Access to PoE power w/ 802.3af adaptor board?

Our Ethernet Shield combines LAN connectivity, a microSD slot, Power-over-Ethernet support, and even a prototyping area for you to add your own parts.
Ohmware
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Access to PoE power w/ 802.3af adaptor board?

Post by Ohmware » Fri Dec 09, 2011 4:19 pm

Greetings,

I am using a Freetronics Ethernet Shield with the 802.3af adaptor board, and I need some higher-voltage DC supply for my project. Can I tap the two 802.3af pins on the shield directly to get this? Or should I use a step-up converter and draw the power from the 5v supply?

swordfishBob
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Re: Access to PoE power w/ 802.3af adaptor board?

Post by swordfishBob » Fri Dec 09, 2011 11:22 pm

Interesting question.
802.3af involves signalling before power is applied by the switch/injector. Once the power is brought on though, it should be possible to tap the supply. There may be limits and assumptions on current draw, including inrush.

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jonoxer
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Re: Access to PoE power w/ 802.3af adaptor board?

Post by jonoxer » Sat Dec 10, 2011 12:02 pm

You have two options: take power straight off the PoE input (which may be 50V+, depending on the injector, so be careful with what you attach to it!) or taking power from the regulator module output. I set the regulator output to about 7.5Vdc because it's then fed to VIN and therefore to the input of the onboard regulator on the Arduino, which needs a couple of volts above its expected output voltage (5V) to allow for its own voltage drop.

So if you connect your additional devices to GND and VIN you'll effectively be getting the output of the PoE regulator module before it hits the onboard reg, so you'll have about 7.5V to use. Not much above 5V so I don't know if that helps you, but it's something.

This is something I've found a bit of a pain, too, because my use-case for this would be to have an EtherTen in a box with a bunch of 12V relays, and I want to power it all over the LAN including the relay supply. I can't do that with the 802.3 PoE module at the moment because the output isn't quite high enough to reliably drive the relays: 9V would do OK, but 7.5V isn't enough. Right now both my home automation switchboards have EtherTens in them being driven using home-brew PoE with our new switchmode 28V voltage regulator modules (www.freetronics.com/pr28v) and 4-channel injectors (www.freetronics.com/poe-injector-4ch) because I can send a solid 12V down the wire, tap it off the input to the PR28V, and send it through the relay coils that are low-side switched by transistors on the EtherTen.

My ideal would be a PoE module with both 12V and 7.5V outputs, one for feeding to the Arduino and the other for miscellaneous other loads.
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Ohmware
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Re: Access to PoE power w/ 802.3af adaptor board?

Post by Ohmware » Sat Dec 10, 2011 2:08 pm

It was some time ago that I looked over the PoE specs, but IIRC part of what you can do with that complex signaling setup is negotiate how much power you want. Is that true? What exactly does the 802.3af module negotiate with the switch? Is there a schematic and BOM for the 802.3af module online somewhere?

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Re: Access to PoE power w/ 802.3af adaptor board?

Post by LukeW » Mon Dec 12, 2011 3:17 am

The schematic is available from the Freetronics GitHub repository, here:

https://github.com/freetronics/PoERegul ... 8023af.sch

Unfortunately there are no PDFs or similar, only the Eagle files, so you'll need to download those and have Eagle installed.

The Freetronics 802.3af board is based on a Silver Telecom Ag8100 802.3af module, and there's really nothing else on the Freetronics board except for that module, a capacitor, a power LED, the configuration wiring to set the voltage output to 7.5 V by connecting ADJ to +VCC, and the headers.

http://www.semiconductorstore.com/pdf/n ... Ag8100.pdf

You might be able to "hack" the output voltage by modifying the voltage regulator feedback on that module, by cutting the PCB track connected to ADJ.

To be honest I was a little bit surprised with the current iteration of the design of the Freetronics 802.3af interface board, because the use of the proprietary off-the-shelf pre-assembled board isn't very open, and it would be kind of nice if a future revision was re-implementing the Ag8100 module in a more open way, using (for example) an NCP1090 and a high-voltage-input buck converter.

