RE: current limit on 48 VDC
I have opinions on a couple of things that have washed by this morning.
1. Bob, as does Rick, I disagree with your assertion that 350ma was not our
intention as a worst case number. Most of the discussion that I recall has
said 350 ma max. I read that as 350 ma +0% -?% with the minus tolerance yet
to be determined. I do think that our 48 volts is not 48 volts worst case
either way. We haven't settled on a number for that yet but I have bee
using +/- 8% in the meantime.
2. In the discussion about using a PTC for current limiting it should be
obvious but is worth pointing out that the secondary safety device comes
into play only when we have a downstream failure. Since the device is
broken the interest at that point is all safety. We can reasonably use a
non-recovering device such as a fuse since the beast will have to go out
for repair or replacement anyway.
3. We haven't even started talking about what it means if we say "350 ma
max". What that means in terms of obligations on the source to provide a
current limit at that level and what the time constants associated with it
are. One thing is clear to me is that the sink must be able to come up with
a current limited inrush where there actually is a hard current limit of
350 -?% ma. We did some work on this when we revisited the spec for the 10
Mb/s AUI (Ref: 126.96.36.199 see the cautionary text in the 2nd half of the
paragraph, also 10.4.2.2)