Choosing a MagJack - Page 1
Forum section: electronics / beginners

author: brendan-petrol
author_rank: Contributor
timestamp: August 09, 2012, 11:30:59 am
content: Hello people!I just have a quick question! I'm encorporating the Wiznet W5100 into an AVR board for a friend (yeah, I know - a beginner routing ethernet, not a good mix. The first thing I said was "you know it's probably not going to work first time, right?!"), and since I have no experience with ethernet, etc. I thought I would start off with the schematic for the WIZ811MJ (their premade module). It has some resistors and capacitors just off the RJ-45 jack, so now I start looking at MagJacks on the DigiKey!However, I notice that according to the schematic (well, the one I looked at anyway), it contains the resistors and capacitors as well as the magnetics! Huh!Does this mean I don't put the magnetics from the base schematic on mine, or will it not hurt putting both in?The MagJack I saw was thishttp://www.digikey.com/product-search/en?x=19&y=14&KeyWords=380-1119-ND- the reason I chose that one was I want to be able to run passive PoE (which was one of his requirements, the premade board can't do it).Hope that all makes sense!Brendan

author: brendan-petrol
author_rank: Contributor
timestamp: August 09, 2012, 11:50:01 am
content: Just realised I forgot to put in the base schematic I was using!http://www.sparkfun.com/datasheets/DevTools/WIZnet/DEV-09473-Datasheet_V_1.1.pdfIt's on one of the later pages, and it's not the best copy, but it gives you some indication!

author: kripton2035
author_rank: Super Contributor
timestamp: August 09, 2012, 11:53:24 am
content: beware a real poe device is difficult to make, and poe hubs are very expensive...you'd better make a "home" poe like explained here :http://www.tuxgraphics.org/electronics/200903/hobby-poe.shtmlhere is a schematic of a real poe power supply by olimex :

author: brendan-petrol
author_rank: Contributor
timestamp: August 09, 2012, 11:55:51 am
content: Hey,The passive PoE I was planning to do was to get the two spare pairs (if I remember correctly they are "differential pairs"?) to send +24v Passive across, to be then fed into a Switchmode and used in my device.Does this present any additional issues?

author: gxti
author_rank: Frequent Contributor
timestamp: August 09, 2012, 06:43:22 pm
content: Regardless of whether you're doing passive power or 802.3af you will need a jack that brings out pins from the media side, e.g. the far side of the transformer, because of course DC power can't flow through the transformer. But the one you posted does have this so you're fine. Running the DC pairs outside the plug like the tuxgraphics link above is also an option but, well, eww.With regards to the additional components needed: that jack contains magnetics (transformer and choke) and EMI suppression capacitors, but does not include termination resistors (the 51 ohm ones in the schematic). Jacks don't come with built-in termination because the details often vary depending on what circuitry the PHY uses internally to drive the line. Some use push-pull drivers, others use only open-collector drivers and rely on externally-provided bias. Just follow the datasheet for the part.Also, that Olimex schematic is not "real PoE" -- it features neither 1kV isolation nor the necessary part to signal to the upstream injector that it is present before applying power. Here's their injector:https://www.olimex.com/dev/mod-poe.html"MOD-POE doesn't follow the recomendations set by IEEE 802.3af-2003 standard."

author: brendan-petrol
author_rank: Contributor
timestamp: August 10, 2012, 12:03:09 am
content: Ahh thanks people!I cannot work out the purpose of the Common CT P2 in the magjack datasheet - what does that connect to? Also, the same with VC+ and VC- on P8 and P9 - I can't understand their use. The ones on 10 and 11 I will be using for passive PoEThanks,Brendan

author: brendan-petrol
author_rank: Contributor
timestamp: August 10, 2012, 07:52:02 am
content: I really hate doing this, but *bump*.That RJ-45 jack is the last thing I need for my project, and i'm really excited to finalize the PCB, then start ordering stuff!

author: gxti
author_rank: Frequent Contributor
timestamp: August 11, 2012, 01:00:47 am
content: P2 is the center tap and needs to be connected to 3.3v. It's called TCT in the wiznet schematic.P8 and P9 are for drawing power from the data pairs when used in an 802.3af application, you should leave them unconnected.

author: brendan-petrol
author_rank: Contributor
timestamp: August 11, 2012, 03:05:47 am
content: Ahh thanks!I just read your answer and had one of those 'I need to kick myself - of course that's the answer' moments!