But remember not to take that voltage too high, if you do you'll break compatibility with the Freetronics ethernet boards because the voltage will be too high for the 5V regulator, which is OK, as long as you don't plug it directly into a freetronics ethernet board.

An 802.3af module like the Silver module essentially consists of two components. Firstly, you've got a chip which communicates and negotiates with the 802.3af power injector, tells it how much power it wants, etc.

The NCP1090 is an example of a chip that does this. http://www.onsemi.com/pub_link/Collateral/NCP1090-D.PDF

(Note: This is just an example, this chip has got nothing to do with the module used by Freetronics).

Secondly, you've got a voltage regulator (a switchmode buck converter) which can accommodate a relatively high input voltage, up to the 48V or so which you may have.

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Re: Access to PoE power w/ 802.3af adaptor board?

Post by jonoxer » Mon Dec 12, 2011 10:16 pm

The Silvertel module has some configuration options that I set on the carrier board, including the power rating requested from the PSE (power sourcing equipment) and the output voltage of the module.

I configured the power rating to be the maximum possible under the 802.3af spec, which is (from memory) 12.4W.

The output voltage is configured to be 7.5V, which as LukeW noted was chosen to not overheat the voltage regulator on the host. It's high enough to be comfortably over the minimum input voltage required for a 7805, but low enough to give the 7805 very little power to dissipate so it stays cool.

Regarding the module, yes, ideally I'd like to design a new one. That would probably be a week or two of work, though, for essentially no gain. I decided the pragmatic thing to do was treat the Silvertel unit as if it's an IC: it's a monolithic module with a defined pinout and specific functionality, even if it's not in a single block of plastic. I don't bother reimplementing ICs using discrete transistors in order to be more "open" with the design, so I did the same with this.

I agree, it would be good to design a new one. Right now I'm sleep deprived enough though!
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Ohmware
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Re: Access to PoE power w/ 802.3af adaptor board?

Post by Ohmware » Wed Dec 14, 2011 9:56 pm

So, could I theoretically put my own 12V regulator circuit in the prototyping area left on the shield after the PoE daughter board is installed? Tapping the power from the PoE power inputs directly? Or would this be ill-advised for some reason? (Like heat maybe…)

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jonoxer
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Re: Access to PoE power w/ 802.3af adaptor board?

Post by jonoxer » Wed Dec 14, 2011 10:38 pm

Yes, you could add your own voltage regulator, but just be careful about the input voltage. Commercial PoE power sourcing equipment delivers a nominal 48V (can be 56V or more) so your regulator will have to be able to handle that. A linear regulator would need to dissipate a *lot* of heat, which is why power supplies used for PoE are typically switchmode supplies instead.
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Jon

Ohmware
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Re: Access to PoE power w/ 802.3af adaptor board?

Post by Ohmware » Sun Feb 12, 2012 8:52 pm

I have recently circled back 'round to this project, and so I downloaded both the schematic for the adaptor board and the Eagle software to look at it. I'm confused, though, because when I look at the board in Eagle it looks as though the GND pin isn't connected... Is there a ground plane that doesn't show up in Eagle? (Obviously I'm new to the software...)

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jonoxer
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Re: Access to PoE power w/ 802.3af adaptor board?

Post by jonoxer » Mon Feb 13, 2012 12:15 am

There's a ground plane on both sides of the PCB, and until it's "poured" in Eagle you'll see airwires for anything that relies on it for a ground connection. There's a "ratsnest" button near the bottom left of the Eagle toolbar in PCB view, so click that button and you'll be all sorted.

Unfortunately there's no easy way to "unpour" the polygon to hide it again, so in the command line at the top of the window you have to type:

Code: Select all

ripup @;
That rips up the polygon pours while leaving all your other nets in place.

Yes, obscure. Welcome to Eagle.
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Jon

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