Selecting an RJ45 socket for PoE - Page 1
Forum section: electronics / projects

author: homebrew
author_rank: Frequent Contributor
timestamp: May 15, 2025, 07:59:41 am
content: HelloI'm currently designing a little PoE-Powered board (think dev-board for a specific purpose) that will be used most likely embedded in another enclosure or plastic box or will just run unenclosed on the bench. In any case, I'll not have a reliable, conductive chassis of any sort. Thus I have two questions:Question 1) What connector could you suggest? It must be an *upright* connector with integrated magnetics (10/100 MBit/s) and PoE capable 802.3af mode A or B.I found at least 1 (pricy) candidate: 7499410221 (https://www.we-online.com/components/products/datasheet/7499410221.pdf)However, what I don't get is how to build a somewhat standard compliant system with connectors like these. Ethernet requires 1.5kV isolation between any of the signals and the device. Given that the PHY-Side pins are only 2.54mm spaced apart from the PoE pins (cable side), I've really no idea on how to achieve sufficient distances, here. Slot milling between the 100mil spaced pins? Having a pad there will restrict the slot-width significantly.So if you know any better suited connector, please let me know.Question 2) What to do with the connector/cable shielding?Currently my plan would be to connect the Ethernet-Cable-Shield to a little local "chassis-plane" on the PCB that has no overlap with any system GND plane and maintains a 2mm gap to any other conductor. At one point I would use an SMD Y1 cap of 1nF to provide safe AC coupling between grounds. That said, there is a third "ground" hidden in the PoE circuitry (primary side) that also has to remain isolated from any other ground. Does this need to be AC-coupled as well or is it really just free floating? Or am I completely off?

author: Terry Bites
author_rank: Super Contributor
timestamp: May 15, 2025, 04:35:16 pm
content: 13F-67 series from yuandean? About $3

author: voltsandjolts
author_rank: Unknown
timestamp: May 15, 2025, 04:45:20 pm
content: For your reference;https://www.eevblog.com/forum/projects/poe-ethernet-gpib-adapter-open-source-project-now-public/

author: Hydron
author_rank: Super Contributor
timestamp: May 15, 2025, 06:42:02 pm
content: Pretty sure the isolation is just a functional spec, not a safety one. As long as you can pass the 1.5kV test you're good to go, and 2.54mm is probably just enough for that, though more would be nice!

author: homebrew
author_rank: Frequent Contributor
timestamp: May 16, 2025, 05:34:22 am
content: Quote from: voltsandjolts on May 15, 2025, 04:45:20 pmThank you very much for the pointer to the GPIB-Project.Maybe I completely mistake the concept of (functional) isolation, or that project is quite relaxed in that perspective. From what I can see the minimum distance between MDI (cable) side to GND is only about 0.25mm.Also, both GPIB shield and ETH cable shield are bonded to the signal GND-Plane. Isn't that problematic in terms of ground-loops especially on sensitive measurement equipment?This (somewhat random) document I found on the subject seems quite strict, with 1.25mm (50 mil) trace-to-shield distance requirement:https://www.pulseelectronics.com/wp-content/uploads/2020/12/Pulse_Layout-Considerations-v7.pdfBut hey, maybe I'm just overcomplicating things ...

author: Kofen
author_rank: Regular Contributor
timestamp: May 16, 2025, 10:13:13 am
content: Hi, I am the designer of the PoE adapter.You are right in calling this out, as the design does not meet IEEE 802.3af Isolation Requirements as this specify 1500v for 60s at 50 or 60 Hz. If this design will sustain this, depends on the humidity that day and or the contamination of the boards.The thing with the ethernet specification is that it calls for the same requirement on both the PSE side AND the PD side, so you are not dependent on the PSE you are connected to.What makes this difficult in many cases is the use of integrated mag-jacks where the signal pairs for isolation is inside the connector, so when it is bought out on the pins, you already have an isolated and non-isolated domain, that breaks the isolation requirement as soon as you add an IPC compliant footprint!Now, this is how I chose to handle it, and some/many might not agree here.The specification is strict to ensure safety and EMC, but if we can rely on the PSE for isolation, we are only left with a need for functional isolation on the PD. And this is done a lot even on commercial devices, simply due to lack of mechanical isolation. And this is also what I did on the adapter.My rule of thumb is that my designs should withstand 400vac permanent as this is worst case gnd fault, this design does not do it according to IPC-2221(that is quite strict), and if you managed to lift the switch at that potential, some ground current would flow, but likely not arc over and create damage or danger. This is a very worst-case scenario, and a lot of things need to go wrong for this to happen.But I will give you this, I could have improved the isolation a bit more, but it is functional isolated for the maximum input voltage potential we will see in normal use.When it comes to the shield, it is correct that this needs to be connected to chassis, this gives the correct return path for noise current and should be on the same plane as the gpib connector. The gpib connector is then connected to the chassis of the test instrument, ensuring compliance.This can give ground loops, since the same rule apply to the PSE, the shell has been connected to the cassis, but normally shielded cables are not used, and shielded cables should only be used where you need to use them on a case-by-case basis.Commonly one uses un-shielded cables and that ensures that you don’t create ground loops. One benefit if you are very worried about isolation requirements would be to use a shielded cable as this puts you on the same potential as the PSE(but I would not worry).Now since you are making a dev kit sort of device, I would not worry above functional isolation, maintain as much isolation as you can and absolutely ensure you have a complete ground plane as a chassis where you terminate all common mode filtering, as close to the connector/source as possible.But do your due diligence before deciding to move away from the specification as it is now up to you to ensure that the design is safe to use in all use cases. Just becouse I could decide is was correct for my design, does not mean the same apply to yours as I do not have all the details.

