Protecting from ESD - Page 1
Forum section: electronics / beginners

author: joeyjoejoe
author_rank: Frequent Contributor
timestamp: January 30, 2022, 08:49:11 pm
content: I've got an electronic scoreboard I made.Notice the 3 momentary switches at the top left, top right, and top center. These go to the microcontroller. The threaded button is also in contact with the bare aluminum extrusions that form the frame.It's powered by USB-C.I had this setup, and my battery bank was on the ground. I touched the frame, ESD happened, and it instantly died. My thought is that the path was the metal frame, to the button housing, and then down one of the wires (signal or ground), to the battery bank which was sitting on the ground. My best guess.Anyways, I've picked up a new micro, and some bi-directional ESD diodes. I was planning to put them between the 2 input wires on each button, but I'm not quite sure if that covers me fully?First time ESD has ever killed something.

author: Manul
author_rank: Super Contributor
timestamp: January 30, 2022, 11:22:49 pm
content: I guess it is also possible that ESD event caused damage through the USB port or other periphery, not through button inputs. Without seeing the whole construction it is not possible to say for sure. Probably the most correct and simplest approach would be to connect the metal frame to circuit ground. Also connect USB cable shield to circuit ground (if it is not yet connected). That way ESD current will flow outside and not through sensitive circuit parts. Sort of like Faraday shield. Fun fact: people survive in a car hit by a lightning just fine (with possibly bursted eardrums).Quote from: joeyjoejoe on January 30, 2022, 08:49:11 pmYou should use unidirectional diode here. I presume that the signal on these lines is unipolar (like 0-3.3V or 0-5V), so you want to clamp above the high level and everything below low level. Bi-directional diode would allow a significant (possibly destructive) negative voltage before it will start to clamp.On the other hand even unidirectional diode might not help fully, because if device is unpowered, then there will still be positive transient which will exceed maximum allowed by microcontroller (usually 0.6V above VDD). Also diode clamping voltage might not be very accurate.My standard ESD solution (if feasible) is a series resistor and dual schottky (like BAT54S) diodes going to VDD and GND. It is not a good protection against general overvoltage events, because of current injection into power rail, which might cause power rail voltage to go up, but for ESD events that is not a problem, so it works perfectly. You may search the forum and find different approaches and related discussions.All of this is likely unnecessary, because in this case it is more simple to just "ground" everything what can be touched.

author: joeyjoejoe
author_rank: Frequent Contributor
timestamp: January 31, 2022, 12:27:01 am
content: I thought for ESD bidirectional is recommended as you don't know what polarity the static discharge will be?

author: Manul
author_rank: Super Contributor
timestamp: January 31, 2022, 12:54:25 am
content: Quote from: joeyjoejoe on January 31, 2022, 12:27:01 amSimplified example: lets say we have a TVS which clamps at 5V. Bidirectional will clamp at -5V and +5V. Unidirectional will clamp at -0.6V and +5V. So unidirectional diode clamps both ways too, just not at the rated clamping voltage, but at a standard diode voltage drop. And this is what you want for unipolar signal lines. You don't want to clamp at -5V, you want to clamp immediately when it starts to go negative.If you have a bipolar signal line, then it is a different story. For example RS232 line which use levels from -12 to +12V. You need bidirectional diode, because otherwise you will clamp your signal (negative is a valid level here).