Self-made galvanic isolation, in the common people - lightning protection

During the assembly of the galvanic decoupling I had the idea to write about the process and the results of this epic.

To whom it is interesting, I ask under the habrakat.

It all started with the fact that the old Internet provider was “fed up” (tariffs, support service, etc.) and I decided to change it to another provider. In the process of laying the cable, it turned out that it would pass over the roof, in the open air (the equipment of the new provider in the first entrance, I live in the second), and this is an ordinary twisted pair (UTP), in addition, a lightweight version. Two pairs instead of four, and all this without a screen (only two pairs are used anyway, so no difference). I did not like the noodle variant on the roof by default, the alternative was a shielded cable for outdoor installation on the same roof at UAH 3 per meter, and these meters are 30 and not the fact that it would not be stolen later. I nevertheless agreed to the default option, if the provider doesn’t care about protecting my equipment, all the more so if the Internet works, I’ll think of something with the protection of my equipment.
')
And I started searching for low-cost protection, eventually came across this wonderful post, where all the threats that may affect cable and equipment are perfectly described.

I tried to repeat the wooden core transformer, my attempts were unsuccessful, the link did not work, although the indicators blinked, as usual. As a result, I bought a factory lightning protection, set it and calmed down a bit.


Air transformers of their own making, the finished device did not work

I was interested in the idea of ​​galvanic isolation, I began to look for various options for implementation. As a result, I found a website where in the post “How to make a trance (briefly)” (in other posts there are explanatory diagrams, photographs) I found instructions for making a transformer on a ferrite ring.

I will describe the moments of manufacture in order to bring more clarity to the understanding of what was written in the original (from my point of view):
1. We take one twisted pair from a 1.5 m UTP cable.
2. Fold in half and twist evenly to get a four-core symmetrical wire. It is necessary to look so that the conductors of the same color are located opposite each other.
3. We take a ferrite ring with a size of approximately (uncritically) 30x8x8 mm, preferably high-frequency (you can take any), sharp edges are processed with emery paper (more convenient with a file). I had the ferrite rings from the CoolerMaster cases (supplied in the kit, to reduce interference in the wires from the front panel), dimensions 28x16x7 mm, I took them.


CoolerMaster Ferrite Rings

4. We fold in half the previously obtained four-wire cable, and evenly tightening the two ends, we wind them together side by side (in parallel) on the ferrite ring until it is filled in one layer. I got 8 pairs of turns on this ring.
5. Check that the pair have the same number of turns and cut off the excess ends of the wires, leaving 30 mm each.
6. In each four-core wire connect the wires of the same color together (they are opposite each other). Each four-core wire, 1st and 2nd, turns into a symmetrical line, where: wire A (pair of one color) and wire B (pair of a different color).
7. The beginning of the wire A of the first line is connected to the end of the wire A of the second line.
8. Connect the beginning of the wire B of the second line with the end of the wire B of the first line.


Ready Transformer

The result was a symmetrical broadband transformer with medium points, matched, the input and output impedances of about 100 Ohms and the insulation breakdown voltage is much higher than in the separation transformers of network cards.

For the manufacture of galvanic isolation, you need to make two such transformers:



I think everyone remembers which pairs are used to transfer data at a speed of 100 Mbit, so I’ll quote a picture of the assembled device (first “on snot” to check):



The device started working immediately, after which it began to assemble the entire structure on a wooden ice cream stick (the first thing that caught my eye) with the help of a heat gun:



Between the stages, I checked for performance, to eliminate the possibility of error. Here I shortened the conductors for compactness:



Here are close-up connections, if anyone is interested:





And finally the finished device (sorry for the unattractive look, because of the glue - a kind of isolation):



I have the galvanic isolation according to this scheme: provider -> purchased lightning protection -> self-made electrical isolation -> router -> computer. The length of the line from the provider somewhere 50-60 meters. The connectors were borrowed from non-working network cards. No worsening in terms of speed reduction, no increase in response time.

The device was made and installed in January 2013. From the direct hit of lightning, most likely does not protect, but from interference and static completely. So calmly waiting for a stormy summer.

Update 07/21/2013:
Half a year has already passed, and the galvanic isolation both worked and works, despite the fact that there were several large thunderstorms (the closest lightning "thrashed" within a radius of about 500 meters). Communication with the equipment of the provider for all this time has not been lost. So the device was successful and regularly performs its function, despite the not very attractive appearance.