Powering devices over Ethernet cabling seems easy, but there’s more to it
than meets the eye. In this article, Eddie Insam explains how it all works.
So you’ve designed a brand new Ethernet-based device. Perhaps it’s a clock, a weather sensor, or an industrial controller device. You plan to hang it proudly on your wall and connect it to a RJ-45 wall socket. But how are you going to power it? Where will the system get its juice? Surely, you aren’t going to disgrace your design with a brick wart. There must be a better way!
Why not feed power over the CAT-5 cable? Well, you’re not the first person to consider this technique.
Standard CAT-5 cable has four pairs, and only two are used for data in a typical 10- or 100-Mbps installation (see Figure 1a). So, it sounds obvious to stick a few DC volts down the spare pairs. Oh, yes. But hang on, life is never so simple. This is technology, remember? There has to be a catch somewhere. So, sit down and relax, I have the story.
It may not come as a surprise that the wise men at the IEEE thought about this for a while and came up with a standard (IEEE 802.3af). This standard has been around since 1999, but progress has been relatively slow. It started to take off only recently, mainly because of the availability of inexpensive specialist components. Tom Cantrell and Jeff Bachiochi have covered some of the available components and modules (Circuit Cellar 165 and 187). A wide range of parts are now available, including dedicated switching transistors, isolation transformers, and high-quality nonsaturating magnetics, making power over Ethernet (PoE) a practical proposition.
The IEEE document covers two main methods for sending power down the CAT-5 wire. One involves using the spare pairs. The other involves sharing with the existing data lines using center-tapped transformers (see Figures 1b and 1c). The latter method is beneficial when spare cable capacity isn’t available.
The method involving spare pins allows a decent amount of current to be drawn because the two spare pairs are paralleled together to increase capacity by reducing the total DC resistance. The present IEEE specifications allow up to 13 W of power to be transferred this way. This may not be enough for some heavy-duty devices, but it’s quite acceptable for medium-size and small items such as TV cameras and VoIP phones. An updated PoePlus standard is currently being considered. This will allow for up to 30-W capacity, while still remaining backwards compatible.
Transmitting power with center-tapped transformers is more limited. Pulse transformers and other magnetics in the Ethernet controller must be designed to take the full DC power load current without saturating. That isn’t an easy task for miniature surface-mounted components. The advantage of this alternative is that it leaves the extra pairs alone, an essential consideration in higher-speed gigabit Ethernet, which requires all four pairs to carry data.
Why can’t you just stick any old power supply across the spare wires? Because you don’t know what’s at the remote end, and you may run the risk of blowing up sensitive equipment. If you don’t believe me, take a look at Figure 2, which is a typical Ethernet terminator. This kind of circuitry is sometimes contained within a single metal enclosure called a MagJack.
Note the two 50-W resistors R3 and R4 across the center taps of transformers T3 and T4. They are branched in series to form an effective 150-W DC load across the input lines. Also note the two 50-W resistors R1 and R2 right across pins 7 and 8 and 4 and 5. These present a controlled impedance load to the otherwise non-terminated wires. They are there for robustness and noise reduction. This hookup is sometimes known as a Bob Smith termination.
If you connect a 48-VDC raw supply into such a socket, you will be driving a good third of an amp through these tiny resistors. This is guaranteed to vaporize them to kingdom come. Tiny SMD resistors are not built for such treatment.
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