Power over DSL - Page 1
Forum section: electronics / projects

author: rounin
author_rank: Regular Contributor
timestamp: April 14, 2021, 09:42:53 pm
content: I'm testing some COTS DSL/ethernet bridge devices, and realized they have DC block capacitors for -48V telecom phantom power. I'd like to add the DC bias on one end and pull it off on the other, but am not quite sure if it as simple as hooking up a supply (these bridges are not DSL line powered). I kind of assume some kind of filter is needed to block the ~200MHz DSL signal from leaking all over.Some references to POTS telecom circuits show a 200R / 5H inductorlow pass filter /current limit(~6.3Hz low pass?)between the source and the world side of the block capacitors.https://www.epanorama.net/circuits/teleinterface.htmlWhat kind of source connection / filter do I need?What kind of load connection / filter do I need?Is it as simple as putting a big low pass filter with a few Hz cut-off on source and sink, on the side of the transformer with the block capacitor? I'd like to have less R than POTS, targeting 802.3at levels of power ~ 600mA @ -48V (although also interested in non-standard ~300V bias later).

author: rounin
author_rank: Regular Contributor
timestamp: April 14, 2021, 09:59:04 pm
content: Oh I guess the POTS circuit is series LR not a LR low pass, so its more like a choke blocking high frequency. Do I just get a series inductor with SRF a bit above DSL frequency ~200MHz? Is this what a bias T is?

author: dom0
author_rank: Super Contributor
timestamp: April 14, 2021, 10:35:49 pm
content: Considering this is transformer-coupled, I'd wager phantom power is a better idea here than T-power. If you want to keep the transformer AC-coupled, you will need to use inductors (i.e. low pass filters) to couple the power into and out of the signal lines. If the transformer may be DC-coupled, and can withstand the supply current, you can pretty much just use the center taps instead.

author: NiHaoMike
author_rank: Super Contributor
timestamp: April 15, 2021, 01:06:14 am
content: Take a look at Homeplug adapters, they're generally cheaper than DSL modems and are specifically designed to be powered by mains voltage.

author: rounin
author_rank: Regular Contributor
timestamp: April 15, 2021, 01:33:21 am
content: Yup, actually trying to make an alternative to a homeplug system. homeplug is slow, sub-100Mbps real world.So for inductor selection, just as high of inductance as is possible at an SRF that is above the DSL freq of ~232MHz, with enough Isat for the application?Looking at some inductors, SRF > 260Mhz, current rating ~ 1 amp, 1uF - 3uF inductance because thats all I saw at digikey with a high SRF and current capacity.Eg BRC2012T3R3MD‎, BRHL2518T1R0M‎, 74406032010‎, 74406032010‎, B78148E1102K000‎. Not trying to optimize selection yet, just breadboard atm.Then just splice in power through the inductors like this, I guess? (Probably would have more protection circuitry later, esp for HVDC)

author: NiHaoMike
author_rank: Super Contributor
timestamp: April 15, 2021, 01:54:20 am
content: Quote from: rounin on April 15, 2021, 01:33:21 amNot true of AV2, reviews report real world performance over 400Mbps. Probably could do even better on a dedicated wire pair with minimal noise.https://www.smallnetbuilder.com/archives/lanwan/lanwan-reviews/32662-trendnet-tpl-420e2k-powerline-1200-av2-adapter-kit-reviewed?start=1