Hi
All,
2A @
10V = 20W
The
"proper" PowerOverEthernet (POE) specs can supply somewhere around that
amount
of power .....
BUT......
the
way it works is to use a higher voltage somewhere between 40 - 55volts and
then use a switchmode power supply at the distant end to providfe the
correct
regulated voltage. The benefits of this are twofold
1.
The current for a given power is less at a higher
voltage
2.
Lower current means lower volt drop and with the higher starting voltage
the
percentage of power lost is much less.
There are TWO serious flaws with your idea.
1.
You need 2 AMPS - CAT5 is only rated somewhere between 250-350 mA.
Admitedly
you can use 2 wires to double the current rating but you have to consider
what
happens if one wire breaks or a plug has a bad connection, the other wire
is
then overloaded.
2.
Not only do you get voltage drop - but if the power supply is grounded on
its
0V rail you also get ground lift at the distant end.
Assuming a 12V power supply and 2V drop the distant end will receive
10V but this will be across the pair.
The
true voltages with respect to the power supply ground will
be
Epod
0V will actually be +1V
Epod
10V will actually be +11V
Thats fine as long as no other connections exist
to
the Epod. But as you are using ethernet there will be a 1V offset sitting
on
the RS422 connection to the hub. This probably wont have any harmful
effects.
If
you then connect something else to the Epod that has a grounded connection
eg
an Amplifier etc, this will cause the Epod 0V rail (currently at +1V) to be
dragged down to ground possibly giving a nice hum to the sound. Because the
voltage drop occurs due to the resistance of the cable and you have
effectively reduced the resistance by providing an alternative path back to
the PSU there will be less voltage drop. This means there will be more
voltage
delivered to the Epod.
Dont
forget the safety aspect as well, to power 5 Epods you need a power supply
capable of 10A. Connecting a cable with a 250mA rating to a supply capable
of
40X that is ill advised without careful fusing arrangements. It is
effecctively the same as connecting an electric kettle directly to the
incoming mains feed with lighting flex!
Keith
www.diyha.co.uk
www.kat5.tv
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
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