Static Electricity creating pulse on ground that is clearing shift register?

[Oaklandishh]

So recently due to the low temperatures and in turn the low humidity I have been seeing this weird thing where if you wiggle too much in a chair near the circuit-board and power system I have been working on, then stand up, you create a pulse in the ground of the system which is strong enough to toggle the clear pin on the boards shift register.

The chair is according to my DMM isolated from the floor, and the person standing up needs only to either zap themselves by touching the chair or sometimes just walk away from the chair in the correct direction to recreate this.

I am not an amazing circuit board designer so I am guessing I made some simple error that anyone with experience would know not to do.

All of my ground planes are connected. They were in a web style connection earlier, but I tried moving the shift register to a point style connection on solder-less breadboard and still had issues.

All of the grounds are also connected to earth ground through the 120VAC 3 prong plug GND as well as the chassis holding everything.

The 120VAC comes in and goes to a switching 24VDC PSU which has negative jumpered to earth ground. That then goes to a motor controller made by ZABER with 4 DO pins and an analogue out with a 5VDC pin which I use to control the shift register.

I have tried adding caps between the 5VDC and GND, I have put a resistor and caps between the 5V and CLR pin and CLR pin and GND respectively.

The shift register in question is a TPIC6c595 although I don't think its particularly important to the question.

Any suggestions?

Thanks.

 

[IRstuff]

Here's the web page I found:

http://www.ti.com/product/TPIC6C595/datasheet

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[BrianE22]

Because it's easy to do...try putting a copper sheet below your breadboard and ground it to your circuit. Try one common ground and try multiple distributed grounds. Try caps between the copper and the breadboard +5 volt supply.

 

[itsmoked]

Or aluminum.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Oaklandishh]

I tried the ground plane without success. I was unable to implement the one common ground point so I did not try that, but it seemed like it might have gotten worse when I used caps between the sheet and gnd and when I just connected each bus to it directly.

I have recreated the issue using an arduino so that is what I am not testing with.

I have taken the board and the arduino and put them in a box that I covered with aluminum foil and grounded and still had the issue.

I added some weird Diode low pass filter combination to each input pin and that seems to have helped, but not fully removed the issue. I haven't fully tested weather it is the low pass filter or the diodes that are making the difference, but from the little I have tested I think the diodes have helped the most.

here is what I can still get on each input pin using the oscilloscope.

 

[Oaklandishh]

Related, my back is getting chaffed from rubbing in the chair and shocking myself so many times, is there a more repeatable way to create the ESD then wiggling in the chair and touching it?

 

[itsmoked]

ESD guns

Or a glass rod and cat fur. (I'm serious.)

But then, I'm serious that you're wasting your time here dinking with these lame setups that don't come even close to reality.

Layout a correct board with logical ground planes and protect the board from power supply transients and this issue will be a mute point.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[BrianE22]

You could try opto-isolators on the input pins. That possibly would help if the noise coming into the register is from the input pins.

 

[BrianE22]

Just a general question to all - how do you measure the potential of the ground plane ("ground bounce")? Do you put the scope leads between earth ground and the circuit "ground"? How good is a power cord plugged-in scope at isolating the inputs from its own ground?