PoE Voltage

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.
spyclub
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PoE Voltage

Post by spyclub » Thu Jul 18, 2013 8:04 am

I have been looking at starting a new Ethernet project using PoE for a monitoring environmental conditions around my house (similar set-up to http://superhouse.tv/). My concern at the moment it heating caused by the electronics itself. Having a quick look at the Ethernet shield with the PoE module, I don't understand why you are using a switch mode to take the 48V from the PoE to 7.5V when you can get modules to take it down all the way to 5V. The immediate disadvantage I see is that the PoE will pass through the Arduino's linear regulator and hence kick out a fair amount of heat.
What is the advantage of the higher voltage? Surely the regulated 5V output from a PoE module would be more efficient and hence less heating?

angusgr
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Re: PoE Voltage

Post by angusgr » Fri Jul 19, 2013 12:54 am

spyclub wrote:I have been looking at starting a new Ethernet project using PoE for a monitoring environmental conditions around my house (similar set-up to http://superhouse.tv/). My concern at the moment it heating caused by the electronics itself. Having a quick look at the Ethernet shield with the PoE module, I don't understand why you are using a switch mode to take the 48V from the PoE to 7.5V when you can get modules to take it down all the way to 5V. The immediate disadvantage I see is that the PoE will pass through the Arduino's linear regulator and hence kick out a fair amount of heat.
What is the advantage of the higher voltage? Surely the regulated 5V output from a PoE module would be more efficient and hence less heating?
Hi Spyclub,

You're entirely correct. The 7.5V choice for the 802.2af module output is a design tradeoff. The PoE input on the shield drives the Vin pin for the Arduino, meaning that it has to supply at least 7V to overcome the regulator dropout from the linear regulator.

You're right that the module could produce 5V and drive the 5V pin directly instead of the VIn pin. However a shield with this design would not be suitable for some other applications, such as jumpering the inputs directly and providing "cheap DIY PoE" in place of 802.3af, by feeding (say) 12V through the ethernet cable. This is actually a popular choice, especially for people with shorter cable runs and low power requirements.

The other benefit to providing 7.5V onboard is that if you have a device that requires a slightly higher voltage then you're able to drive that as well by sourcing from the VIn pins on the Arduino.

The downside is obviously that, as you say, the linear regulator is only 66% efficient here compared to the switchmode which is probably 90% efficient or more. If you're concerned about the specific heat output then I recommend you calculate how much 5V current your application needs (about 220mA should power the Ethernet Shield and Arduino), and decide whether the additional heat produced is enough to be a problem.

Cheers,

Angus

spyclub
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Re: PoE Voltage

Post by spyclub » Fri Jul 19, 2013 12:52 pm

Thank you for your reply, I thought I was going a bit mad when I has a look at some other companies and they also used a greater than 5V PoE module.

My aim was to place these units either inside air tight boxes, and some potentially even inside of a wall (or roof insulation) so I really don't want it to heat up at all (I know that is not realistic). In your experience, how hot would something like the etherten with the PoE module, or and ethernet shield with a standard arduino get when left in a sealed box? Assuming worst case, a linear regulator is only 66% efficient, so will I actually need more like 330mA into the system with 110mA (0.55W) lost to inefficiencies/heat? and if that is the case, how does that 0.55W loss translate to actual increase in temperature?

cacycleworks
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Re: PoE Voltage

Post by cacycleworks » Mon Jul 22, 2013 8:17 am

A company I worked for (making HDTV broadcast equipment circa 1999) had a mechanical engineer on staff for the sole purpose of thermal dynamic calculations.

IMHO, the easiest way to answer your question is to set up a resistive load to simulate the output in a sealed box and observe the temperature changes. It seriously wouldn't be mroe than a day's work to accurately answer your question. 0.55W can be achieved with a 5W or less resistor and the correct environment around it. IMHO, if you're engineer enough to ask that question, you can engineer a practical answer to same...

A little wood, a little drywall, and the same box you intend to seal your circuit into. I suggest testing with a measured 1W heat output. Very easy to set up a voltage divider with a 1W resistance in the middle of it. You can measure its resistance expertly and also measure its current...

I'm replying because personally, I'd like to know the result of said experiment. But that's not my issue and work is busy enough that this is about all the time I can devote ...

:D
Chris

spyclub
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Re: PoE Voltage

Post by spyclub » Mon Jul 22, 2013 11:18 am

I like the idea, resistors are certainly cheaper than ordering the PoE equipment and then realising it all runs to hot and needs redesigning. Will let you know when I have got round to actually doing the tests.

