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Isolating/terminating a 24V PoE port

Bit of an odd one, maybe... I've got a NanoSwitch at the end of a long Cat5e feeding a Nanostation and a Unifi AP, and want to hang a non-PoE device off the spare port.  The device can be powered separately, I have mains power nearby so I just want an ethernet connection.  But the NanoSwitch is always putting out 24V on all the ports, which I assume may fry anything I plug into it which is not specifically designed for that.

 

Any way to safely isolate the 24V?  (Obviously you're thinking, just run another Cat5e cable, yup, thanks, but I want to avoid that because it's a long and awkward run and I have a switch right there with a spare port so it seems uber wasteful not to use it)


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SuperUser
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Re: Isolating/terminating a 24V PoE port

[ Edited ]

To retain Gb speeds on that port in this situation,

I use a Gb-rated passive PoE adapter in reverse.

So the port that has PoE injected in it is connected

with a jumper to the switch, and the non-powered

port goes to the user device.

 

Yes: you can isolate PoE to a 100Mbps device simply

by removing lines 4,5, 7 and 8. But it won't work for

Gb devices--which use all lines. They need the

transformer isolation provided by the PoE injector.

 

I suppose you could use a spare Ubiquiti PoE power

supply, but you would be injecting PoE power into its

internals. Maybe not a problem.....

 

A much better solution would have been a disable switch

on each port. And F-PoE has the same issue.   Dave


> HQ in Seacoast region New Hampshire U.S.A.
> Ubiquiti Certified Trainer [UCT] for:
     UBWA [AirMax] / UEWA [UniFi] / UBRSS [routers]
UBNT.NH@gmail.com

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Established Member
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Re: Isolating/terminating a 24V PoE port

[ Edited ]

If you only need a 10/100 port, just don't terminate the 4,5 + 7,8 on the cable feeding the non-poe device.

SeaKay Broadband
http://seakaybroadband.net
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Re: Isolating/terminating a 24V PoE port

I use an extra POE for this purpose, just not plugged into the wall.

SuperUser
Posts: 42,794
Registered: ‎06-23-2009
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Re: Isolating/terminating a 24V PoE port

[ Edited ]

To retain Gb speeds on that port in this situation,

I use a Gb-rated passive PoE adapter in reverse.

So the port that has PoE injected in it is connected

with a jumper to the switch, and the non-powered

port goes to the user device.

 

Yes: you can isolate PoE to a 100Mbps device simply

by removing lines 4,5, 7 and 8. But it won't work for

Gb devices--which use all lines. They need the

transformer isolation provided by the PoE injector.

 

I suppose you could use a spare Ubiquiti PoE power

supply, but you would be injecting PoE power into its

internals. Maybe not a problem.....

 

A much better solution would have been a disable switch

on each port. And F-PoE has the same issue.   Dave


> HQ in Seacoast region New Hampshire U.S.A.
> Ubiquiti Certified Trainer [UCT] for:
     UBWA [AirMax] / UEWA [UniFi] / UBRSS [routers]
UBNT.NH@gmail.com
New Member
Posts: 8
Registered: ‎08-13-2018

Re: Isolating/terminating a 24V PoE port

Awesome, thanks for the replies.  Didn't occur to me that the adapters would work unpowered, but I've dug out a GigE PoE adapter from the bottom of the office junk pile and it works perfectly, all the magic smoke has stayed inside my Disposable Ethernet Analysis Device (a raspi obviously) and it successfully connects at 1000BaseT.  The LED on the adapter even lights up!  (Wondering if it will produce 240V output too?...)

 

 

 

SuperUser
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Re: Isolating/terminating a 24V PoE port

[ Edited ]

Welcome! Yes: in this case, the dc that usually is

provided externally to the adapter comes out of it

instead. It's the power provided by the switch port.

 

But sorry: only 24V--not 240!   Dave


> HQ in Seacoast region New Hampshire U.S.A.
> Ubiquiti Certified Trainer [UCT] for:
     UBWA [AirMax] / UEWA [UniFi] / UBRSS [routers]
UBNT.NH@gmail.com
New Member
Posts: 8
Registered: ‎08-13-2018

Re: Isolating/terminating a 24V PoE port

Hah!  Great.  So now I feel the need to jerry-rig a C5 cable to a 5V step-down regulator and hide this somewhere powering some USB device to confuse the **** out of some poor future sod, should be totally safe and nobody will get killed right?

SuperUser
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Re: Isolating/terminating a 24V PoE port

Right.  Dave


> HQ in Seacoast region New Hampshire U.S.A.
> Ubiquiti Certified Trainer [UCT] for:
     UBWA [AirMax] / UEWA [UniFi] / UBRSS [routers]
UBNT.NH@gmail.com