but it seems to work for now!