Tricky question -- power over RJ45 - Page 1
Forum section: electronics / projects

author: blueskull
author_rank: Unknown
timestamp: April 10, 2018, 12:23:06 am
content: Disclaimer: this is not about transmitting 220V through UTP.I'm designing a network connected audiophile DAC that has an Ethernet port. Since I only use 2 pairs of the Ethernet port (Fast Ethernet with GbE jack), I have 2 pairs doing nothing.I would like to use the rest 2 pairs for power and AES3, and by power, I mean to power an SPDIF receiver that consumes ~200mW max.However, I can't just dump DC power to one of the spare pairs because that will fry the peer device if it has a GbE magnetics.I also can't dump high frequency AC power to the spare pair because that will fry the peer GbE device's PHY.One way to do such is to use inter-pair voltage to carry power, but then I will either fry or get fried by a PoE enabled device.My goal is to supply a small amount of power through an RJ45 port while being 100% compatible with any existing Ethernet devices.Do far I have 2 ideas:1. Send a low level ping signal sequence through one spare pairs for a short amount of time, if the peer responds, then I know it is my device, not a generic GbE PHY. If not, stop bothering with it.2. Use a RJ50 10P10C connector. I don't want to do this because this requires on-board Ethernet magnetics which I don't want to have. I want to keep things small and integrated.Any suggestions?

author: ogden
author_rank: Super Contributor
timestamp: April 10, 2018, 01:01:16 am
content: Quote from: blueskull on April 10, 2018, 12:23:06 amThis is whatPoE IEEE 802.3af does. Extra ICs, proper (PoE-capable) tranformers and some other components are needed, but this is the way you shall design your DAC to get broad compatibility w/o sparks or smoke flying. For power sourcing side use any PoE IEEE 802.3af switch.

author: ogden
author_rank: Super Contributor
timestamp: April 10, 2018, 01:11:12 am
content: Quote from: blueskull on April 10, 2018, 01:05:40 amRight. Make sure you ship your DAC with 10pin cable crimping tool as well I am not sure that you can plug 8pin connector into 10pin socket. Leftmost/rightmost pins probably will be irreversibly bent.

author: llkiwi2006
author_rank: Regular Contributor
timestamp: April 10, 2018, 01:21:39 am
content: Quote from: blueskull on April 10, 2018, 12:23:06 amHow would using a RJ45 over a RJ50 eliminate the need for magnetics?

author: ogden
author_rank: Super Contributor
timestamp: April 10, 2018, 01:36:30 am
content: Quote from: blueskull on April 10, 2018, 01:16:20 amDon't overcomplicate. Just implement "passive PoE", preferably 12V, in standard connector as many cheap Ethernet devices (point-point radios, cameras) do and that's it. Just make sure to have diode so u do not feed power from DAC back into Ethernet cable in case it is powered from wall-plug, not PoE. Obviously you shall supply passive PoE Injector "Y cable" aswell.Mikrotik & Ubiquiti uses such all over the world and customers usually don't blow ports of their upstream devices.

author: ogden
author_rank: Super Contributor
timestamp: April 10, 2018, 01:45:48 am
content: Quote from: blueskull on April 10, 2018, 01:39:38 amGeez, somehow missed that. Sorry. - Then RJ50 and ribbon cable and Bob's your uncle

author: djnz
author_rank: Regular Contributor
timestamp: April 10, 2018, 03:36:21 am
content: I've used a RJ45 cable in a RJ50 socket and I don't think the mechanical stress is something to worry about.How will you handle straight-through vs. crossover cables in your system? It's not unfathomable that someone will use a crossover cable when you designed it for straight-through connection. Won't that be a problem?

author: ejeffrey
author_rank: Super Contributor
timestamp: April 10, 2018, 03:58:17 am
content: Quote from: blueskull on April 10, 2018, 01:39:38 amI don't understand this. Why does "passive 12 V PoE" of the type used by ubiquiti and friends not do what you want? The power is coming from your ethernet connected DAC (which has its own power supply?) and going somewhere other than the ethernet peer? In that case, why are you trying to put the power on the ethernet port?In terms of not blowing out devices, 802.11af solves this by a negotiation sequence similar to what you described as option 1. The peers negotiate by connecting resistances to the center tap and don't apply power until the detect a compatible device. You could do something similar either using the common mode path like PoE or using the unused pairs in 100 megabit. Just apply a small test current that won't blow anything up and apply full power only if the result looks sensible.

author: ejeffrey
author_rank: Super Contributor
timestamp: April 10, 2018, 04:30:45 am
content: Ah, the Apple school of designSeriously though, that sounds reasonable given the constraints. I can't say if the constraints make sense.I'm not a big fan of mismatching RJ sizes. I know it is designed to "work", but when I have used RJ11 plugs in RJ45 jacks there is just too much slop and it doesn't feel right. RJ45 in RJ50 should be better due to the wider body, but I would get one and try it out to see if the "user experience" matches what you want in an audiophile DAC.This is particularly a concern as I assume this will be the "normal" use case. I assume the dongle users will be the minority, that would be the whole justification for using the dongle rather than building those connections into the device. So by using a loose fitting jack for the common case you are giving everyone a sub-standard experience, rather than only the SPDIF users.I would try to go for the sensing option rather than the larger jack.

author: tautech
author_rank: Super Contributor
timestamp: April 10, 2018, 05:10:34 am
content: Can you put a DC offset on just one of the RJ45 conductors and use shielded RJ45 plugs that use the metal shell for the 9th wire and DC return path.The P to P microwave guys use this heavy CAT6 9 wire cable with an additional bare copper conductor to earth bond exterior HW.https://www.jaycar.co.nz/plg-rj45-8p-8c-strnd-cable-scrnd-pk10/p/PP1437This isn't the stuff but maybe it would do:https://www.jaycar.co.nz/9-way-computer-cable-sold-per-metre/p/WB1578