author: ajb
author_rank: Super Contributor
timestamp: May 16, 2025, 05:57:23 pm
content: Strictly speaking, there is no fixed relationship between isolation voltage and clearance distance or isolation material/thickness. There is a bare minimum distance where the isolating material (whether that's air, plastic, or vacuum) will break down at the required voltage, but you would never design to operate at that point just like you'd never design to operate at the abs max ratings for any component. So appropriate isolation for a given application needs to include an adequate safety margin, which depends on manufacturing and material tolerances, operating conditions, and the consequences of an isolation failure.In the case of PoE, you (should) have at least two isolation barriers: In the PSE between mains voltage and the 48V supply, and in the PD between the cable and local sides of the transformer. The former separates the hazardous mains voltage from the relatively accessible unearthed SELV on the ethernet cabling, so the consequences of failure are significant (death!): therefore it requires 'Reinforced' isolation. The isolation in the PD separates the already-reinforced SELV from the local circuitry, and is primarily there to prevent common-mode currents caused by the ends of the cable being at different locations, induced junk, etc, and the impedance is relatively high, so the consequences of failure are much lower (mostly equipment malfunction): therefore it requires only 'Functional' isolation. (If your PD's local circuitry involves hazardous voltages, like a mains-referenced energy monitor or something, you're back to needing Reinforced isolation somewhere between that and the SELV.)Beyond that, the details depend on engineering considerations (materials, overvoltage classification, pollution degree/other environmental factors) which are generally wrapped up in the various applicable standards for specific products or components. For personal/dev board type use, you can make an informed decision based on your personal risk tolerance and use cases. Is a dev board or PoE GPIB adapter that probably never leaves the bench really likely to experience 1500V across the isolation barrier, or even a fraction of that? Hopefully not! If the likelihood of having more than 60V across the isolation is practically zero and the consequences of a failure are minimal, you might decide that 0.25mm is all you need. For commercial products on the other hand, given the potential for liability and compliance/certification requirements, you need to figure out which standards apply and start working through them, hire a consultant to do that for you, or buy in components/modules that already have the required certifications where possible.This TI appnote has a decent overview of the whole subject, with references to more docs:https://www.ti.com/lit/ml/slup419/slup419.pdfMore specific to the question, if you can't find a magjack that you like, you always have the option for separate jack + magnetics. Ethernet transformers generally have the MDI and line sides on opposite sides of the package, so you get better clearances, and have more part choices -- including transformers designed for higher degrees of isolation.

author: Kofen
author_rank: Regular Contributor
timestamp: May 16, 2025, 06:16:34 pm
content: Thank you ajb!You summed this up a lot better than me, and we are 100% aligned!

author: voltsandjolts
author_rank: Unknown
timestamp: May 16, 2025, 06:28:20 pm
content: Ethernet has become ubiquitos of course, and has found it's way to outside of buildings, say for outdoor wifi points, extenders, or cameras etc. In such cases, there is risk of lightning strike and thusprotection devicesshould be used. But I bet most such installations have nothing of the kind!

author: Kofen
author_rank: Regular Contributor
timestamp: May 16, 2025, 06:34:44 pm
content: Agree, and that is a big part of the problem, there is a spec, but then you get another layer and all hell breaks loose. It is not easy to navigate to ensure that your own device comply. Tricky stuff. Hard for us that actually try to make the best of it, while most OEM seem to do not care.

author: homebrew
author_rank: Frequent Contributor
timestamp: May 18, 2025, 07:44:48 am
content: Thanks for these very insightful answers. While I'm just designing stuff for my personal use at the moment, I trie to do it as perfect as I can. Maybe there is an opportunity some day to turn it into a product and then this effort will pay off.Anyway - when I'm done designing the board, I'll post the results here. In the meantime, this is an RJ45 Jack with magnetics and PoE that I think enables me to keep the required distances. The price point with many distributors is even worse (in single quantities) but at least it seems to support all my requirements.https://www.belfuse.com/products/ethernet/magjacks-icms/v8br-1ax1-gh

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 18, 2025, 12:31:48 pm
content: Are you going to bring out any interfaces besides the Ethernet? If not, the idea of isolation is irrelevant as it's just a matter of enclosing the device in a case. If there are, might it be easier to isolate those interfaces instead?