joelones
Posts: 12
Joined: Fri Jan 16, 2015 4:01 pm

Powering Pi over active POE switch

Fri Jan 16, 2015 4:28 pm

Hi,

I'm kind of new to electronics, but would like to power the pi over POE. I have the following switch which is what I assume an active POE switch:

NETGEAR 8 Port w/ 4 ports PoE Switch (GS108P)

I also bought the a LM2596 DC to DC Step-Down Transformer.
http://www.gearbest.com/development-boa ... 7AodwmQAbQ

After doing the research, I'd like to use ethernet pin 4 for VDC+ and pin 8 for VDC-. Pins 1-2, 5-6 are used for data. This would work I assume if you had a passive switch. I believe that active switches do some sort of auto-negotiating before sending power.

Would I still be able to accomplish this with an active switch.

Thank you

joelones
Posts: 12
Joined: Fri Jan 16, 2015 4:01 pm

Re: Powering Pi over active POE switch

Sat Jan 17, 2015 9:45 pm

Any thoughts?

fruitoftheloom
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Location: Delightful Dorset

Re: Powering Pi over active POE switch

Sat Jan 17, 2015 10:04 pm

Rather than negativity think outside the box !

Asus ChromeBox 3 Celeron is my other computer.

joelones
Posts: 12
Joined: Fri Jan 16, 2015 4:01 pm

Re: Powering Pi over active POE switch

Sat Jan 17, 2015 10:57 pm

Thanks for the suggestion and yes I realize I could use a POE splitter (that converts down) to get both data and power but I'd hope there would some way into fooling the active switch into sending power such that I could to the converting with my own circuit thus bypassing the need to buy a poe splitter.

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aTao
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Location: Howlin Eigg

Re: Powering Pi over active POE switch

Sat Jan 17, 2015 11:36 pm

joelones wrote:I believe that active switches do some sort of auto-negotiating before sending power.
Pretty sure I read (Wikipedia maybe) that the power negotiation was all about measuring resistances, so not sure how "intelligent" the powered device needs to be.
>)))'><'(((<

Tarcas
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Location: USA

Re: Powering Pi over active POE switch

Sun Jan 18, 2015 11:40 pm

I'm not a PoE expert, but I am a network engineer and know more than a little about it.
802.3af and 802.3at are the IEEE PoE standards. They specify 48V DC, and do require quite a bit of intelligence on the endpoints. I have seen that there are Pi adapters for these, and they are quite expensive. If you want something that "just works" get one of these and be done with it.
If you want a power injector, that's more of a DIY sort of solution, where you would use the brown and blue wires to inject current into the cable, and the take it back off at the other end. The Pi takes 5v, but 5v won't travel very far down an Ethernet cable and remain 5v. Your best option is to inject a significantly higher voltage (hence the standards using 48v) and step it down to the 5v that you need near the Pi. Again, you can buy ready-made solutions for this. These will be MUCH cheaper than the IEEE compliant ones. You can also make it yourself if you're good with DC and Ethernet.

joelones
Posts: 12
Joined: Fri Jan 16, 2015 4:01 pm

Re: Powering Pi over active POE switch

Mon Jan 19, 2015 4:41 pm

Tarcas wrote:I'm not a PoE expert, but I am a network engineer and know more than a little about it.
802.3af and 802.3at are the IEEE PoE standards. They specify 48V DC, and do require quite a bit of intelligence on the endpoints. I have seen that there are Pi adapters for these, and they are quite expensive. If you want something that "just works" get one of these and be done with it.
If you want a power injector, that's more of a DIY sort of solution, where you would use the brown and blue wires to inject current into the cable, and the take it back off at the other end. The Pi takes 5v, but 5v won't travel very far down an Ethernet cable and remain 5v. Your best option is to inject a significantly higher voltage (hence the standards using 48v) and step it down to the 5v that you need near the Pi. Again, you can buy ready-made solutions for this. These will be MUCH cheaper than the IEEE compliant ones. You can also make it yourself if you're good with DC and Ethernet.
Thanks for the reply. And yes, the idea was to step it down to the 5v that near the Pi.
I guess I should have simply bought a power injector rather than an active switch as from what I gather there appears to be no way (for me at least) to treat the active switch as a passive switch.

Tarcas
Posts: 741
Joined: Thu Jan 09, 2014 5:38 am
Location: USA

Re: Powering Pi over active POE switch

Tue Jan 20, 2015 4:48 am

joelones wrote:
Tarcas wrote:I'm not a PoE expert, but I am a network engineer and know more than a little about it.
802.3af and 802.3at are the IEEE PoE standards. They specify 48V DC, and do require quite a bit of intelligence on the endpoints. I have seen that there are Pi adapters for these, and they are quite expensive. If you want something that "just works" get one of these and be done with it.
If you want a power injector, that's more of a DIY sort of solution, where you would use the brown and blue wires to inject current into the cable, and the take it back off at the other end. The Pi takes 5v, but 5v won't travel very far down an Ethernet cable and remain 5v. Your best option is to inject a significantly higher voltage (hence the standards using 48v) and step it down to the 5v that you need near the Pi. Again, you can buy ready-made solutions for this. These will be MUCH cheaper than the IEEE compliant ones. You can also make it yourself if you're good with DC and Ethernet.
Thanks for the reply. And yes, the idea was to step it down to the 5v that near the Pi.
I guess I should have simply bought a power injector rather than an active switch as from what I gather there appears to be no way (for me at least) to treat the active switch as a passive switch.
If you can return it, the most cost-effective method would be a power injector; ether DIY or ready-made, plus something like a 5v UBEC. After a very quick search, I found this:
http://www.adafruit.com/product/435
You probably want to pass at least 12v through the Ethernet cable and then regulate it down to 5v, but for $6 for the injector, that leaves a lot of room in the budget for extras like the 12v power supply and 5v UBEC before you'd be better off buying an active PoE device.

Bruny
Posts: 159
Joined: Fri Feb 06, 2015 5:22 pm

Re: Powering Pi over active POE switch

Thu Feb 12, 2015 2:01 pm

so RB750UP is best passive option with 24V?

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