POE bump-in-the-wire conversion

Fri Dec 31 19:08:04 UTC 2010

On Fri, Dec 31, 2010 at 10:49 AM, Robert E. Seastrom <rs at seastrom.com> wrote:
>
> I was aware of this device (being a big Ubiquiti fan), but have yet to
> find anyone who has direct experience with using them on a 3524-PWR.
>
> Have you actually tried this (on a 3524-PWR, not a 3550 or anything
> later-but-pre-standard)?  The equipment will be quite happy with
> 16v...

I've actually used them in other applications.  They're a standard
802.3af device, and they just step-down to 16V @ 0.8A (max) though
they seemed to get a bit warm at 0.8A but worked fine, haven't had one
die yet.  To the switch they are a 100% 802.3af device so may not work
with the 3524-PWR.  I've not tried any 802.3af devices with the
3524-PWR, I have gone the other way (802.3af injector/switch with
pre-standard devices that accepted 48V) -- You might be better off
upgrading to an 802.3af switch or using a seperate 802.3af power
injector device/devices, enterasys for example makes a 20 port
injector (last I checked) among others.  Most almost all 802.3af units
will also do a Cisco compatible 'pre standard' mode for the older 7900
series phones that aren't 802.3af.  Pre standard cisco POE is limited
to about 10W, as IIRC, it uses only one pair (pins 1,2) for DC power,
the device has a low pass filter to get rid of the DC component for
the ethernet receiver hardware.  802.3af doesn't define which
wires/pins to use but generally will use the unused pairs, 4,5 and 7,8
for DC+ and DC-, unless it's gig-e, then it uses 1,2 and 3,6 (again
this is just my experience with some Netgear and HP gear and doesn't
necessarily represent anything else).

The use of pins 1,2 for power is possibly also why you don't see
pre-standard to 802.3af because there's far less available power, AND,
you'd have to build a low pass filter and possibly regenerate the
Ethernet signal to make it work too.  Combine that with cheap 802.3af
injectors (either rack/multiport units, or single units) there's not a
lot of incentive for hardware manufacturers to build such devices
either.

>
> -r
>
> Philip Dorr <tagno25 at gmail.com> writes:
>
>> The Ubuquti Instant 802.3af seems to do what you want (as long as the
>> equipment can handle 16v)
>>
>> http://ubnt.com/8023af
>> http://ubnt.com/downloads/instant8023af.pdf
>>
>> On Fri, Dec 31, 2010 at 9:00 AM, Robert E. Seastrom <rs at seastrom.com> wrote:
>>>
>>> Perhaps someone from this august list can offer a clue here.
>>>
>>> Have:  Cisco 3524-PWR  (paleo-POE, pre-802.3af Cisco standard).
>>>
>>> It runs the 7960Gs great.
>>>
>>> Have:  Wireless AP stuff that wants 12v on the unused pairs for
>>> passive POE.  48v will let the magic smoke out.
>>>
>>> Might buy:  phone that does 802.3af
>>>
>>> Want to run these with the 3524-PWR.
>>>
>>> I can't imagine that nobody makes a bump-in-the-wire converter for
>>> this application, but haven't been able to find anything other than
>>> 802.3af to the passive POE use case.
>>>
>>> Anyone got a pointer for me?
>>>
>>> Thanks,
>>>
>>> -r
>>>
>>>
>>>
>
>