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I'm looking for ways of waking my RPi 4B on lan. I read that the PoE hat allows switched mode power which seems like what I'm looking for. Now my router doesn't support PoE, so I may need to use also a PoE injector as well. My question is: could I avoid using a PoE injector and power my RPi with the normal power adapter and still use the PoE hat capability to remotely start up and shut down the RPi? (I guess not: chances are that the switched mode power only works through the PoE, but I couldn't find the info...)

Thanks!

cainbantam
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2 Answers2

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PoE (Power over Ethernet) has nothing to do with WoL (Wake on LAN). They are complete different things.

PoE is used to power the RasPi with its ethernet cable so you do not need a power supply and a connection for it near by the RasPi. There are simple passive PoE adapter that can be used with the normal 5.1 V power supply near by the ethernet switch but restricted to 100 MBit on the ethernet cable. I have tried to power my Raspberry Pi with it, without success. The RasPi complains undervoltage. The brownout on the ethernet cable is to big. So you have to use the active PoE hat that works with 48 V on the line and for that you need a 48 V injector near by the ethernet switch - no way to use the hat without it. With a power switch on the injector you can power up/down the RasPi, if you mean that.

WoL is a complete different solution. When the computer is powered off, at least its ethernet device is in a very low power standby so it can still listen to a magic ethernet packet. If it receive that packet it will wake up the computer. But as far as a know (please correct me) WoL is available on the ethernet device but not supported by the RasPi.

Because a Raspberry Pi draws so little power, I would say it isn't worth the effort and investment in additional hardware. Just leaving it on will be cheaper.

Ingo
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Thanks for the answers! From the side of the PoE (power over ethernet) hat, I wrote to one of the brand supplying such hats and they told me that indeed PoE hats don't add a WoL capacity to the Pi. Of course, if you have a PoE switch and plug your Pi on it with the help of a PoE hat, then if you turn the switch on/off, you can control the Pi's energy supply and knd of wake it on lan, without the "magic packet".

Yet beware that the solution consisting in adding an ethernet controlled relay, althouch it certainly works to remotely control the Pi, probably drains more power than the Pi itself. I couldn't find precise information, but simple relays like this one (https://www.antratek.nl/eth002) need an AC adapter supplying at least 500 mA with 12V DC, so that would mean a consumption of 6 W, which is twice the consumption of the Pi4 in iddle regime. So good to remotely control the Pi, but not to reduce its consumption.

Finally, wrt my question as to the possibility of using a PoE hat to add the WoL capacity, while still using the AC adapter to supply the power, the answer is no since PoE hats do not add the WoL capability. I don't know what the answer would have been if PoE hats would indeed offer this capability... but they seem not to. Basically, they only make sure that the roughly 48V supply from the Ethernet power cable supporting PoE is transformed to the Pi's requirements (5V and 3A I think...) and protect it from voltage variations or the such...

cainbantam
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