RJ45 with integrated magnetics - different configurations

[peter-h]

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?

[woofy]

I purchase thousands of magjacks from CONNFLY and Würth 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.

[peter-h]

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.

[thm_w]

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.

[nctnico]

I think the real question here is: how bad is it when the 75 Ohm resistors blow up? If that doesn't affect the wiring inside the magjack in any way, then I'd assume there is not a big (showstopping) problem.

[thm_w]

I think the real question here is: how bad is it when the 75 Ohm resistors blow up? If that doesn't affect the wiring inside the magjack in any way, then I'd assume there is not a big (showstopping) problem.

Good point, just tried applying power across Blue pairs to Brown pairs, blew the resistors inside. Port still works, interesting.
So I guess in my case it was always lightning failures and not passive POE injection.

Although the loss of resistor might make it more susceptible to other failures.

[nctnico]

PoE is not only injected on the blue & brown pairs, but also on orange and green (the signal pairs) and I've seen that causing problems with blown ports as a result as well. The best way to avoid problems is to used a rugged phy, series beads / resistors and TVS diodes. Many ethernet phys only allow 6V, maybe close to 7V on their pins so using the right TVS diodes is very important.

[thm_w]

OK, tried orange and green which is called "802.3af Mode A" here: https://pinoutguide.com/Net/poe_pinout.shtml
Port still working.

[Whales]

This is fascinating to read about.  Passive & non-standard PoE is more common than people imagine, thanks to IP cam installers (and early ubiquity products, but they're 802.3af compliant now).

If those 75 ohm resistors blow then the termination impedance of the ethernet cables ends up wrong.  In MANY cases things will still work, but depending on cable lengths & the devices being used you might not be able to get a reliable link (or your devices might downgrade to a slower link speed).

Next time I have a device with weird ethernet behaviours: I'm going to buzz out the resistance between pins

[ajb]

A friend of mine had an HDBaseT midspan injector that fried the upstream, non-powered device when the upstream, non-powered port was connected after the powered device—twice.  A little reverse engineering revealed that Microsemi had followed their own application schematic to the letter in this capacitively isolated midspan injector. Turns out that 57V through 100nF will wallop a PHY even with a magjack and bob smith termination in the way.

The magjack, including termination resistors, seemed to be no worse for the wear, but the PHY ended up with a low-ish resistance connection between the rx and tx pins.

[radar_macgyver]

and early ubiquity products, but they're 802.3af compliant now).

That's true of the Unifi series, but their EdgeMax devices (meant for small ISPs) use passive PoE at 24V, mainly for their radio products. EdgeMax PoE switches allow you to manually turn on 24V injection on each port.

[T3sl4co1l]

Yep, never underestimate the impulse of a mechanical connection discharging a capacitor.  The risetime is very fast (subnanosecond) and the peak current can be, well probably just amperes in this case, on up to 100s or 1000s of A for electrolytics and, say, ones' hapless screwdriver (or breadboard, or..).

Note that pins in general will mate in some random order (well, maybe it's relatively consistent for a given pair of mates, but in general, you can't know which), so you're essentially guaranteed to get some full-magnitude unbalanced surge across one or both pairs.

Note that an unbalanced discharge does flow through the common mode chokes, so the risetime will be attenuated some; but within a few hundred ns, the core will saturate, and the bulk of the pulse will continue onward to the PHY.  Which will in turn saturate in some ~µs, but in that time, the damage has been done to the PHY.

So yeah, ESD diodes are a must with PoE!

Tim

[peter-h]

For actual intended POE, the RJ45 is a different P/N anyway. It contains diodes and other stuff, IIRC.

I am not implementing POE because it increases the cost a fair bit. My product has two separately isolated-powered sections so I would need two DC-DC converters. Or one driving the main DC input. Then EMC certification stuff to be done all over again. It also BTW needs to be done so that if you have POE and the external power unit (or DC IN) is/are also connected, nothing blows up.

I think POE is big with "remote" devices (wifi APs and such) but not so much industrially which is what I am doing. POE switches can work out very expensive.

And any proper POE switch (e.g. Netgear, Cisco, etc) will not blow anything up.

