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USB Ethernet

A project log for Collection of Misc Small Project

Here is a collection of some of my small hacks. They take minutes to an evening and even then procrastination gets the better of me.

K.C. LeeK.C. Lee 02/14/2018 at 19:328 Comments

I bought a couple of "cheap" $2-ish USB Ethernet adapter from China.  The come in the usual generic cheap white plastic case.

Here is what's inside.

They cheap out on the usual Ethernet transformer and instead on your luck that the other device it connects to is implemented correctly.

The back side isn't better either.  The soldering is sloppy as there is a thread of solder at the connector pins.  The layout person obviously doesn't understand the Ethernet 1000V isolation nor how to route differential signals.  

The two terminals of those 0603 jumper are supposed to be isolated to1000V RMS in the IEEE specs.  I supposed I could have improved somewhat on the isolation by cutting and peeling off the now unconnected traces.

All I can hope for is to at least have some minimal ground isolation - probably good for 50V.  There is a lot to be said for Ethernet jack with integrated magnetic as it remove some beginner level incompetence from the equation. 

I ordered the cheapest 1:1 Ethernet transformer in SOIC16 (Pulse H1102NL) I could find for $3.17 at QTY 10.  Not sure what I am going to do with the other spare ones.

After removing the four 0 ohms resistors, I reflowed the transformer.  Here is the PCB looks like.  

I accidentally pushed R7 during reflow and the PC had some difficulty recognizing the dongle.  Upon resoldering R7, the dongle works fine.

BTW here is what's inside the transformer:

There are the two center tapped 1:1 for Tx and Rx side and their common mode filters (4 toroids below).

Discussions

Ted Yapo wrote 02/14/2018 at 21:42 point

You can see why they left out the transformer - it would double the cost :-)

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davedarko wrote 02/14/2018 at 23:16 point

it was a ten pack of thingamagingies, so only a factor of 1,2 :)

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Ted Yapo wrote 02/15/2018 at 13:13 point

True, that's not as bad.

On the other hand, you have to wonder if you opened up the cheap transformer you'd find actual magnetics or just a few 0603 jumpers....

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davedarko wrote 02/15/2018 at 13:42 point

@Ted Yapo I like the idea of that :D

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K.C. Lee wrote 02/16/2018 at 11:03 point

Actually there are real individual magnetics in the transformer I ordered.  They didn't fill those shells with goop.

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ɖҿϝիɟթվ wrote 02/15/2018 at 10:57 point

Still, you have to admire the foresight of including the footprints for the 0Ω resistors in case you want to skip the transformer. And I suppose also the two missing parts on the bottom, whatever they are.

I'm triggered by that trace going along the edge of the PCB on the back, though.

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K.C. Lee wrote 02/16/2018 at 10:55 point

The problem is that the pad distance between the two terminals of 0603 aren't really rated beyond 50 may be 75V.  The specs call for 1000V.  The "foresight" is where it will fail for the transformer option.  

Also common mistake: Noobs like to route traces between the two opposite side of isolating transformers/optos and not realize that there are creepage requirements for high voltage isolation.  Apparently no one caught on to make restrict area between isolation area in foot prints.

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K.C. Lee wrote 02/16/2018 at 11:10 point

Those would be the usual EEPROM where the MAC addresses are stored.  I don't know if the Ethernet chp comes with their own (e.g. Microchip ENCxx parts).  Not too much an issue as you can set them at driver level these days.

I would assume there is a power LED in the part of the PCB that got cut.

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