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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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    what does PoE have to do with WoL? – jsotola Aug 09 '20 at 22:17
  • Well, PoE allows to manage power, and I thought maybe PoE Hat would add the WoL capacity to RPi, I thought that it is what the switch mode power feature was supposed to do. But maybe I mixed up. Isn't it possible to switch off the PoE with the ethernet switch? I now realize that it would not be WoL properly speaking though. Thanks – cainbantam Aug 10 '20 at 19:20
  • yes, you can turn off PoE ... it is simply a power supply that is connected between the twisted wire pairs using a center tap of the ethernet magnetics ... one twisted wire pair would connect to one side of the power supply, a second twisted wire pair would connect to the other side of the power supply ... the arrangement is invisible to the data portion of the circuit – jsotola Aug 10 '20 at 19:32
  • you would shut down Linux and then turn off PoE circuit – jsotola Aug 10 '20 at 19:33
  • thanks! But then I need a real PoE switch, I guess. Cannot do it with a non-PoE router associated with a PoE injector, right? – cainbantam Aug 10 '20 at 19:40
  • all you need is come type of a switch to turn off the power to the PoE injector ... use an ethernet cotrolled relay, if you have no physical access – jsotola Aug 10 '20 at 21:39
  • Yeah, I saw that options, but I'm not sure a relay consumes less than a RPi! Since my router doesn't support PoE, I think I'm bound to leave my RPi on and not be able to restart it once the OS is shut down. But I may still reboot it in case of need... – cainbantam Aug 16 '20 at 02:45

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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  • Thank you Ingo. I know these are 2 different things, but it seems there is a way of using PoE to do WoL, which is indeed not supported on the RPi, as mentionned here: (https://raspberrypi.stackexchange.com/a/58162/123980). I'm also trying to find out if it is worth it since the RPi 4B consumes only 2.7 W idle. In particular, I've seen a solution on the same post using an ethernet switch (https://raspberrypi.stackexchange.com/a/61979/123980), but it seems that the switch consumes more than the RPi (it requires at least an adapter that provides 12V and 500 mA)! – cainbantam Aug 10 '20 at 13:25
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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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