Power Over Ethernet

Power Over Ethernet (PoE) Adapter by eLab

Power over Ethernet or PoE, is the technology used for power transmission in network equipment, via network UTP cable, together with data. PoE is useful in situations when we want to connect network devices that are far away from a power source. In this situation we take advantage of the Ethernet cable, because there are 4 pairs of wires, but for the majority of networks, only 2 pairs are used for data transmission. PoE uses 2 unused twisted pairs, for electrical transmission needed to power the device.

There are several architectures of PoE and many of them existed before the technology being standardized, due this fact not all types of PoE are compatible. PoE devices receive power directly from the network cable, but should be noted that according the manufacturers, specs like nominal values of voltage and current, and even pairs used to transport electrical energy may be different.

Source here.

Step 1: Description

Applying a voltage across terminals not dimensioned for that, may damage the network device. To avoid this, the best way is to make a PoE adapter, separating the two input signals, via two simple circuits, an injector and a splitter. The injector will feed the network cable with power and this circuit is placed near the origin of the installation, where we have a modem or a switch, and of course a power plug. Then we have as outputs of the Ethernet cable, data and power that will connect to the device. Now we connect the splitter circuit, which receives data and power. From this circuit leaves an Ethernet cable and a DC plug avoiding

applying unknown voltages to the devices.

As example consider the eLab case, which have a switch on the back of the room and we want to feed a 12V wireless router which is at the opposite end of the room, to obtain greater coverage by departmental area, in a place near a windows without power sources nearby. The network diagram is the one above.

Step 2: Material

• 2 RJ45 Female connectors• 2 Plastic boxes• 2 Ethernet cables (or UTP cable + 4 RJ45 crimping connectors)• 1 DC male connector• 1 DC female connector• Perfboard or PCB

In this project we used only recycled components, from old devices, so we had a total cost of € 0.

Step 3: Connectors

In our case, we used old phone connector boxes as shown in the following image.

We replaced the RJ11 connectors by RJ45. Starting from two plastic boxes, it is necessary to open a hole to fit the RJ45 connectors and secure them with hot glue. In one of the boxes it’s also necessary to make an opening to DC female connector, for power input, this box will be the injector.

Step 4: Ethernet data

Now to understand the connection, we must understand how data flows through an Ethernet cable.

For 10 and 100Mbps network we have the previous table of the configuration in the twisted pairs.

As seen in the table, pins 4, 5, 7 and 8 are not used, so let’s use them as positive and negative terminals for power. We use pins 4 and 5 (blue and blue/white) as positive and pins 7 and 8 (brown/white and brown) as negative.

Regarding the standard of colors, it doesn’t matter what standard is used, however all project should be done with the same standard. In our case we used the standard 568B.

Step 5: Schematic

We cut the Ethernet cable to obtain two ends with 15 cm long. This also can be made with UTP cable crimping the RJ45 connectors.

One of the tips will be the injector connection to the Switch and the other will connect the splitter circuit to the Wireless Router.

In these two tips, pairs blue and blue/white and brown and brown/white are cut and left open. In no situation they should be connected DC power at the risk of damaging the Switch or Wireless Router, so make sure they are left opened!

The connection should be made according with the schematic.

The 2 remaining pairs are normally plugged to the female RJ45 connector in the correct pins. The power supply comes from the female DC plug and will connect in the place of the pairs that were left opened. The positive will connect in pins 4 and 5, and the negative in the pins 7 and 8 of the female RJ45 connector. In the splitter circuit, the same will happen, being the male DC plug the only difference.

Step 6: How it looks

In the first image you can see the tip of the cable that will come from the switch and the DC input connecting to the RJ45 plug. If you look closely you can notice that the RJ45 connector is a little melted, this is because

the material is reused from old electronic devices, the same goes for the perfurated PCB. There are however connectors with mechanical contacts that don’t require any soldering.

In the second image you can see the RJ45 connector that will receive the PoE signal, where then the data will be separated from the power supply. The data will be sent through that tip of Ethernet cable, which will connect to the Wireless Router. The power supply is sent through another cable and will connect the DC plug to power the Router, however it’s not visible in this image for it is behind the Ethernet cable.

