Power-over-Ethernet negotiator/regulator - Page 1
Forum section: electronics / projects

author: LukeW
author_rank: Frequent Contributor
timestamp: April 13, 2014, 08:58:40 am
content: Can I get a little peer-review or another set of eyes to look over this schematic please?A NCP1090 and a LM5008 are used to implement 802.3af POE, regulated down to 3.3V for a small 3.3V load, a total system load of only about 220mA at 3.3V.The system should power up from either Ethernet or USB, if the USB is connected without a powered Ethernet connection.There is a microcontroller and some other stuff not shown on this schematic, but it's not really important, the POE is the main part I haven't worked with before, so that's what I'm interested in as the "risky" system. Thanks

author: T0mn0mn0m
author_rank: Newbie
timestamp: April 13, 2014, 02:34:01 pm
content: Your PDF is a little confusing, but as long as you are using 10/100 base ethernet you should be fine. Traditional PoE runs at 42v and around 1 amp if I remember correctly, so that shouldn't be a concern. Some PoE devices carry out a handshake before delivering power to make sure the PoE injector isn't plugged into a 1000Mbit device.

author: Kjelt
author_rank: Super Contributor
timestamp: April 13, 2014, 03:34:09 pm
content: Is that MagJack just a RJ45 connector or does it also include the powerdiodes and Lan Ethernet Transformer? Missing a lot of parts otherwise

author: LukeW
author_rank: Frequent Contributor
timestamp: April 13, 2014, 03:39:52 pm
content: The jack has built in magnetics, diode bridges and LEDs.http://katalog.we-online.de/pbs/datasheet/7499210121A.pdfOh, and yes - this is intended for 10/100 use only, not gigabit.

author: gxti
author_rank: Frequent Contributor
timestamp: April 13, 2014, 04:07:11 pm
content: Looks OK. Note that in order to be compliant you would have to use an isolated switcher. Depending on the product and how it connects to the outside world (or doesn't) it may not really be necessary though. The USB connection would be one such issue, because it's going to tie your circuit ground to mains earth if it's plugged into a PC.

author: ve7xen
author_rank: Super Contributor
timestamp: April 13, 2014, 07:16:20 pm
content: Quote from: T0mn0mn0m on April 13, 2014, 02:34:01 pmStandards-based PoE (which OP here is implementing) is nominally 48V, typically 52+V. It works fine with GigE equipment as it works by putting a common mode voltage between pairs, which is removed by the magnetics before reaching the PHY. Magnetics which allow recovering this common mode voltage (ie. bring out the primary centre taps or provide a bridge recitifer like the one used here) are required for PoE. In addition, the PSE is required to negotiate with the PD before delivering power. I believe this means putting a low "sense" voltage on the line and detecting the resistance at the other end before going to full power, this to protect devices not designed for PoE.Some manufacturers or DIYers make "PoE" systems by putting a random voltage on random wires (usually the unused pairs in a 10/100 system), and maybe connecting them directly at the PD side without magnetics, but that's not what we're talking about here.As far as the OPs design, I'm not an expert, though I have considered doing something similar myself. I don't see any glaring errors.

author: LukeW
author_rank: Frequent Contributor
timestamp: April 16, 2014, 01:27:04 am
content: Actually, now that you mention it, I wonder if it might be a good idea to have some sort of support for non-negotiated el cheapo 10/100 spare-pairs non-802.3af POE.This may be done by putting a jumper in to connect POE- to ground, bypassing the internal FET inside the NCP1090, and we don't care about negotiation or lack thereof, the NCP1090 is essentially always "switched on" and power is supplied to the regulator. What do you think?

author: Teledog
author_rank: Regular Contributor
timestamp: April 16, 2014, 05:40:46 am
content: I agree with ve7xen, {73's neighbor, BTW}If one is using a true PoE switch (PSE), it interrogates the device (PD) and looks for a valid signature (resistance/capacitance within a time frame), if it's out of spec., the switch should ignore/not supply power.There are lousy switches that supply power- no matter what, ..and there are certifiably correct PoE switches.Tested racks of PoE switches in the past, some are perfect..and some manufacturers have absolutely no clue..If you want PoE regardless of any standards..then just use an injector (cheap on eBay or DX)

author: Kjelt
author_rank: Super Contributor
timestamp: April 16, 2014, 07:01:21 am
content: Quote from: Teledog on April 16, 2014, 05:40:46 amYup smoked my brandnew HP Elitebook 1000Mbps, now it only supports 100MbpsShouldn't have happened is the only thing that I heard, well it did happen.