Dual Power a Pi from POE and USB input?

Question

I have a Pi4 with the standard raspberry-pi branded POE hat, and it works fine.

It is installed behind a monitor, which has USB ports. So I thought to add a USB-A to USB-C cable from monitor to Pi as a second power source. Again this worked fine, until I turned off the monitor.

My expectation was that the Pi would still get power from the POE hat. However it lost power immediately.

This is different to putting a UPS on the POE switch, or having a dual PSU POE switch. I'm specifically meaning to have a local power via USB, and a remote power via POE.

Is there a way to "dual-power" a POE-enabled Pi so that both sources work and can survive either dropping without warning?

Answer 1

AFAIK, there is no "standard" PoE HAT. But even if there is, this dual power scheme will depend almost exclusively upon the design of the PoE device itself.

My suggestion: read the documentation for the PoE, and/or ask the PoE manufacturer.

That said, there is no fundamental or physical reason why this redundancy cannot work - it's simply a matter of proper design.

Answer 2

I suppose the monitor powers its USB ports from an internal 5V bus. When you switch off the monitor, this bus goes down to 0V, and brings the Pi down as well. The PoE supply is simply not powerful enough to back-power the 5V circuit of the monitor, which can easily draw several amps.

This cannot be fixed by changing the PoE supply design. Using a proper 5V supply instead of the monitor might work though.