Before trying it, we tested the conductivity of each pin, for that we connected the injector and the splitter with an Ethernet cable and then checked if pins 1, 2, 3 and 6 had continuity, from the injector to the splitter, and the same with the DC connectors. We also tested if pins 4, 5, 7 and 8 didn’t have continuity, once they are cut in the adapters and left opened in each side. Finally we also tested adjacent pins to make sure there were no short circuits anywhere.

Step 7: Results

The injector on the right and the splitter on the left, as you can confirm by the DC output.

For well executed adapters, there is no influence of the power supply on the data cables, there is no interference in the transmission of data packets or in the speed itself.

In the following images you can see the adapters in the place where they were destined to be installed. The eLab network has been working well and there hasn’t been any kind of problem.

Step 8: Observations

Once again we remind that in the injector and splitter, the tips of the twisted pairs 4, 5, 7 and 8 (blue, blue/white, brown/white and brown) must be cut and left in open circuit. If this is not done or if there is any kind of anomaly that imposes voltage in these pins, the network devices might be damaged. It’s necessary to guarantee that these pins are left opened, preferably even isolated.

As mentioned before, the PoE technology uses the 2 twisted pairs that are not used on the ethernet cable, however, this is only true for 10 and 100Mbps networks. 1Gbps networks already use all the twisted pairs for

data transmission, so the PoE technology is applied differently, through a technique called Phantom Power, therefore this adapter is not suited for those networks.

To avoid the risk of damaging network devices, it is never advised to inject the PoE signal directly anywhere but the splitter, unless you are absolutely sure about the device’s characteristics and about what you are doing. The injector and splitter should be always used together.

According with the application, voltage drops should be considered. For most uses, specially for low wattages, voltage drops in a PoE cable are ignored, however, for longer distances they start having considerable values which can change the performance of the network devices. If the voltage drop is relevant, then you should consider increasing the voltage supply in order to compensate the drops through the cable and have the device’s nominal voltage in its terminals. In our case, the router is less than 5 meters way from the switch, so the voltage drop is irrelevant. For longer distances they should be calculated and considered. Maximum voltage used in PoE systems is 57VDC.

If you are not sure about the network, about the devices, about how they work or about what you are doing, then it’s advised not to do any changes, under the risk of damaging the network devices, and we are not responsible for any kind of damage. Whatever you do, do it at your own risk.

36

AndyGadget says:Apr 21, 2013. 1:35 AMReply

 Nice write-up with plenty of information and the appropriate warnings. I'll be using something similar to run a remote IP webcam. Be aware that in some countries 568A is the norm so check the connections carefully. (I love standards - There's always so many to choose from #;¬)

adillbeck says:Apr 25, 2013. 9:09 AMReply

Curious, does the standard you choose really matter? So long as you use the same pin out on both adapters it seems like the wire in the middle wouldn't matter all that much. I took a short Leviton certification course 12 years ago, and if I remember correctly when making your cables you can choose either A or B, so long as you use the same one on both ends of the cable. (if you don't, you end up with a cross cable) 

If this is incorrect then let me know, as I'd love to try this to free up an extension cord. :)

6

eLab (author) says:Apr 25, 2013. 1:46 PMReply

As long as you use the same norm in both adapters, it's all fine ;)

36

AndyGadget says:Apr 25, 2013. 9:36 AMReply

You're absolutely right - As long as you're connecting the same both ends the standard won't matter. My comment was more of a 'heads up' in case anyone was using a mix of off-the-shelf components which could be marked differently.(If you use anything other than one of the recognised standards, ensure the signal pairs are consistent otherwise the transmission characteristics would be affected.)

6

eLab (author) says:Apr 21, 2013. 6:02 AMReply

Thank you! We wanted to keep it simple but with all the necessary info.

Gelfling6 says:Apr 25, 2013. 12:14 PMReply

A few years back, there was a pair of dongles, One, a USB-A Male on one end, an RJ-45 on the other, and the other, a USB-A Female on one end, and a RJ-45 on the other.. The device was using a length of CAT-5E up to 250-feet, to transmit USB-1.1 from one end to the other, Including power, but it, too, had a limit on how far it would reach.. Also Giga-bit standards use different wires other than the standard ethernet wires, despite using the same or Cat-6 cabling. Still a good idea, to save on cable for power.. 

