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Ethernet CAT5 and CAT6 cables in their traditional role of transmitting high frequency data were missing a trick. It's electronically easy to separate DC from a high frequency signal, so why not use the existing wires to provide a DC potential at the other end of the cable? No mains power supply needed, ideal for cameras and many remote devices already connected via Ethernet cable. The 8 wires in the cable aren't very thick though and 100M is a long way. There's all sorts of specs to do with heating effect due to the resistance of the wires which need to be formally observed so the net result is two official standards, IEEE802.3af and IEEE802.3at, able to deliver 12.95W and 25.5W respectively at the PD.
There are a number of proprietary extensions to these specifications and some manufacturers don't use them at all. They may still offer the ability to transmit DC over the cables so it's still Power over Ethernet, just not necessarily compatible with the standards. Some products claim to be compatible with the 802.3at standard and offer up to 51 W by utilising all 8 wires in the cable, but strictly the 2009 standard prohibits the use of all 4 pairs in this way.
PSE / PD / Midspan
Generally there are 3 types of devices used in a PoE environment. A PSE, a PD and a Midspan. Poe functionality can be incorporated into most Ethernet devices. The PSE (Power Sourcing Equipment) is simply a specially configured power supply designed to provide power safely to a valid PD (Powered Device).
A simple handshake is used between the PSE and PD before power is allowed to flow from the PSE. The PSE is most often part of a switch or hub, or a variety of up/down converters which include an RJ45 connection and can usefully include a PoE facility. A Midspan is a type of PSE and in simple terms is typically a pass through RJ45 box which adds PoE power and acts as a PSE.
How Much Power?
Perversely, the relatively low powers that can be drawn through the thin wired cable are far more useful today because devices are becoming more energy efficient.
IEEE802.3af - maximum 12.95W at the PD.
IEEE802.3at - maximum 25.5W at the PD.
Stepping outside the standards brings a large increase in power for special applications, with jumps to 51W, 60W and beyond with the current max at 100W. Mention should also be made of the HDBaseT Alliance who have developed the HDBaseT specification aimed at multimedia and offering up to 100W of power via PoE.
Many higher power PSEs or midspans also include outputs that meet the lower power level standards.
DC unbalance between the centre taps of transformers used to transmit the PoE compliant power can degrade data link integrity but can be difficult to measure. Sifos Technologies offers a highly unique, plug 'n play, and fully automated approach to DC Unbalance tolerance analysis in a power sourcing Ethernet port
The IPW10ID3S-H1-PoE-PCT features an Intel® Atomô Dual Core Processor N2600 1.60 GHz, Power over Ethernet (IEEE802.3at), and Projected Capacitive Touch (PCT) Technology in an industrial grade LCD flat panel
The POE-171S is a pass through Gigabit PoE splitter that meets existing PoE standards and also has a high power mode capable of 60W at 12V, 19V or 24Vdc.
Models IPS-1080A, IPS-1080-24V, IPS-1042FX-24V and IPS-1042FA Series all feature PoE+ with two models aimed at the automotive/automation market providing a 24Vdc PoE power supply input
This new model in Phihong's 'Ultra PoE' range is designed to allow the user to operate powered devices designed with two separate detection circuits for each of the two pairs
Planet's POE-165S provides passive PoE at 12, 19 or 24Vdc and with a temperature range of -40 to +75 degC it's a fit and forget solution
The IEEE 802.3 1-pair Power over Data Lines (1PPoDL) Study Group meets September 2nd and 3rd in York, England and will consider a new single pair, Ethernet based powering standard
PLANET VC-203Px series of PoE over Coaxial Extender is based on IEEE 802.3at (PoE+) and delivers up to 25 watts of power output to af/at compatible PDs such as PoE IP cameras or PoE Wireless Access Points
RocketLinx ACS7106 is a Poe+ (IEEE 802.3af/at) Industrial switch with four PoE Plus ports and two Gigabit ports, UL294B certified, and with a 12/24VDC voltage boost
In a recent release of 10 new NGFW (next generation firewall) products from Fortinet, 2 feature PoE for use with Fortinet PDs
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PoE is Green
If you haven't heard that we're all supposed to save energy then you really might be on a different planet. And PoE must continue to expand it's reach on this planet because of that imperative alone. Sending power down the same cable as data makes too much sense to ignore - versus the alternative of trailing wires, remote power supplies, power management and maintenance. Of course there are practical limits to the amount of power that can be provided in this way (long, thin cables provide those practical limits) but as devices continue to be built to use less power then PoE becomes increasingly more viable.