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Re: PoE Voltage

Post by angusgr » Tue Jul 23, 2013 10:37 pm

Hi spyclub,

Sorry it's taken me a few days to get back to you.
spyclub wrote:In your experience, how hot would something like the etherten with the PoE module, or and ethernet shield with a standard arduino get when left in a sealed box?
This really depends on a lot of things - what the box is made of, how large the box is, how thick the walls are, what the ambient temperature and airflow outside the box are, etc, etc. As Chris has pointed, thermal modelling is a full time professional job and the only way to be sure is to simulate and measure.
spyclub wrote:Assuming worst case, a linear regulator is only 66% efficient, so will I actually need more like 330mA into the system with 110mA (0.55W) lost to inefficiencies/heat? and if that is the case, how does that 0.55W loss translate to actual increase in temperature?
The efficiency of the linear regulator is because the current in and current out are the same, but the voltage drops. So if you need 220mA @ 5V (1.1W) then you're putting 220mA @ 7.5V (1.65W) in. But your overall figure of 0.55W extra heat loss is correct, though.

(Because the heat wasted is proportional to the voltage drop, this gets worse as the voltage drop goes up. If the linear regular was capable of accepting 48V directly - which it is not so don't try this - then you'd be putting in 220mA @ 48V (10.5W) and the regulator would only be 9% efficient!)

The PoE regulator is probably around 90% efficient at taking in 48V and outputting 7.5V, you can make a worst case estimate that your total power dissipation is 1.65W / 90% = 1.83W. Call it 2W to be sure. :)

(Remember to add on extra power requirements if you plan to run anything else from the Arduino.)

Making a test rig like Chris suggests is a great way to estimate if it will work or not. However after you've done that I still suggest bench testing a final prototype, because that way you can account for things like differences in where the heat is produced, how it dissipates inside the box, etc.

Remember that if it's sitting in your roofspace the ambient temperature may get very high, so it's worth testing in a similarly hot ambient environment if you can.

The chip on the 802.11 regulator has a thermal shutdown feature so you should in theory see the entire unit switch off if it gets too warm. This is not something you want to rely on, though.

A couple of other suggestions that might help:
- Try mounting in an aluminium box if you can get one (if you're in Australia then Jaycar sell diecast aluminium boxes), to conduct heat more effectively to the outside.
- Build some temperature monitoring into your design. Depending on the exact model microcontroller you're using, some Arduino/compatibles have an internal temperature sensor already. It's not super accurate but would work for an "am I overheating?" monitor you can keep an eye on in summer.

- Angus

dukman83
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Re: PoE Voltage

Post by dukman83 » Wed Aug 19, 2015 11:22 am

Hello fellow enthusiasts and professionals...

I thought I'd piggyback onto this topic regarding my issue, since it is related to the voltage and heating of the Arduino using PoE.

The issue I’m having is that with the 803.2at regulator, my EtherTen runs very hot - too hot to touch for more than 1-2 seconds (I know that in one of Jon Oxer's videos he said if it’s too hot to touch for 3 or more seconds that it’s not good).
I am pretty concerned that it could be doing some damage to the board, particularly since my NetGear GS728TPP switch indicates that it’s supplying 3A to it, which is not something I can change.

I was hoping to get some advice on this, as I don’t think that I can deploy the unit if it’s going to be running this hot, and potentially risking permanent damage or potentially causing a fire inside my wall cavity.

Apologies if I haven't provided enough information, if there's anything I may have omitted, I'd be happy to advise.

I really appreciate all/any advice and help with this.

Kind thanks,

Alex

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Re: PoE Voltage

Post by angusgr » Fri Aug 21, 2015 6:37 am

Hi Alex,

I think you probably have a faulty 802.3af regulator if it's running that hot and using that much current.

Which components are the ones which are getting hottest? Are you able to measure the voltage between the VIn and GND pins on the EtherTen?

If you purchased the components recently and you think they are faulty then email us and we'll arrange an exchange.

Angus

dukman83
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Re: PoE Voltage

Post by dukman83 » Sat Aug 22, 2015 10:55 pm

Hi Angus,

Thanks for getting back to me and sorry for the delayed response.

The voltage measured between Vin and Gnd is 9.96V and the hottest area is noticed on the underside of the EtherTen board, directly below where the regulator sits - the regulator itself doesn't seem to get as hot tho - it gets quite warm, but no way near as hot as the underside of the mainboard.

Sincerely,

Alex

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Re: PoE Voltage

Post by angusgr » Mon Aug 24, 2015 7:21 am

Hi Alex,

The nominal output voltage from the 802.3af is 10V, so 9.96V is within the range we'd expect.

The linear regulator on the board talks the 10V and regulates it down to 5V, and is probably the hottest component on the board. It's the large flat component located under the 802.3af PoE regulator, near the IOREF/RST/3V3 pins.

Even though it gets hot it still shouldn't be too hot to touch for a few seconds. So it's possible your EtherTen is faulty.

Are you able to try powering it with the PoE disconnected, using a "wall wart" 12V DC adapter directly connected to the DC jack, and tell us how that compares?


Angus

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