802.3af Fun



So the story goes like this, my roommate needed to tap some
IP phones for diagnostic purposes. I found a netgear HUB that would
serve that purpose. This was not ideal as the IP phone needed
power from the network, or a 48V wall-wart. He asked if I could get
PoE to pass through the hub. Easy, I'll just loop the unused pair and
viola! Wrong.

What really happened is I had to do a lot of reading, and found a
few key things. According to 802.3af standards, the PSE or
Power Supply Equipment, decides how power is going to be
sent, while the PD or Powered Device has to accept BOTH modes.

This might be a good time to explain that 802.3af can send power one
of two ways. Using phantom power through pins 1,2,3 and 6 (mode A)
or through the unused pairs, pins 4,5,7 and 8 (mode B). In all my
testing, Cisco favors mode A power. Probably something about
gigglebit standards?

So, in order to pass power through to an IP phone, I had to loop
mode A power through, so while I had 48V passing through the hub
I thought it would be a good idea to power the HUB from PoE too.

So here I am. I am going to show how I built a PoE powered switch
using a netgear GS105v3. I am not looping power through on this build
but if I was, It would be REALLY easy on this model. Also, this build
will NOT be 802.3af compliant. I would need one more bridge rectifier
and one more 25Kohm resistor added to the 'unused pairs' for the device
to be compliant.

I chose netgear for the metal case, SMALL form factor, and usually
the netgear product line is 5VDC friendly. Also, I chose the GS105v3
over the GS105v1 for a few reasons. It has been my experience that
later versions have smaller more refined components, and probably
more room in the case for extra stuff. Second, the packet transfer
times on the v3 were WAY improved over the v1.

Down to business.


Above is the newly purchased GS105v3. I was happy to see that the
ethernet magnetics were exposed and NOT integrated into the RJ45
jacks.

Below you can see the under side of the logic board and again
I was happy to find that the magnetics were through-hole solder
types. This makes soldering SO MUCH easier. Also, I found there were
12 pins per port magnetics. 8 for data and 4 for center taps, which
is crucial for PoE ;)





Above you will see that I removed R47 and R48. These were 75ohm
resistors connecting the RJ45 side center taps to ground.
That probably serves some purpose, but as far as I was concerned
they were causing a "short" for my PoE purposes.

Below you will see I have connected 4 wires to the under side of
the board. Orange to the center tap for pins 1 and 2. Green to
the center tap for pins 3 and 6. White-Orange to DC + and
white-green to DC -.





Above is the extra stuff I will add to power the switch. We have
a bridge rectifier, solid state relay, general purpose PNP
transistor, 25.2Kohm resistor, and a 10W 48VDC-5VDC power supply.

Below is the top of the board with the power supply mounted with
double stick tape. It was just enough room to snug the supply against
the magnetics and the heat sink for the main processor.





Above is all the components mounted to the DC-DC power supply. One
thing not pictured is the lead from the 5VDC ground to the ground
leg of the relay. Aside from that, the 25Kohm resistor is placed
between the two "AC" legs of the rectifier, this tells the switch that
an 802.3af device is waiting for power. next the negative side of
the rectifier is soldered directly to the 48V ground input on the PS.
A rectifier is required because 802.3af states that power can work
with a straight through cable OR a crossover cable. Polarity doesn't matter.
The +48VDC side is a bit more complicated. The +48V comes and connects
to the common leg of the relay. The PNP transistor's emitter and
collector are attached to the relay's common and normally open leg.
The purpose is to keep the load off of the PoE until a full 48 volts
is supplied by the switch and saturates the transistor. The relay then
takes the load off of the transistor once 5V is created by the power
supply. The normally open is then connected to the +48V input on
the power supply.

Below is the switch connected to a cisco 3560, receiving PoE.




Also, the switch comes with a 12VDC 1A power supply, yet the logic
voltage is 3.3VDC. I did a quick test before to see if the voltage regulator
was linear or smart, good thing it turned out to be smart. 5VDC input
still provided 3.3V to the logic. So based on this, I now have a PoE or
USB powered gigglebit switch. You could attach a few diodes to the DC-DC
power supply so you could still use the 12VDC wall-wart, but, I don't plan on it.
This switch will stay in my laptop backpack and never see a wall-wart ever again.

Now you might be asking, how a switch that wants 12W of power can use a
10W DC-DC power supply or a 2.5W USB power, and I have an answer for
this. Netgear claims that the MAX power consumption by the device is 3.5W
or 700mA @ 5VDC. This is the MAX draw of the switch using ALL PORTS at
FULL TRANSFER. Nominal usage is not going to draw that much current, so
we should be safe using USB power. You could always use a double USB to
power adapter and get a full 5 watts. Also, this means the 10W supply I
used is more than enough for the switch draw. I could have used a 5W DC-DC
converter and would have been much smaller.

The DC-DC power supply, solid state relay, etc., were sourced an an
electronics surplus store. You could find a DC-DC power supply from
DigiKey for about $60. Lucky I found mine for $15. It was also luck that this
switch had center taps on it's port magnetics. You never get to see the
inside of things like this unless you look for yourself.