Static electricity knocking out the RGB LCD

[vegasbrad]

Hey everyone. Recently I've had some issues with static electricity knocking out my LCD on one of the units I've built, it used a clear hinged cover with a metal sub face to which the LCD was mounted to. Upon touching that metal face the LCD would display crazy characters and lockup. I power cycle the EVSE to bring it back. I put my fluke multi-meter on the leads to the menu button it's about 5 volts, I was able to figure out if you just plug the main power cable in and stand back things look okay... it stays at 5 volts...as soon as you rub your feet on the carpet and touch the aluminum mounting panel the LCD and button are mounted in then suddenly the voltage drops down to 3.5v and you put your finger on the back of the chip and within 2 seconds if its hot! I used my IR gun and it was 212 degrees F after about 30 seconds so I unplugged it and tried this test several times and it seems like if you're very careful and stand back it won't react. If I touch the plate after boot-up is complete then it triggers the event right away even without actually getting a static static shock...something very interesting is happening inside.

I did fix the problem. I ran two ground wires, one from each hinged corner of the metal sub-face  to the ground block and even connected those wires to the hole directly next to the main ground wire on the block for good measure. This way where ever the plate is touched the shortest patch to ground is one of the 14 ga. ground wires. problem solved! I can scuff up some static and touch the face, generating a nice spark sound and the EVSE isn't phased.

Interestingly there is a solder pad on the RGB LCD to tie one of the mounting holes to the I2C ground wire, it is un-bridged by default.

This  this latest EVSE build uses the traditional poly case and has a little bit different of an issue when you press the metal menu button occasionally the LCD will lock up. it is pretty random standing still, but I can get it to do it consistently by rubbing my feet across the carpet then coming back and touching the button at which point the LCD is still fine and then after you press the button the characters disappear, the  back-light LED is still on and the illumination on the menu button is dim so I unplugged the unit and took the cover off and put my finger on the back and sure enough the large IC is extremely hot, the solution for this one was to solder a wire to the threads of the push button and connect it to the main incoming ground line, now I can move across the floor and even get a good spark sound when touching the menu button and nothing happens to the LCD and I press the button and it still continues to function as normal.

So finally here is my question, why do the LCD seem to be more sensitive now.  I've never had to ground a button before in dozens of builds. I inspected the soldering job on the header pins and they all seem fine, I even reflowed the ground pins just to be sure but that did not help.

[gnuarm.del...@gmail.com]

I know this post is a bit old, but my two cents worth...

All CMOS devices are sensitive to static.  It can fry them or create internal shorts that may not impact the way they operate, but will cause excessive current draw.  It can also just upset them so they don't work right until power is cycled as you seem to be experiencing. 

When a pin on a CMOS device is pulled too far outside the power supply range a internal parasitic diode starts to conduct current.  This current is injected into the substrate where it can turn on a parasitic SCR structure that is part of all CMOS devices.  Once on this SCR structure can draw enough current that it destroys the device, or at least impact the operation of the CMOS device.  The only way to shut off this current is to remove power. 

I can't say why one module is more sensitive than another, but all external metal attached to any internal circuits should be grounded or otherwise protected against static discharge.

  Rick C.

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[Civic Turbo]

Rick, Thanks for your feedback! Following this post I was advised to add pull up resistors to the I2C data lines, the target was not less than 1k ohm which did help the LCD not be soo sensitive to static shocks.

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