[time-nuts] Embedded NTP server ideas and feature requests

Bob Camp lists at rtty.us
Sun Feb 10 13:39:08 EST 2013


If you go the DIY route to PoE, be careful of the converter bricks you decide to use. Switchers by nature put hash on both the input and output terminals. You want to avoid back feeding crud onto the cable. It's common mode, but the chokes and transformers only isolate to a limited degree. Some of the common mode will indeed get into the receiver.

You don't have to go crazy selecting un-obtainable parts. Grab a couple to look at. A check with a scope and possibly a couple bypass caps / series coil(s)  likely will do the trick on one that's not awful to start out with.  


On Feb 10, 2013, at 12:42 PM, Jim Lux <jimlux at earthlink.net> wrote:

> On 2/10/13 8:22 AM, Chris Albertson wrote:
>> On Sun, Feb 10, 2013 at 12:40 AM, David J Taylor
>> <david-taylor at blueyonder.co.uk> wrote:
>>> - although I've never used it, I do like Chris's suggestion of power over
>>> Ethernet.  I see dozens of choices in Amazon's lists - is there a standard
>>> for the power adapter and level?
>> Yes there is a standard but there are also may non-standard implementations.
>> I was a little worried about the comment regarding "isolation" being
>> hard to implement.   The standard for Ethernet required galvanic
>> isolation on all Ethernet ports by use of a transformer.  So POE
>> simply buts a DC bias on the wire.  I think it can provide 12 Watts of
>> power
> More like, it can provide 0.25 Amps.. that's 12W at the nominal 48VDC. I suspect that current is the limiting aspect.  Actually, Wikipedia says 350 mA at 44V at the load..  48V at the source end, probably.
> Yes, what PoE does is put a common mode voltage on each of the pairs, with the assumption that you will have a *galvanically isolated* DC/DC converter at the load end.
> The catalogs are full of little DC/DC bricks intended for just this sort of application.  And, the transformers that go between the PHY and the jack to provide the isolation that Ethernet already requires, but which have the center tap for the PoE.
> (and, I assume, though I haven't looked, there are probably integrated PoE power injectors which do the same)
> I think PoE is a great thing, because it allows you to get rid of all those wallwarts..
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