Electronics > Microcontrollers

RJ45 with integrated magnetics - different configurations

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I posted it here since the other forum sections are not as responsive on complicated matters :)

The "classic" config, of the "classic chinese HR911105A RJ45", is this

The above is also heavily counterfeited. The Hanrun part is 2x the price of the fakes but much cheaper than say Wurth parts.

It works but has a weakness: if somebody plugs in a cable which is fed by a cheap chinese POE injector (one which simply sticks 48V across the two wires - I forget which two - without any POE negotiation) that melts the 75R resistors instantly.

That leads to this one e.g. Hanrun HR913322A:

which solves that problem, but costs quite a bit more.

Does anyone bother with the capacitor version?

I purchase thousands of magjacks from CONNFLY and W├╝rth and they are both of the 1st type, with the single capacitor.
So far there have been zero reports of damaged magjacks, so not sure how big an issue it really is in practice.

Nominal Animal:
Typically these "passive injectors" put the positive voltage on J4 and J5, and the negative/ground/return on J7 and J8.
For example, many MikroTik routers can be powered using these, as they are similar to MikroTik's own RBGPOE.

Whenever this kind of "passive injector" is described, it is usually mentioned that this is not proper PoE, i.e. IEEE 802.3af-2003 (PoE), IEEE 802.3at-2009 (PoE+), or IEEE 802.3bt-2018 (PoE++ or 4PPoE), and not compatible with those.  Even in the hobbyist/Arduino environments, it is "known" that using these "passive injectors" can damage standard equipment.  (That is, anyone who asks in a forum or mailing list, will be told it may damage their equipment.)

I would only bother with the separate capacitors version, if I wanted my device to be compatible with these cheap "passive injectors", even though not able to be powered via one of those.

Interesting - thanks.

I could have also used a jack with magnetics but no resistors, and then adding all these externally costs nothing. But it's too late now; I don't want to change the PCB. I used this RJ45 because the ST 32F4 dev board had it, and that board was used as a starting point.

Using one similar to the first type, a few failures where you can measure the resistance between pins and you'll get some value other than 150R which means its blown yeah. But very rare, 1 in 500 maybe. More common to see ESD/lightning blow outs. I suppose those could have also caused the issue.

The annoying thing is there are at least 4 or 5 different Magjack pinouts. Some are even the same internal config, just external pins have been swapped around.


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