POE project questions - Page 1
Forum section: electronics / projects

author: SF
author_rank: Newbie
timestamp: January 06, 2021, 03:35:10 pm
content: Hi all,It's my first post so thanks for all the help you could provide.I've built a few PCB's with ethernet connections and now I'm trying to make my first POE powered PCB.While looking at manufacturers data and schematics I've got confused and now I've got more questions:How does the ethernet balance network work in this case. Won't the resistors make a short circuit?How should the resistors be connected?I'm sure this have a simple answer but I'm scratching my head and I can't reach a conclusion.

author: fordem
author_rank: Regular Contributor
timestamp: January 06, 2021, 07:25:03 pm
content: I'm assuming that the "ethernet balance network" you mention is a reference to the fact that UTP (unshielded twisted pair) is a form of balanced transmission line - if that is correct - that is a reference to the fact signals on each pair of wires is balanced with respect to one another - so the transmit pair is taken as single entity, and the receive pair as a separate entity - this allows for any noise picked up on the cable to be cancelled out in the receivers.The resistors, being connected to the center tap of the coupling transformers will have no impact.Going back to the "ethernet balance network" - ethernet was originally run over a 50 ohm coax cable so there is no requirement for it to be "balanced" - speeds have increased, physical media has been changed, but as far as I know the underlying "collision detect" mechanisms that allowed "multiple access" are still in place.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: January 06, 2021, 08:00:23 pm
content: Resistors and capacitors are not needed when using PoE. Take a look at the SILICON LABS SI3402-B-GMR CHIP. And on a HANRUN HY931147C or WurtElectronik 74984104400 socket.

author: SF
author_rank: Newbie
timestamp: January 06, 2021, 09:06:26 pm
content: Thank you for your replies.I think you replie put me in the right path but,I still have a question on the second schematic there's a D2 (HD01-T) bridge. Is it acting as termination instead of the resistor/capacitor network? I believed that is good practice to use it, at least in "normal"/non POE ethernet circuits.I considered using the SILABS IC you mentioned but unfortunately the circuit I'm designing needs short bursts aprox. to 2A (for a few uS) wich with the rest of the circuit would put the POE close/over it's limit so it could cause odd resets of the circuit.But back to the question. I will have a look into what you mentioned.My english is not the best eitherand again thank you

author: viperidae
author_rank: Frequent Contributor
timestamp: January 07, 2021, 06:02:45 am
content: Are you planning on using gigabit Ethernet or just 10/100?Gigabit uses all 4 pairs for data so you need to take that into account.You then need to decide if you're going to use passive poe or go full IEEE 802.3

author: S. Petrukhin
author_rank: Super Contributor
timestamp: January 07, 2021, 06:51:14 am
content: In the 100Base-T (100Mb/s) network, 2 pairs are used, resistors and a capacitor are installed on the free pairs to remove static charges so that free metal does not fly in the air.SI3402 controls the current in the primary circuit (I consider only an isolated converter) and this is 470mA. With a power supply of 52V, this is 24W, which corresponds to IEEE802.3at class 4. But you do not use all 52V of the PoE input voltage for your needs, you have a transformation coefficient. For 3.3V, this is about 1:15, hence the current you can get at 3.3V is much larger, about 5A continuous consumption.Short pulse consumptions should not stop it if the storage capacitor is correctly selected.Look apendex notes:https://www.silabs.com/documents/public/application-notes/AN956.pdfP.S. It is a good practice to look carefully in the datasets for references to appendix notes. Developers and document designers are also people, they make mistakes, forget, do not immediately understand what to say to the user about the product, probably sometimes even hesitate and then fill in the gaps in appendix notes.

author: SF
author_rank: Newbie
timestamp: January 11, 2021, 06:55:53 pm
content: Hi,I've been looking into datsheets, etc and I think I'm getting it now.It will be a gigabit ethernet so all the pairs will be used. A bit different than if it was only 10/100.Any advice is very welcome.Thanks for your help.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: January 12, 2021, 07:14:05 am
content: Quote from: SF on January 11, 2021, 06:55:53 pmYou just need to use another connector with a built-in transformer or another external transformer for all 4 pairs for 1000BASE-T.PoE will not change.