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Like in similar questions, I have my Pi on my TV and it currently draws power from the TVs USB. Fingers crossed, it currently works, but surely the Pi should have a better supply.

Now, the good point of the current config is that the Pi is powered on exactly when and if the TV is switched on.

How can I reproduce this when powering the Pi externally? Normal USB Relays seem to use the "data" on the USB to configure the relays; I would need one that most simply switches on when the TV powers up its USB and switches the Pi off as soon as the TV is powered off and consequentially its USB out goes down. Like an USB relay simply controlled by USB power, not data.

Thanks for any ideas!

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    "Fingers crossed, it currently works, but surely the Pi should have a better supply" -> Just because it is a USB jack does not mean it is a subpar supply. While USB 2.0 jacks only have to supply 0.5A, this is not enough to boot most models, so if it is running at all, it is not limited that way (which is not unusual, eg., with 2.0 hubs individual jacks may not be regulated at all, so you can draw anything up to the limit of the entire hub). If you aren't getting low voltage warnings now they are not going to suddenly start happening for no reason, so probably you do not have to worry. – goldilocks Jan 2 '20 at 17:21
  • Well I think I see something like low voltage warnings in form of the yellow flash symbol in the upper right corner of the libreelec kodi.... – Stefan Hegny Jan 2 '20 at 17:40
  • Not close to a solution but interesting comments here: electronics.stackexchange.com/questions/474327/… – Stefan Hegny Jan 3 '20 at 20:22
  • You should never just cut the power of a running RasPi. It may corrupt your operating system (OS). Do you shutdown the RasPi gracefully before switching off the power, or do you have a read only OS? If not then crossing fingers doesn't help. – Ingo Jan 4 '20 at 8:49
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    Most of us simply never poweroff the RasPi. It really doesn't hurt with its low power consumption and I think it would solve all your problems very simple. – Ingo Jan 4 '20 at 20:43
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First, if your RPi works fine (you don't get undervoltage warnings), then leave your setup as it is. Don't try to repair that ain't broken.

Second, it's actually not such a great idea to power down the RPi without using the "shutdown" command. So, if you're going to get a separate power supply for the RPi, consider leaving it powered at all times, even if the boot time is not relevant.

Finally (if you wish to ignore the advice above), you don't need a USB relay for controlling a power supply. You need a dumb 5V relay wired to USB:

schematic

simulate this circuit – Schematic created using CircuitLab

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