6

eLab (author) says:Apr 25, 2013. 1:45 PMReply

Yes, USB usually cannot reach very large distances, there are even some USB extensions for sale that sometimes are a ripoff because the data will be lost in the middle.. You're right, this adapter is only for 10 and 100 Mbps networks because they only use 4 wires, it will not work in 1Gbps networks as they require all 8 of them.

99guspuppet says:Apr 25, 2013. 7:25 AMReply

This scheme can also be used to ship data or audio on the unused pairs. YMMV

6

eLab (author) says:Apr 25, 2013. 1:41 PMReply

Sure ;)

whitewolf499 says:Apr 25, 2013. 11:03 AMReply

Good Afternoon From UK, Linda. I have a query, although I am no technical wizard, but is there no danger of this device generating Radio frequency interference, given that the cables over which the power travels may act as an antenna. This has happened many times here in UK, Europe and North and South America. I am not saying this WILL happen, only quoting the experience of what is called Power Line Transmission devices, which uses power cables to pass data between distant points in the same building. You are to be praised for your efforts posting, and this is a genuine concerned enquiry. Thanks for youre time, Best Regards, Gary

6

eLab (author) says:Apr 25, 2013. 1:39 PMReply

Thank you! Well, that is a common concern about PoE adapters but Ethernet communication is "kind of immune" to this type of electromagnetic radiation, besides the power lines being used in these cases are very low power. Therefore using some wires for power and some wires for data in the same cable, should not affect the transmission of data packets or the speed itself.

N1XRR says:Apr 25, 2013. 9:17 AMReply

Very nice write-up! A few things to note: 1) This should only be used over semi-short distances (maybe 30-40 feet). PoE uses 48v DC for power transmission so it can be tolerant of the natural voltage drop over long distances. 

For 12v DC/500ma at 50ft of 24AWG Cat5e, the voltage on the other end would be 10.8v, which is very close to the cut-off point of the Voltage Regulators in most home routers. For 12v DC/500ma at 50ft of 26AWG Cat5e the voltage would be 10.1v. At 5v the cable length should be restricted to 10-15 ft. YMMV 

2) Ethernet is, by design, noise-tolerant. Like you said, data packets and speed should be unchanged. 

3) To avoid soldering, you could use off-the-shelf Keystone jacks and simply punch-down the correct power wires in the right spot.

6

eLab (author) says:Apr 25, 2013. 1:31 PMReply

Thank you! Yes, those are some notes to keep in mind.

pachai says:Apr 25, 2013. 8:46 AMReply

This is a good instructible. I had seen a similar thing once, and I found another trick. If you take an RJ45 coupler, many of them are made in 2 pieces that can be snapped apart and back together. I snapped open the RJ45 coupler, cut a notch on either side for the wires to come out, and reached into the tight bundle, snipped the 4 wires, soldered on the leads, taped it and closed it back up. My hope is that the Cat5 rating is least affected by this approach.

6

eLab (author) says:Apr 25, 2013. 1:24 PMReply

Thanks!

DragonFired says:Apr 25, 2013. 8:04 AMReply

Thanks for the instructable. I was planning on building one of these myself, the only thing I would change is to use 2 female RJ45 jacks, to eliminate repairs when the pigtail breaks. Thanks for taking the time to document and post the relevant information.

6

eLab (author) says:Apr 25, 2013. 1:23 PMReply

Thank you!

99guspuppet says:Apr 25, 2013. 7:22 AMReply

Some types of ethernet use all 8 wires and will not work with this scheme. Gigabit ethernet uses all 8 wires and is not compatible with this scheme. There are ways to build a DIY POE adapter for gigabit ehternet.... but they are more complex.

6

eLab (author) says:Apr 25, 2013. 1:23 PMReply

You're right, this adapter is only for 10 and 100 Mbps networks.

1

AJMansfield says:Apr 25, 2013. 12:43 PMReply

Reminds me of RFC 3251, Electricity over IP.

1

Gabse says:Apr 21, 2013. 9:42 PMReply

can i plug also the arduino with ethernet shield and PoE module to this adapter?

Cracknel says:Apr 22, 2013. 11:33 AMReply

Of course :)