What has to be the strangest electrical problem I've seen.

Nite Shadow

pi Model or other hardware: Raspberry Pi 3 model B
Power Supply used: Generic USB 5.5V
RetroPie Version Used : How do I retrieve this data?
Built From:
USB Devices connected: Unknown brand pen drive (Mounted to load roms), Generic Xbox controller.
Controller used: See above.
Error messages received: None
Log found in /dev/shm/runcommand.log (if relevant):
Guide used: none
File: None
Emulator: None
Attachment of config files:None
How to replicate the problem: Unhook the power supply.

So, this just has me baffled.
For some unknown reason, when I sever the power to my Raspberry Pi 3, it remains running. This is actually disconnecting the power supply entirely. Still powered on, still displaying video, still operational. No batteries present. I've concluded that it is drawing power off the HDMI out somehow, since unhooking that cuts the power. My question is how is it doing this? and if I should be concerned. If it is a major problem, then how do I fix this?

jonnykesh

@nite-shadow HDMI has been known to back-feed power. Sometimes just enough to keep the Pi ticking over. Try editing your /boot/config.txt file and rebooting. Maybe this config_hdmi_boost=4 4 is default, maybe.

Nite Shadow

Changing that setting to 1 seems to have done the trick. The undervoltage icon is appearing now, but that could just be due to my PSU.
Thank you!

jonnykesh

@nite-shadow It's really odd behaviour and depends on so many factors... Your TV, the cable. the length, even your individual Pi. Also, get a decent PSU!
Even try a different HDMI port on the TV. Some tend to supply more than others. MHL and ACS ports tend to give a bit more. On my TV (LG) they are marked, on others they are not.

Nite Shadow

Tell me about it. 1 TV and an HDMI to AV adapter in my house seem to trigger this behavior. I didn't realize there was that much power running through the HDMI.

jonnykesh

@nite-shadow There is meant to be only about 50mA via HDMI but it usually doesn't matter. The Pi is just so sensitive to power that it will take what it can. It is designed that way.

jonnykesh

@nite-shadow I bet you thought your Pi was haunted! "How do you kill this thing??? It's gone sentient!"

Nite Shadow

Nah, I found out where it was receiving power from pretty fast. this was just a matter of "Huh, that shouldn't be happening"
Although the term "This thing has just gone creepypasta on me" did come up.

jonnykesh

@nite-shadow Ha! If I pull the power on something and it keeps working... It's haunted as far as I am concerned! In the time I have been messing with Pis it has taught me so much more about other electronics.

Nite Shadow

This thing is most definitely different than the typical electronic device I end up fixing.
It's not like reprogramming an XBox so I can play Super Mario Bros, or fine tuning a windows computer. No... this is unique...

jonnykesh

@nite-shadow It is, It's a unique piece of hardware. (to most) It's not designed for a retail plugandplay consumer.

Nite Shadow

This is part of what makes it so powerful. While not for novices, the same aspect makes it highly flexible. If you know what you are doing, you can do literally anything with it.
By my perspective: Very easy in many aspects, but much that I simply do not know. It's not often that I have to call tech support. Needless to say, this is a learning experience.

jonnykesh

@nite-shadow It is a learning experience. When you know it all you get to retire.

Nite Shadow

Then I pass the knowledge on, so that my legacy may continue.