I don't think there will be any problems with DC over the contacts. You
should have a good look at
http://grouper.ieee.org/groups/802/3/af/index.html
Then you will see that not only you plan to power RS485 nodes (and I
plan to power CAN Nodes) over CAT5 cable:
The highly official IEEE commitee is developing a standard to power
network nodes on the ethernet (!) through CAT5 cable. Of course fully
downward compatible... And they use DC to do it...

Florian

Martin Peach wrote:
>
> I thought of doing the same kind of thing. There are plenty of extra wires
> in Cat5 cable. The only issue I could think of is possible corrosion of
> dissimilar metal contacts using DC, so I was leaning to towards low-voltage
> AC. The RJ-45 jacks are usually rated in amperes so there would be no
> problem with current-carrying capacity. Also daisy-chaining means that a
> failure anywhere brings down everything beyond the fault.
>
> /\/\/\/*=Martin
>
> ----- Original Message -----
> From: "Kenneth Lumia" <klumia@ADELPHIA.NET>
> To: <PICLIST@MITVMA.MIT.EDU>
> Sent: Monday, December 17, 2001 4:21 PM
> Subject: [EE]: How can I power a small RS485 network?
>
> > Hello,
> >
> > I need the lists knowledge of RS485 to answer a hopefully simple
> question -
> > can I use extra pairs in a Cat5 cable to power several RS485 nodes? More
> > details below (long).
> >
> > I'm creating a small RS485 network in my home to monitor temperatures,
> > humidity, water leaks and a few security sensors (using PICs of course).
> > The nodes are relatively simple, however I am somewhat at a loss on
> > how to power them.  The simplest solution is of course a wall wart on
> > each node, however I want the nodes to look "clean", that is no wires
> > hanging outside the walls, very similar to an air conditioning thermostat
> > housing.  I plan to run a daisy-chain of Cat5 cable to each node.
> > Now the question.  Has anyone ever run power to RS485 nodes on
> > unused pairs of Cat5 cable?  Is this a no-no? The power requirements are
> > relatively low, < 100mA each node during transmission, maybe
> > 10 - 20 mA at other times, x 6 to 8 nodes.
> >
> > I'm thinking a small supply, either AC or preferably DC (to reduce parts
> > count on each node) at the start of the cable is a possible solution,
> i.e.,
> > a DC supply of sufficient voltage to cover power losses, L di/dt, etc.,
> > with a LDO regulator at each node.  Is this poor design Vcm wise, etc?
> >
> > I've seen many RS485 designs use 100 ohm resistors in series with the
> ground
> > connection to limit common mode current.  Are they really needed on a
> > "small" installation (300 feet, internal to one building).  If needed,
> they
> > sort of mess up the idea of using DC.
> >
> > Any thoughts, suggestions?
> >
> > Thanks in advance.
> >
> > Ken
> >
> > --
> > http://www.piclist.com hint: To leave the PICList
> > mailto:piclist-unsubscribe-request@mitvma.mit.edu
> >
> >
>
> --
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