I would expect blown-up 75R terminators to badly compromise operation, other than with 30cm cables I know USB is different but it totally breaks if you strip the shielding from just 3cm of the cable (as if e.g. wiring a toggle switch so you can use a USB connection for data or for just external DC power).

[mon2]

Sharing some good contacts for RJ45 with magnetics - each builds for major brands under private labels.

Jason Cai   |   Sales Engineer
Jason <jason[at]link-pp.com>
LINK-PP INT'L TECHNOLOGY CO., LIMITED
Tel:  +86-752-3322917       |    Mobile: +86-13434796102
Wechat :  jasoncai1221    |       Skype: link-pp6
Add: Bldg A-B No.54, South of Jinhu Road,Chengjiang Town, Huizhou City, 516229, China
RJ45 Connector | Lan Transformer | Optical Transceivers  | SFP+ cage & Conn

sherry
Company:Trxcom INT'L Technology Co. Limited.
Tel:0086-755-81755632
Fax:0086-755-81752963
Cel:0086-135-5491-8626
Email: sales1[at]trxcom.com
Skype: bettytrxcom
Web: www.trxcom.com
Trxcom is the official vendor for Raspberry Pi 4 PoE project
Reference Link: https://www.raspberrypi.org/products/raspberry-pi-4-model-b/

[westfw]

If those 75 ohm resistors blow then the termination impedance of the ethernet cables ends up wrong.

huh?  If they're using passive PoE, then by definition those pairs already had horrible termination impedance, and it didn't matter because nothing was using them for data anyway, and the devices were by definition not using them for any kind of PoE, either (or they'd need a different magjack.)

I suppose there is potentially a problem if you move the devices with the blown resistors to a proper network without the passive PoE; a switch might get confused.  But the device is still not using those pairs, and I'd expect the normal switch behavior would be to ignore them as well (after all, they ought to ignore a less-than-4pair cable that was only doing 10baseT, right?  Or a full cat6 cable connected to a 10Mbit only device that doesn't implement those pairs at all (should be electrically the same as the Magjack with blown resistors, right?)

[cdev]

I recently got my first POE switch which I plan to use for IoT ish stuff, Its got some issues so don't plan to use it for my main LAN, I have yet to thoroughly check it out, are there any failure modes that crop up often? One nice thing it can do is NTP, it appears! (one can even connect a GPS to it) It can do hardware time distribution which I have wanted for a long time. And of course it can power VOIP phones.. and do QOS

[Whales]

and do QOS

N.B. have a read up on bufferbloat.  QoS is basically useless if the packets are intended to be sent over the internet and your internet link is externally saturated. 

Learning this magic made all the difference in the world for me, and explained lots of weird oddities I've always blamed on my home router or ISP over the years. 

[peter-h]

one can even connect a GPS to it

Which model is it?

POE is indeed very good for VOIP phones, and WIFI APs and such like. But all those are "proper POE" devices which don't need the cheap chinese POE injectors which are the real problem. Also any POE switch will be doing proper POE so no power comes out unless negotiated.

[peter-h]

For amusement, real and fake.
The funny thing is that the real Hanrun, being chinese, is probably a counterfeit of another one

The fake one is marked F. The real one also has "Hanrun" inside it at the far end (visible in photo).

The fake one is 5.5g and the real one is 6.0g.

Both seem to work equally.

[Emo]

Hi Peter,

The GPS is connected to the USB port. Mikrotik is then able to use it as NTP

Eric

[peter-h]

Ah it must be a special GPS, with USB. Which Mikrotik box are you using and which antenna? I can't find them on their website.

[nctnico]

Ah it must be a special GPS, with USB. Which Mikrotik box are you using and which antenna? I can't find them on their website.

Likely all you need is a USB to serial adapter between the GPS receiver and Mikrotik.

[Doctorandus_P]

It's not really related because the photograph below does not have integrated magnetics, but it does show there is some weird stuff out there.
Presumably some manufacturer read somewhere the wires should be twisted.

The picture is not mine. Just found it on the 'net some years ago.

[Nominal Animal]

Presumably some manufacturer read somewhere the wires should be twisted.

Considering the numerous different pinouts for RJ45 connectors, I think it is more likely that the manufacturer created a connector without magnetics using the footprint and pinout of an existing RJ45 connector that does have the magnetics.