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Ethernet RJ45 jack - what do the transformer centre taps do?

Started by Peter November 19, 2019
This is a common type of jack with integrated magnetics

http://www.kosmodrom.com.ua/pdf/HR911105A.pdf

It connects the txf centre taps via 75R resistors to a 1000pf cap
which then (pin 8) goes to the circuit ground.

What does this do?

There are versions of the 10/100 jack which don't use the centre taps
at all e.g.

http://peter-ftp.co.uk/screenshots/20191119201261014.jpg

Peter <nospam@nospam9876.com> wrote:
> This is a common type of jack with integrated magnetics > > http://www.kosmodrom.com.ua/pdf/HR911105A.pdf > > It connects the txf centre taps via 75R resistors to a 1000pf cap > which then (pin 8) goes to the circuit ground. > > What does this do?
The center taps are for Power-over-Ethernet. Apparently this device does not support that. Instead they terminate the line with resistors. Likely another version exists that does not do that but feeds the center taps to connector pins.
 Rob <nomail@example.com> wrote:

>Peter <nospam@nospam9876.com> wrote: >> This is a common type of jack with integrated magnetics >> >> http://www.kosmodrom.com.ua/pdf/HR911105A.pdf >> >> It connects the txf centre taps via 75R resistors to a 1000pf cap >> which then (pin 8) goes to the circuit ground. >> >> What does this do? > >The center taps are for Power-over-Ethernet. Apparently this device >does not support that. Instead they terminate the line with resistors. > >Likely another version exists that does not do that but feeds the center >taps to connector pins.
Indeed it does but it has more pins and isn't compatible with the more standard integrated-magnetics jack. http://peter-ftp.co.uk/screenshots/20191119591275514.jpg However, it cannot be quite that since the HR911105A doesn't give you access to the centre taps. So why are the centre taps there?
On 2019-11-19 15:10, Peter wrote:
> This is a common type of jack with integrated magnetics > > http://www.kosmodrom.com.ua/pdf/HR911105A.pdf > > It connects the txf centre taps via 75R resistors to a 1000pf cap > which then (pin 8) goes to the circuit ground. > > What does this do? > > There are versions of the 10/100 jack which don't use the centre taps > at all e.g. > > http://peter-ftp.co.uk/screenshots/20191119201261014.jpg >
It is called a Bob Smith termination, and should minimize EMC problems, see e.g. <https://katalog.we-online.de/pbs/download/Tutorials_Connecting+LAN+Transformers_EN+%28rev1%29.pdf> It causes problems with PoE, see e.g. http://grouper.ieee.org/groups/802/3/at/public/2005/11/robbins_1_1105.pdf Lots of Chinese 'man in the middle' PoE supplies have no proper PoE control circuis and will cause magic smoke to escape from the RJ45's. The best termination value is under discussion: https://electronics.stackexchange.com/questions/161436/why-is-bob-smith-termination-for-ethernet-recommended-if-its-wrong https://www.eetimes.com/document.asp?doc_id=1277940# Regards, Arie de Muijnck
Arie de Muynck <no.spam@no.spam.org> wrote

>On 2019-11-19 15:10, Peter wrote: >> This is a common type of jack with integrated magnetics >> >> http://www.kosmodrom.com.ua/pdf/HR911105A.pdf >> >> It connects the txf centre taps via 75R resistors to a 1000pf cap >> which then (pin 8) goes to the circuit ground. >> >> What does this do? >> >> There are versions of the 10/100 jack which don't use the centre taps >> at all e.g. >> >> http://peter-ftp.co.uk/screenshots/20191119201261014.jpg >> > >It is called a Bob Smith termination, and should minimize EMC problems, >see e.g. ><https://katalog.we-online.de/pbs/download/Tutorials_Connecting+LAN+Transformers_EN+%28rev1%29.pdf> > >It causes problems with PoE, see e.g. >http://grouper.ieee.org/groups/802/3/at/public/2005/11/robbins_1_1105.pdf >Lots of Chinese 'man in the middle' PoE supplies have no proper PoE >control circuis and will cause magic smoke to escape from the RJ45's. > >The best termination value is under discussion: >https://electronics.stackexchange.com/questions/161436/why-is-bob-smith-termination-for-ethernet-recommended-if-its-wrong >https://www.eetimes.com/document.asp?doc_id=1277940# > >Regards, >Arie de Muijnck
A very useful and interesting reply. One can achieve the BST while protecting the 75R resistors getting blown up by cheap POE injectors, by putting capacitors in series with them (22nF 100V in one jack I have seen, by Hanrun). You can't draw power from the centre taps then but at least your product won't get blown up, so if not using POE this seems to be a no-brainer. Hanrun jacks are not cheap (though nowhere near as madly priced as the "western brands" but are popular, and are widely counterfeited. One thing I wonder about is whether, on a 10/100 product (not gigabit), there is any issue with having no connection to pins 4,5 and 7,8. These are used on gigabit networks only. However it may be that the consequent lack of the BST on these two wire pairs might compromise the EMC angle. I have not seen this mentioned. Probably it *is* a problem otherwise one could get power via these pins *alone*, if the product is only 10/100. This is one attempt http://peter-ftp.co.uk/screenshots/20191119551282618.jpg but the diodes mess up the BST on the 10/100 mode, and there is no gigabit support. You get POE via both modes. This one seems to cover all bases, again only for 10/100. No POE of course. It is quite cheap http://peter-ftp.co.uk/screenshots/20191119201293018.jpg OTOH there is criticism of the BST patent. The reasoning appears to be poor. It just seems better than nothing, so everybody is using it.
On Tuesday, November 19, 2019 at 9:22:41 AM UTC-5, Rob wrote:
> Peter <nospam@nospam9876.com> wrote: > > This is a common type of jack with integrated magnetics > > > > http://www.kosmodrom.com.ua/pdf/HR911105A.pdf > > > > It connects the txf centre taps via 75R resistors to a 1000pf cap > > which then (pin 8) goes to the circuit ground. > > > > What does this do? > > The center taps are for Power-over-Ethernet. Apparently this device > does not support that. Instead they terminate the line with resistors. >
more specifically, the center tap terminations, terminate the COMMON MODE mode of the line. The DIFFERENTIAL MODE of the line is terminated by the secondary. m
makolber@yahoo.com wrote

>> The center taps are for Power-over-Ethernet. Apparently this device >> does not support that. Instead they terminate the line with resistors. >> > more specifically, the center tap terminations, terminate the COMMON MODE mode of the line. The DIFFERENTIAL MODE of the line is terminated by the secondary.
Yes; I did a lot of reading on this. The common mode termination is really for EMC and ESD performance. It does nothing for the data transfer. There are some cheap RJ45 integrated-mag jacks which don't have any centre taps; you can get them for USD 0.30. Presumably they are used on products which fake the EMC compliance i.e. most stuff made in China :) On the assumption that the 75R is the right value (which many question, but others say it is a reasonable compromise, especially as you don't know if cat5 v/ cat6 cable will be used) you cannot draw power from the taps. Well, you could draw a tiny amount of power, but probably not enough to power an ethernet device :) The other approach is to draw the required/available power while emulating the 75R resistance over the required frequency range. This is possible but not trivial. I haven't read up on it but probably some chips exist.