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I have a setup using a Raspberry Pi 3B. I use the PoE hat on top and I power the entire setup over PoE from a Unifi switch.

The setup has quite a lot of peripherals that need power and I use the 5 volt rail of the Raspberry to provide the peripherals with power. This has worked perfect for a year or so however lately I am having a hard time booting the RPi. Most likely because the peripherals have a high surge during startup. Sometimes I manage to start the RPi when I try it a few times and when it finally starts it can run for 6 months no problem.

Now I am thinking of possible solutions. Either I should find a separate power source for all the peripherals, but it also feels a bit complicated with different potentials and other issues. Also I would prefer not to put a plug in the wall.

The second solution I am thinking about is to swap the RPi 3 for a RPi 4 and hope that it could provide a little bit more power for my peripherals.

I find little or no info on the RPi 4 together with the PoE hat. It seem to be supported though.

Anyone who can shed some light on this for me? Is there any chance the RPi 4 would provide more amps on the 5V rail when powered over PoE? Or is there anything I can do to limit the current dras during bootup?

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While the 4 is technically capable of providing more power to usb devices, if your setup was working before and is not now (without changing the USB power load), the bottleneck may be upstream. If it wasn't, then you could draw power from the PoE to power a usb hub (avoiding the wall wart).

In my experience (using PoE cameras), PoE switches have been the culprit in most setups. (in which case getting a 4 won't make a difference as it'd be powered from the same faulty source).

If you want to try pi-centric solutions:

Some of my pi setups with less-than-stellar power supplies have done well with a large capacitor on the 5v (same cap could/should have been built into the power supply).

I've also sometimes used uhubctl to kill power to usb devices unless I needed them (although I haven't specifically tried at boot). The tool is pretty easy to install from a package manager; you could turn them off in your startup sequence and turn them on later if you believe you just need to weather that initial load at pi bootup.

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  • Thanks for the reply. I dont have any usb periphersls. All of them are connected to the 5v rail. The suggestion regarding the large cap might be good though. What size of cap have you used and where did you connect it? Since I am using poe I have no idea. Maybe to the 5v rail? But caps usually creates a large surge during startup so I wonder if it will even make the situation worse Sep 20 '20 at 6:31
  • I used a 1000 uf cap going from 5v to ground (no resistors) but with nothing but the pi on the power rail. I don't believe it causes a greater surge, but thats with my power supply. If they're on the 5v rail, take a look at your devices and see if you want to add some power mosfets to let the pi control when they are powered.
    – Abel
    Sep 20 '20 at 15:53
  • I am going to accept this as the answer since it seem like a reasonable solution. However the problem I had seemed to be more complicated. My current theory is that the SD card was near end of life and that it failed totally when voltage dropped during boot. When I switched the SD-card everything worked smooth again. This was really odd because sometimes it also worked with the old card so really hard to investigate the cause. Sep 22 '20 at 20:59

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