Static and Lightening protection

3rd-Party Sensors & Devices
1

It seems like a bad idea to directly connect long alarm lines directly to MCU pins.

The diodes sold for protection don't appear to be implemented correctly. 
At a minimum you would want reverse biased diodes between the pin and each rail (GND and 3V).
Better would be that in addition to a series resistor and a capacitor to ground.

Am I being over paranoid?  Have people have units fail unexpectedly (assuming most likely cause is ESD), or is this thing strong enough to take the ESD from long wires strung around the house?

Here is a good discussion on the issue and the possible solutions.
https://www.digikey.com/en/articles/techzone/2014/jun/protecting-mcu-io-lines-from-esd-and-other-transients
https://www.digikey.com/en/articles/techzone/2012/apr/protecting-inputs-in-digital-electronics

2

The diodes being sold are being used as blocking diodes instead of a traditional ESD protection diode. Because these pins are only used as an 3.3v output to the sensors we can use a simple blocking diode in order to protect the inputs. Future designs will have fully featured ESD, reverse voltage, and lightening protection built in.

3

Neil, interesting post!

I am sensitive to this since the reason I'm now using  konnected is because a nearby lightning strike took out my old panel! As I discovered during the re-install the strike also welded closed all my upstairs window sensors!


On sensor inputs series resistance is not really possible as you have a voltage divider effect with the series resistance of the wiring and the input may not trigger.


I also doubt the blocking diodes would be that effective, although I have no empirical data to support this. Along the lines of what you mentioned I am going to try some 3.6V zeners across the zone inputs (anode to ground and cathode to zone input). My gut feeling is that this is a better protection scheme, since it will afford some degree of protection regardless of the polarity of any induced voltage.


Of course, in. the event of a direct strike no amount of protection is going to save you!


Regards,

Steve 

4

many thanks