Ethernet Shield with PoE module providing too much power for Arduino regulator?

I built a PoE powered gas sensor for rooms that have no power sockets but can easily be equipped with a Cat5 Ethernet cable. Running Arduino, Shield and sensors on USB power works: I can poll two sensors (MQ-2 and MQ-7) into cacti via HTTP.

All that does not work however if connected to an HP 2610 PoE switch unless the sensors stay disconnected from 5V/GND.

I also experimented with a 12/2A regulated power supply to the same result. Even connected to 12V/2A input and an Ethernet shield without PoE it cannot run the sensors.

Some research in this forum suggested that in case of the 12V/2A input there was too much power for the Arduino's regulator to handle. The switch puts out up to around 48V that should be regulated to safe input voltage for the Arduino by the PoE module. Apparently this is not the case.

My question is what to add or remove between Arduino and Shield to build a working PoE powered sensor?

You need to firstly comprehend the meaning of "power" and "voltage".

You cannot power (the heaters of) gas sensors from the Arduino - you need to provide a separate 5V regulator.

That PoE type is an active PoE, providing power over the data lines. You must use a passive PoE supply for the ethernet shield.

Q: What PoE devices are NOT compatible?

Not all PoE devices are necessarily IEEE 802.3af compliant. Early versions of IP Phones, though they may accept power over the Ethernet cable using their own Mid-Span Power devices, may only accept power over the Non-Data Pairs of wires (Category 5 cable, pins 4/5 and 7/8). The 802.3af specification clearly states that Power Devices MUST accept power over either Data Pairs (1/2 and 3/6) or the non-data pairs. Furthermore, a compliant device must also be 'insensitive' to polarity, meaning that it should power up to either +50VDC or -50VDC.

The 2610 series PWR switches provide PoE power over the Data Pairs with pair 1/2 being Positive DC.

Hi Paul,

you mean powered through USB the sensors don't get powered through the regulator and thereby the sensors work?


Hi SurferTim,

I actually bought a standard compliant PoE module that should well work with the power supplied by the PoE switch. I'm not planning on using an injector.

I'm just looking for a way to get that power either from the shield or from the PoE module and transform it to the 5V input power the sensors will need to operate.

Injection and passive PoE has many issues and introduces unnecessary risks for other network devices - a risk I'm not willing to take.


Just some more details on my setup:

The Board I'm using:

The Ethernet Shield I'm using:

The PoE module I'm using: (IEEE 802.3af compliant, 36V to 48V)

The sensors I'm using:

Everything nicely patched into a simple breadboard and like I said well working on USB power alone.

I think I need to get power from either shield or PoE module and get it down to 5V input power for the sensors and maybe also for the Arduino itself. I really don't know what to look for - a regulator, resistors - if there was a $ 5-15 module to add that would be perfect.

I'm looking at the LM2576HV Step-Down Voltage Regulator Module (LM2576HV Step-Down Voltage Regulator Module - Google Search) but wouldn't know where to put it. My best guess would be to attach its input to the Arduino's/Shield's Vin pin as I suspect the Ethernet Shield to feed the power from the PoE module into the Vin pin. From there I reckon I should output 5V to at least the sensors. Any ideas on how to improve this very simple image in my mind?

Thanks for your interest as well as your answers!


I was able to put a voltage regulator set to step-down from 7.5V to 5V between Vin/GND and VCC/GND of the Sensors. Cheers!