Question regarding the Automation HAT and input signal safety

Support
1

I am currently setting up a Pi with the Automation HAT to monitor some security outputs at my home. The signals are 12V based. I was reviewing the schematics for the Automation HAT and I had a few questions.

I see a protection stage with a 20k resistor and 3V3 zener clamp but nothing afterwards. The signals seem to go straight to the IO pins after the zener clamp.

I have been very careful to ensure the 12V supply is switched off before switching off the 5V supply to the Pi. Unfortunately, information regarding the robustness of the GPIO pins on the Pi seems rather scarce. If I switch off the 5V supply to the Pi before removing the 12V to the Automation HAT signal input, could I potentially damage the Pi as the pins will be driven by a voltage without the proper supply?

Has anyone experienced any problems depowering the Pi with signals still attached?

2

Does nobody have any idea? Should I just hope for the best? Perhaps I’ll just solder an additional clamp diode to 3V3.

3

Niko, our head of engineering, says: “it’s ok to disconnect the Pi 5V first, as the Pi has ESD protection diodes on the GPIO pins, so the extra voltage there will just go to the Pi’s 3V3 rail, and the current will be low enough due to the 20k ohm resistor not to cause any issues”

Hope that helps :-)

4

Thanks for clearing that up.