Ethernet magnetics question

Hi guys,

I know this is a little left field but I am designing a board at the moment to use the USBizi and wiznet chip but really scratching my head over a small problem.

I understand that between the wiznet chip and the actual RJ45 jack there needs to be a set of magnetics.

So my question is this:

Where there heck are they?

Do they
a: Come built into the RJ45 Jacks and if so why do none of them say they have them built in?
b: Have to be added as a separate chip between the 2 items and if so why are there so few of them on the market? (Well that I can find anyway…)

Can please someone point out what the heck I am missing as this is driving me quietly mad… I know it is going to be simple and I just can’t see the solution.

Thanks in advance to all,

Kind regards,


You can get them external to the jack or built in the jack. Most use built in jack as it is smaller and easier but if you are making thousands/millions then external maybe cheaper!

Thanks Gus,

I guess my problem was that I am looking at several major suppliers of RJ45 Jacks and having searched several different ways I needed to sanity check what I was doing as none of them came up that they included magnetics… but then when you look for magnetics there don’t seem to be many…

Thanks again,


Nowadays, they are almost always included inside the RJ45 “filter-jack”.

They sell them at robotshop:
(25$ the 5 pack)

They have a lot of choice at digi-key too :slight_smile:

I like the last one used by GHI in their Fez-Connect, with the status leds embedded !

You might see the term “magjack” those have the magnetics built in.

Here is one I have used from Sparkfun:

Thanks guys. It was those search terms I was missing. It’s the age old problem that they have a specific name and I just didn’t know it!

Either way, every day is a school day, so thank you all for your help!

Kind regards,


Using ethernet is not as easy as including the connector. As far as I know the physical layer is not on the chip itself and need an extra IC.