Backpowering from Arduino on USB

Question

I have connected an Arduino MEGA2560 with the Raspberry Pi B+ V1.2 via USB

The idea is to use the Raspberry Pi as gateway to remote flash the Arduino.

So I switch on the Arduino and the Arduino switches on a relay, which switches on the Raspberry Pi power (USB micro) if I need the Raspberry Pi to flash the Arduino

It works. But when I switch off the relay, the Raspberry Pi keeps on working, because of back powering from the USB-A port connected with Arduino USB-B port.

Question 1: How can I prevent this?

Question 2: What happens, if I use an different raspberry? https://forums.raspberrypi.com/viewtopic.php?t=14059

and

https://forums.raspberrypi.com/viewtopic.php?t=203729

and others

the (poly)fuses can be blown up.

I don't like my Raspberry Pi to be damaged regardless which version I use. And I need the possibility to power off the Raspberry Pi via relay (not only via ssh: shutdown -P).

What do you think about these?

Answer 1

Put a Shottky diode in series with the 5V lead of the USB cable. I made an adapter with a USB plug & socket to do this.

Back feeding was a problem with early Pi model. You can't power a modern Pi from USB (dodgy Zero models excepted) but once booted it permits back feeding.

If you aren't planning to power the Arduino from the Pi you could just cut the 5V lead.

It is possible to safely shut down a Pi with a GPIO pin. All of my Pi can be shutdown by shorting pin 39,40.