[Asterisk-Users] Re: Power Over Ethernet for *any* ethernet switch (or hub); product idea

[Stephen R. Besch]

Doug Meredith wrote:
> "Stephen R. Besch" <sbesch at acsu.buffalo.edu> wrote:
> 
> 
>>As far as I know, it doesn't. The POE source somehow monitors the line 
>>(using impedance, etc) to determine if there is anything connected to 
>>the pairs used to supply power. Since netcards and net connected 
>>equipment are not supposed to use the "power" pairs, this should work in 
>>most cases. If the termination just leaves them unconnected the 
>>impedance is infinity and power will not be applied. Conversely, if the 
>>termination are all grounded, impedance is (near) 0 and power will not 
>>be applied.  For some range of impedances in between, the far end is 
>>assumed to require power and power will be applied.
> 
> 
> So a reasonable (cheap!) alternative would be to only plug devices
> that need PoE into it, and have it always provide power.
> 
> Doug

No doubt. It would even be ok for a lot of ethernet equipment. But I 
have seen stuff that connects all unused pins to ground. At any rate, if 
you supply power to each port using a separate, short-circuit protected, 
regulator, although off the shelf, adjustable regulators capable of 48V 
are not too common.  A better solution would be the maxim IC mentioned 
earlier on the list by Clif Jones 
(http://www.maxim-ic.com/view_press_release.cfm/release_id/925). If does 
the whole power over IP thing, including sensing remote end impedance, 
and only costs about 2 bucks. Given that it implements a switching 
regulator, a few external parts would also be needed, but in reasonably 
small quantities you could probably piece something together for about 
$5-$10.  However, design of a switching regulator, even when most of the 
work has been done already, is not for the feint of heart.

Stephen R. Besch

Stephen R. Besch, Ph.D.