Power over Ethernet, or PoE, describes any of several standards or ad hoc systems that pass electric power along with data on twisted-pair Ethernet cabling. There are several common techniques for transmitting power over Ethernet cabling. Three of them have been standardized by Institute of Electrical and Electronics Engineers (IEEE) standard IEEE 802.3 since 2003. The original IEEE 802.3af-2003 PoE standard provides up to 15.4 W of DC power (minimum 44 V DC and 350 mA) on each port. Advocates of PoE expect PoE to become a global long term DC power cabling standard and replace a multiplicity of individual AC adapters, which cannot be easily centrally managed. Critics of this approach argue that PoE is inherently less efficient than AC power.
The recommended approach for implementing PoE is to use Ethernet switches that support PoE. Mid-span injectors can easily be purchased which will add Power-over-Ethernet (PoE) to an existing Ethernet signal without need to upgrade to PoE switches. The downside, of course, is cost of many known brands. If you are looking to save money, you might want to look what is the “cheap Chinese solution”. I decided to try a cheap “generic” PoE injector often referred to as just POE-48005.
I got my power supply from some web shop. The same product is sold in many places. Here is one description:
Wide input voltage 100VAC-240VAC AC-DC POE Injector 48V 0.5A POE Wall Plug Ethernet Adapter with EU Plug.
Wide input voltage 100VAC-240VAC AC-DC POE Injector 48V 0.5A POE Wall Plug Ethernet Adapter with EU Plug. A POE injector adapter connects to regular Ethernet cable on its one RJ45 connector and injects 48V/0.5A DC power on its output RJ45 connector. On Ethernet data POE injector acts as pass through device that mixes data and power on Ethernet cable. The devices requiring Power from Ethernet cable then can work by use of this POE Injector.
POE Powered IP Cameras and surveillance devices
Arduino POE capable Ethernet Shields requiring Power over Ethernet
Custom Designed POE powered modules / devices such as remote sensors
Provides remote power to equipment through CAT 5 Ethernet cable up to 100M
Autoranging switching power supply 100~240V AC
Short circuit, over-current, and over-voltage protection
Reliable 24W passive POE output
Compact, portable size with convenient wall plug design
Output: 48V DC 500mA – Please do not order if your unit requires more than 500 MilliAmps
Input: 100-240V 50/60HZ
Input Current: 1A
Efficiency: 80% Min
Line / Load Regulation: 3% / 5%
Ripple / Noise: 200mV p-p
Turn-On Delay: 2 Seconds Max
Rise Time: 40 mS Max
Protections: Auto Recover Over-Voltage, Short Circuit, Over-Current
Environmental Standards: RoHs
Operating Temperature: -10 to +40°C (14 to +104°F)
Operating Humidity (RH): 5% – 90%
Storage Temperature: -20 to +85°C (-4 to +185°F)
When I was testing this power supply it seemed to always supply 48V even though the original description I saw mentioned IEEE 802.3af standard, which says that the power supply device should identify that there is PoE capable device connected before supplying that 48V to cable. It seems that this power supply does not meet the IEEE 802.3af standard as power source. It power IEEE 802.3af standard compliant devices though. But because it is missing that IEEE 802.3af identification part, it could damage if you plug in a device that does not expect to receive PoE power.
I was planning to do a more detailed review of this device, but then I found this excellent review of one version of the POE-48005 power supply that describes well that is going on.
Review: Generic 48V 802.3af “Compatible” PoE Injector (XLY-POE-48005)
Posted on December 4, 2022 by lui_gough
I decided to scrape the bottom of the barrel for this “generic” PoE injector (often referred to as just POE-48005) for about AU$11.20 each by stacking some coupons. I was curious – just what will I get?
POE power supply, Model POE-48005, accepting 100-240V AC at 50/60Hz and providing 48V 0.5A with positive on pins 4 and 5, negative on pins 7 and 8. The item is Made in China,
At the end, there are two 8p8c sockets – one marked LAN for data input and one marked POE for power and data output to the device. Do not get this the wrong way around!
Key snippets from the sales listing include these somewhat potentially contradictory and misleading statements, especially if skimming the listing:
“CAT5 Ethernet Pin Usage: Data: 1,2,3,6 Power: +(4,5),-(7,8)”
“Meeting IEEE 802.3af standards and more sufficiently is able to provide power to PD devices”
“This Adapter only support 10/100M connection, do not support gigabit ethernet connection, because Data & Power Out port using 4,5,7,8 pin as power transfer, Please do not connect any gigabit ethernet device on “Out” port, this will damage your device.”
“This is an 802.3af compatible device, but it does NOT include the 802.3af compliant detection feature. It supplies power NO MATTER WHAT, so don’t plug the “POE” side into anything that isn’t ready for it.”
As a result, it’s clear that this is not an 802.3af compliant injector. By “compatible”, they mean that it will work as the wiring is the same as Mode B and the voltage is also within the range, so the connected device should run. But it doesn’t have any of the safeguards that 802.3af requires, so plugging it into devices that aren’t expecting PoE is likely to cause damage. It won’t allow gigabit Ethernet either, since no magnetics are used in the injection. Damage to gigabit devices, may only affect certain designs. This is basically equivalent of bundling a “passive” PoE injector with a 48V supply – with the benefit of being inexpensive and relatively neat.
I suppose the interesting thing is that I learned that the PoE load detection procedure can be done by the switch port, but the device doesn’t expect to need to go through the sequence – if power is available, it will just seemingly take it.
From the top, it seems that the design itself is rather minimalist, but is not entirely horrible.
The Ethernet input pins 4, 5, 7 and 8 are completely disconnected from the PCB
While the design didn’t exactly ring any alarm bells, the electrolytic capacitors did leave me some pause for concern.
The results appear to show very stable voltage output of about 49V which is well within the 44-57V range.
The price for skimping on an 802.3af power injector is receiving a product that is a 48V DC power supply and passive Mode B compatible PoE injector combined into the one case. Such a design will work with 802.3af devices and in that sense is “compatible”, but does not have any of the device detection logic that makes the system safe for conventional Ethernet devices. Reversing the connections could lead to harm to conventional Ethernet devices,
The efficiency is not horrible and the power supply design looks sensible enough, but the quality of the electrolytics leaves much to be desired. Perhaps it would have been better to spend those few extra dollars to get something proper in the long run … both for safety and for longevity. Still, at least I was able to satisfy my curiosity!