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Joined: Nov 2007
Posts: 20
M
MMK Offline OP
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Related to "Cat 5 current carrying capacity"
We want to use ethernet as the means of communication to small peripheral control cards. We need however to supply power (24VDC) to each card, at 30W approx per card.
Has anyone done this? Is it wise to use the unused pairs in the CAT5 cable for the power supply? POE does not seem like a suitable solution, complexity, wrong voltage.
Suggestions please,
Michael.

Joined: Jul 2004
Posts: 9,669
Likes: 6
G
Member
The big problem will be the voltage drop. At 100' you will be dropping about 7.5 volts at 1.25a


Greg Fretwell
Joined: Jun 2007
Posts: 301
J
Member
Get some signal boosters. Like the ones for long TV runs.

Joined: Jul 2004
Posts: 625
S
Member
To avoid possible problems with people plugging things in that aren't supposed to be connected to it (and that will happen), I would do it as "PoE on steroids" using the 48V POE standard, beefed up to supply the current you need.

You of course need to make sure that your power injector limits current to a safe value.

Edit: I observe that the TIA specifies that the max safe current for Cat 5 24 AWG conductors is 360 mils. There's no way you can get your 30 watts at 24 volts. Even at 48 volts, using both halves of a pair for each side (as does PoE), you'll be right up at that limit.

Last edited by SolarPowered; 11/05/07 12:37 PM.
Joined: Jul 2004
Posts: 625
S
Member
Dang it, the board won't let me edit my previous post again... frown

There is a reason for the "complexity" of PoE. The people who designed PoE were smart people, looking for a simple solution--they did not build in gratuitous complexity just to make things difficult for people. PoE has the complexity it has because people will inevitably plug any RJ-45 plug into any available RJ-45 jack, and if you don't design for that situation, you will end up destroying things, and quite possibly starting fires.

Joined: Jul 2004
Posts: 9,669
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G
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I wouldn't even bet you could shove 1.25 a through a regular RJ connector.
IMHO these were designed to hit a price point ... for telephone connections. It is far from my favorite connector. I am surprised we don't have more trouble with them, as if it isn't enough.


Greg Fretwell
Joined: Jul 2004
Posts: 625
S
Member
Originally Posted by gfretwell
IMHO these were designed to hit a price point ... for telephone connections. It is far from my favorite connector. I am surprised we don't have more trouble with them, as if it isn't enough.


They undoubtedly were designed to meet a price point--at pre-breakup AT&T. As such, long-term maintenance costs were part of the "price point."

RJ connectors were designed to work, and to keep on working, decade after decade, without an expensive visit from a truck with a "Bell" logo on the side.

Joined: Mar 2005
Posts: 1,213
S
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Would this do it?
[Linked Image from bordergatewayprotocol.net]

Joined: Jul 2004
Posts: 625
S
Member
Bad!

BAD!

Someone please take away his Photoshop for a week.

Joined: Apr 2004
Posts: 811
Member
Originally Posted by SolarPowered
Bad!

BAD!

Someone please take away his Photoshop for a week.


http://www.fiftythree.org/etherkiller/

Ian A.

Last edited by Trumpy; 11/06/07 06:08 PM. Reason: Thread edited to remove double post
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