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POE 802.3af Module question

Posted: Mon Jul 23, 2012 10:06 pm
by NW27

Is the "Power-over-Ethernet Regulator 802.3af" output isolated from the POE inputs?

Some POE devices have different 0V levels depending on whether the POE output is isolated or not. This equates to a fairly large potential difference between 0V on the project and Earth.
This means that 0V on the project could be sitting at 40V and then plugging this into the USB port of the PC that has 0V very close to earth, could cause some damage.


Re: POE Module question

Posted: Tue Jul 24, 2012 8:39 am
by NW27
To answer my question, it would appear that my concerns were warranted.
Looking at the manual for the POE device, it says that "Because the Ag8100 series are non-isolated the +VDC output is (internally) connected to the VIN+ input."
So basically, the Etherten will be sitting at approximatley 40..50V above the earth and therefore the PC USB 0V line.

5.6. Grounding

If the Ag8100 is supplied from a grounded source (e.g. a standard multi-channel PSE) it is
important that the –VDC output is NOT connected to ground.

Connecting both input and output to ground will result in a short circuit that will prevent the
Ag8100 from operating correctly.

So basically, if the EtherTen was plugged into a POE 802.3af switch, there is a very good chance of you damaging a) your EtherTen and b) your PC via the USB port.

I will have a EtherTen and a POE supply in the next few days and will confirm this. If this is the case, then a large warning should be displayed where ever the POE 802.3af devices are sold.