For a lot of modern networked devices, Power over Ethernet (PoE) is a big convenience, allowing the one cable to power a remote device (be it an access point, camera, etc.) and supply the data connectivity to it.
While there is a standardized PoE in the form of 802.3af/at, which specifies negotiation rules and a 48V nominal system voltage, many lower cost devices (especially older ones) cannot justify the increased cost of integrating the necessary logic and power conversion circuitry. These utilize an alternative solution which is termed Passive PoE.
Passive PoE does not involve any negotiation whatsoever and supplies power at all times. In its earliest and simplest incarnation, it used the two unused pairs from Fast Ethernet to carry the DC voltage. This was informally standardized as pins 4-5 as positive and pins 7-8 as negative, similar to 802.3af mode B. Later on, Gigabit Passive PoE used transformers to allow both data and power to share the same pins, although these also come in a few variations.
Most passive PoE installations use a variety of voltages, with 24V being a common compromise but even voltages as low as 12V can be practical depending on run length. Cheap passive adapters (endspan injectors and splitters) can be purchased to convert some 2.1mm DC jack based equipment to run off passive PoE although the cheapest adapters are fast Ethernet type. This is often good enough for older wireless APs.
However, whatever you do, you should not apply passive PoE to a device not designed to take it, otherwise damage to the Ethernet port’s magnetics may occur!
The concept of passive PoE is fairly straightforward and endspan injectors can be purchased in pairs quite cheaply (about AU$2 a pair). But what if you don’t want to inject and remove power at the ends of the Ethernet cable? Maybe you have voltage drop issues, or you have a more convenient source of power closer to the device? In that case, you need a midspan injector.
The Device
On eBay, for about AU$1.59 posted you can have a “wall mountable” midspan Fast Ethernet passive PoE injector.
The unit has a plastic body with a hole on the top for a power indicator LED. The wiring is printed on the casing for reference.
A 2.1mm DC barrel jack receives power from your power pack to inject to the cable. The PoE port supplies the data (two pairs used for Fast Ethernet) and power (over the unused pair) to a device expecting passive PoE or a splitter that you supply. The LAN port provides only the data and is not connected to power.
It is “wall mountable” owing to a small hole eyelet in the casing, although it does seem a little flimsy.
The unit is held together with one screw covered by a QC label I had removed. How does it look inside? Lets see.
Teardown
The screw was the only thing stopping you getting inside.
The unit is built on a paper-type single-sided PCB to reduce costs. Inside, there are the Ethernet jacks, the power jack, an LED, a 10kohm resistor and two jumper wires. This is probably the second simplest design that could be used – the simplest would have omitted the power indication altogether. The PCB is marked XLY-POE2(20150717) suggesting a company by the initials XLY manufactured the unit, this is probably the second version designed 17th July 2015.
A quick calculation seems to show the resistor is well sized – at a maximum 802.11af voltage of 57V, about 55V is put over the resistor resulting in a current of 5.5mA (very safe for the LED). At a low of 12V, with 10V over the resistor, the current is just 1.0mA which should result in a dim but visible glow.
Unfortunately, the unit doesn’t employ any safeties – no polyfuse protection against short circuits, no diode protection against reverse polarity. As a result, be sure to use a limited-power supply which will only source a safe amount of power in case of a fault and confirm the polarity before using it. It would be trivial to add it with a knife and soldering iron though.
However, because of its simplicity, the unit could also be used as a splitter by plugging in the power+Ethernet lead into the PoE port and extracting the power from the DC jack (by making a male to male cable). You can also use it as an indicator as to whether PoE power is being delivered to a passively-powered Fast Ethernet device just by plugging the powered lead into the PoE port and seeing if the LED lights. Or you could use this as a crude Fast Ethernet only cable joiner in an emergency. For the most part, most endspan passive PoE kits can’t do this as they lack an LED.
A look at the underside shows green solder resist and very oddly shaped thick chunky traces. It might guard against delamination during soldering, but it might not have a good effect on the data signals.
Tracing the path of various signals, the positive power is in red, the negative power is in dark blue. The LED to resistor connection is in yellow. The remaining data pins are colour coded green, orange, purple and sky blue. The light-blue overlay shows where the LED, resistors and jumper wires on the other side are.
This is a mess. In fact, this is where endspan units might be better as they are based on a crimped RJ45 end and CAT5 or better cable soldered to a socket which isn’t particularly complex. However, as we can see in this design, the data lines are all different lengths. The traces are also shaped quite oddly with some data lines needing to jump over to the other side and back. This would probably cause both skew and impedance mismatches which could affect the data integrity slightly. Being unshielded, it could also introduce interference into the network in the case of being near high EMI/RFI sources.
Luckily Ethernet is extremely tolerant of dodgey cabling. Low rates of errors will be corrected through retransmission – I’ve even had success running 100Mbit/s Fast Ethernet for a while on CAT3 “voice grade” cabling which was only suitable for 10Mbit/s just on a whim. So I don’t think this design will cause problems per-se, but it’s not the best situation for an Ethernet signal. Of course, using two pairs for power in a dedicated fashion will preclude Gigabit Ethernet from working.
Conclusion
This inexpensive mid-span passive PoE injector is very much as minimalist as it can be, with the exception of the inclusion of a power LED. It can be useful in a pinch as an injector, a splitter (with your own male-to-male lead), diagnostic tool to check for PoE presence or to forcibly strip out any passive PoE to safely connect non-passive-PoE-aware devices or as an emergency Fast Ethernet cable joiner. The design isn’t exactly the safest, so ensure you are using a limited power supply with the correct polarity. The design also isn’t the best for ensuring the integrity of an Ethernet signal, but it should be “good enough” that it would work in most cases. Definitely nice to have as a spare in the toolbox.
