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Sorry, this PoE technology baffles me a bit. I have a lightweight router that can be powered over ethernet; is there a way of providing the power (and ethernet connection between the router and the laptop) from my laptop? I have heard about PoE injectors and I wonder if they could maybe transform the USB socket power into PoE for the router? The idea is to not rely on power sockets in powering that router. Laptop runs linux, but that probably is not important. And lastly the craziest idea - maybe my laptop already provides power over ethernet?

Router has Passive PoE input.

So the main question is - can I power the router from laptop using PoE, maybe using some sort of adapter, but not resorting to wall power socket?

Thanks

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    Does your router accept PoE? If yes, does it expect "passive" PoE or 802.3af?
    – u1686_grawity
    Aug 2, 2018 at 9:47
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    I wonder if they could maybe transform the USB socket power into PoE for the router PoE standard claims 48V (36-57V) up to 400mA. USB cannot provide it. I have heard about PoE injectors It has its own power supply. Use it.
    – Akina
    Aug 2, 2018 at 9:50
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    Aside from what already is mentioned here, it is very unlikely that your router actually supports PoE inwards. Most routers that do support PoE are only PoE outwards, as in, it can power other devices, not being powered by. But that said, why would you want this? Just get a one of those power blocks that give you multiple wall outlets from a single cable, and plug all the devices in there.
    – LPChip
    Aug 2, 2018 at 10:02
  • That is the router. mikrotik.com/product/RB952Ui-5ac2nD-TC - I currently have a router that very satisfactorily works from microUSB from my laptop, so I gather the PoE should be possible. But not likely right from laptop though.
    – r0berts
    Aug 2, 2018 at 10:20
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    Netgear GS105PE 5-port switch can be powered from PoE (802.3af/at), and also pass PoE power through to a couple of attached devices.
    – Craig McQueen
    Nov 14, 2019 at 11:58

4 Answers 4

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Probably not.

PoE operates at 44 volts and (originally) supplied up to 15 watts of power.

USB on the other hand supplies 5 volts with a maximum of 2.5 watts, with some devices being able to output more.

To convert the 5V to 44V can be done, but the efficiency would be bad and a "standard" USB probably wouldn't be able to supply enough power to make your PoE switch work at all. You might get 1 or maybe somewhere near 2 watts after stepping up the voltage.

The problem is then that PoE devices then usually step the voltage back down to their own working voltage which is another power conversion with associated efficiency and power losses. You'd probably lose another half a watt of power, bringing you down to maybe 1 watt of power, if you were lucky. That's not going to be much for any active electronics to work with. I doubt any kind of active Ethernet switch can work with that little power available. Most switches I see need 12V at 1 to 2 amps which is 12 to 24 watts, and you'd be supplying one tenth of that power.

I've never seen a laptop with any kind of built-in PoE hub or switch capable of supplying devices power. It seems counterintuitive and would drain the laptop battery even faster.

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  • Thanks you are quite right. I wonder though from the router specs it says: Max Power consumption 8W, PoE in Passive PoE, PoE in input Voltage 10-28 V. But probably still not feasible, right?
    – r0berts
    Aug 2, 2018 at 10:23
  • Even with the lower voltage requirement it will still be trying to draw far more than a USB port could supply. There seem to be some cheap and nasty "USB powered 5 port Ethernet switch" devices such as this but they make me slightly uneasy and I wouldn't expect them to be blazing fast.
    – Mokubai ♦
    Aug 2, 2018 at 10:45
  • Well, with this last bit - probably the technology has improved as the device I have functions very well - either from wall-micro USB adapter or from laptop usb power. It has Max Power consumption 3.5W - but Ethernet ports of are 10/100 of course.. As it is for the swithc you found.
    – r0berts
    Aug 2, 2018 at 15:33
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From Wikipedia:

The original IEEE 802.3af-2003 PoE standard provides up to 15.4 W of DC power (minimum 44 V DC and 350 mA) on each port.

Later standards change the current capacity, but the voltage requirement remains. This is both too high a voltage and too much power to provide it via USB.

And no, your laptop won't supply PoE

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  • Thanks, so in that case it seems I should get some kind of PoE adapter which would inject the power from the wall socket? Could it be done from a Li battery though?
    – r0berts
    Aug 2, 2018 at 10:26
  • a PoE injector would be the way to go for the PoE route (or just plug the router in directly depending on proximity to a socket. You would be better off finding a battery bank that could power it directly than by using PoE too.)
    – Baldrickk
    Aug 2, 2018 at 10:29
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I would suggest getting mAP lite or mAP which are two lightweight routers that support Power from USB port as well as POE and its own power brick.

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I'd like to make a correction about the a USB port only being able to provide 1 to 2 watts of power after stepping-up the voltage. Watts are a measure of power. So 10V at 0.5Amp, is the same wattage as 0.5V at 10Amps. It is true that the step up modules will lose effency, but cheaper ones look to be in 90-95% range. Standard PoE is 44-57V (ideally 48V), but passive PoE is frequently 24V. And my guess this the case for your switch. I see a standard D-Link 4 port 10/100 network switch only consumes 0.85 watts. So at 24V that would be about 0.035A. Seems a little on the light side for power but could be possible.

The big take away from this is find a step-up module that can produce the voltage you need from 5V (most seem to be able to handle 2 Amps of output voltage) at 90% or better efficiency. Then find the switch that only consumes 2 watts or less. If the network switch is going to sit right next to your computer, just power the switch through its DC port with your computers USB and a step up module.

Sincerely from, The distant future, the year 2000

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