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RE: Re: Power over ethernet and ePODs
- To: <ukha_d@xxxxxxx>
- Subject: RE: Re: Power over ethernet and ePODs
- From: "Kevin Hawkins" <lists@xxxxxxx>
- Date: Mon, 29 Jul 2002 12:42:12 +0100
- Mailing-list: list ukha_d@xxxxxxx; contact
ukha_d-owner@xxxxxxx
- Reply-to: ukha_d@xxxxxxx
Hi Graham
- it's probably
not
a very good idea to try and anticipate a voltage drop down a cable to
reverse
engineer the correct voltage at the other end. A few reasons spring to
mind...
1) The ePods and other devices
take
a varying current and it is the current when passing through some
resistance
that causes a voltage drop (yep ohms law) and so the voltage drop down the
CAT5
will vary with what the ePod is doing, the backlight contributing
significantly.When the unit was in power save mode the voltage may
rise
causing risk.
2) Possibly the biggest
resistance
in the line would be caused by any connectors that were in circuit but
their
actual resistance may be a little variable depending on physical issues. (I
haven't looked at the POE design to see if there are any connectors in
line.)
Where there is resistance, heat is generated.
3) Not all runs to epods would
have
the same voltage drop so you would need different source voltages
anyway.
The ePod appears to require 10V
DC
(an unusual voltage) at up to 2A. Presumably the 2A would be with the
machine running full tilt, all slots occupied and charging the battery as
well.
In typical use this may be much less and you might not have a problem with
the
CAT5 anyway. What would be interesting to know is if there is further on
board
regulation in the ePod in which case it may well compensate for any small
voltage losses anyway. Not sure how well regulated the ePod psu's
are.
The technical solution would be
to
have a spare wire coming back from the ePod end that carried no current and
was
used to sense the voltage at the ePod end and automatically raise or lower
the
source supply to compensate but again this facility is only in the more
expensive PSU's and would not work for multiple loads. e.g. several ePods.
I would be tempted to try it
and
see, feeding the existing supply through a longish length of Cat5 and
seeing if the ePod powered correctly, it is unlikely to do any real harm to
the
ePod (I guess there could be some data corruption potential).What sort of
length
of run of Cat5 are we talking about ?? Avoid connectors if
poss.
The cable will have a spec
somewhere
on resistance per metre although there may not be a guideline to
current
carrying potential as it is intended for data signals only. I know people
use
them for speakers and 2A into 8 ohms would be 32Watts or 16W into 4 ohm
speakers. 1A would be only 8 watts. The voltage drop would sort of be
ignored
though just producing a less accurate sound. All I am saying is that 32W of
power is more than people are using them for so this is pushing it in
comparison. How many conductors are available for the power ? Pairing them
if
poss could help - I haven't looked at the article.
I am sure Keith would have a
view on
a current handling capability. 2A is quite hefty
though.
Kevin
-----Original
Message----- From: graham_howe
[mailto:graham@xxxxxxx]
So
the only remaining question is about the power supply itself. Obviously
I
don't really want to have 5 power supplies at node zero for the 5 ePODs
and anyway I doubt very much that the ones supplied with the ePODs
would
be sufficient over a 50m run. So what I really need is a big power
supply
that can support 5 ePODs concurrently (hence bigger current draw) and
can
supply adequate voltage over 50m of cable (hence higher voltage to
allow
for drop).
Does anyone have an idea where I might get a suitable PSU
and also could anyone with the elecronics knowledge tell me what spec I
should be looking for (I assume it all comes down to Ohms Law, but I
wouldn't trust myself to do the
calculations).
Regards
Graham
For
more information: http://www.automatedhome.co.uk
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