I am working on a project that needs to drive a relay board. I'm looking at the ETHERNET board with PoE and according to the documentation here (http://arduino.cc/en/Main/ArduinoBoardEthernet) the output of the PoE module is 9V.

Is that the voltage to expect out of the Vin pin?

The Relay board that I want to use (http://numato.com/downloads/dl/file/id/29/product/47/user_manual.pdf) specifies that it wants a 7-12V power supply.

I want to confirm that this is the case so I can use the voltage out of the PoE and I don't need to add a separate DC power supply to this system.


I wouldn't expect you to take any power from V-in-, but it actually seems like you can!

If you take a look at the schematic for the Rev.3 Arduino Ethernet, you can see the power rails from Ethernet is connected to Vin. (V1+, V1-, V2+, V2- --> Vin in the leftmost part of the schematic)

  • Thanks! I actually just received the boards and measured it. I got 10V on Vin and the Numato board looks like a shield that shares Vin through the bottom pins. However with the POE module the shield doesn't exactly fit, so I'm just running the wires from Vin to the Numato Vin pin.
    – theproxy
    Feb 3 '15 at 2:32

Your question is a little confusing, so let me give you a couple pieces of information.

First things first - the VIn pin on the Arduino is strictly for power going to the Arduino board, you shouldn't be drawing power from this pin. The documentation in the link you sent is a little misleading on this - it's basically saying 9v is coming OUT of the PoE module, and going IN to the VIn on the Arduino itself. If you wanted to tap into this 9v power, you should connect to the PoE module VOut instead.

PoE by default can deliver 37-57 volts or 42-57 volts, depending on what type of PoE you're using (which 802.3 specification). Wikipedia has a good overview of this information.

That being said, the Arduino Ethernet board you're using can't handle this voltage by itself - unless these boards have changed, they require an additional PoE module, which is basically just a DC-to-DC buck converter that will convert the high PoE voltage down to 9v or 12v. At this point, the Arduino's built-in converter can consume that 9 or 12v and distribute it at 5v across the board.

Now, to your problem - you want to power a relay that requires 7-12v. You could theoretically connect the relay to the VOut of the PoE module, giving you 12v. However, if the relay draws too much power for too long, you will either get brownouts (and your Arduino will restart) or potentially damage the Arduino board. Unless the relay draws a lot of current or is being powered for extended periods, you should be fine. If you want to be more thorough, you could put a large capacitor between your PoE module and the relay.

You should NOT try to connect the relay directly to the PoE VOut, as the voltage will be far too high.

  • 1
    I think the PoE is not following 802.* standards. "the output of the PoE module is 9V." Feb 3 '15 at 9:38

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