12 Jul
2002
12 Jul
'02
5:44 a.m.
On Fri, 12 Jul 2002, Tothwolf wrote:
On Thu, 11 Jul 2002, Eric Smith wrote:
100BT Ethernet coupling transformers should easily be able to
withstand most faults from a telephone line, since they typically have less
than .2 Ohms dc resistance. That makes it hard to dissipate much power in the
transformer unless you have much more than 1A available. 10BT transformers
with a common mode choke are around 1 ohm so your 24 vac power would probably
damage them, though normal telephone line current would not...
As far as damaging the rest of the circuitry goes that depends on
whether the PHY can take the 48V ~1 usec spikes resulting from connection the
isolation transformer input to one (or many) 48V steps as the connectors are
mated...
Peter Wallace
Strange, a 10baseT/100baseT Ethernet interface
should be able
withstand telephone voltages (even ringing), even if the jack has them
wired to the data pairs. Even if it sustained damage, there should
not have been any sparks.
Many of those commercial/digital phone systems often have 24VAC or
something similar available to power the phone on one of the pairs. Most
of them also have quite a bit of current available. I've seen a few cable
pair testers loose their magic smoke due to those types of phone systems.
-Toth