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I have few questions about transformer operation. One of the main points of the transformer is ground isolation.

Let's say if have a transformer in an ethernet transmitter, and my circuit in the primary of the transformer runs at 5 volts.

First quesiton: What is going to be the voltage on the secondary terminals of the transformer?

Second question: Will we be also transmitting a ground reference to the receiving end? Or will it will just be a differential signal?

Third question: Do we always need a reference voltage when we send a differential signal? If not, then how does the receiving end know the reference voltage?

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  • \$\begingroup\$ Transformers are electrical devices. Transforms are mathematical constructs, often to convert values in one coordinate system to another. The fact that you got it right in the title means you know the difference. -1 for sloppiness. \$\endgroup\$
    – Olin Lathrop
    Commented Jan 10, 2014 at 19:15
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    \$\begingroup\$ The reference for a signal in a differential pair is the other signal. That's why we use them in the first place. \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Commented Jan 10, 2014 at 19:18
  • \$\begingroup\$ Does this mean that we can have unit A have a different ground, and Unit B has a different ground, and we can send a differntial signal from A to B, and the signal can be interpreted correctly by unt B, even though it has a different ground? \$\endgroup\$
    – Rudy01
    Commented Jan 10, 2014 at 19:28
  • \$\begingroup\$ @Rudy: Yes, that's the point of differential signals. Transformer isolation is a way to create differential signals with very good balance and high common mode range. This is exactly how regular 10 and 100 Mbit/s ethernet using the common RJ-45 jacks works. \$\endgroup\$
    – Olin Lathrop
    Commented Jan 10, 2014 at 19:38
  • \$\begingroup\$ I just think you know it only pass AC signals. Anyway, for example, auto-transformers does not archive ground isolation, this is not the main point of a transformer. Major types of transformer from the electrical generator to your home are auto-transformers. \$\endgroup\$
    – Diego C Nascimento
    Commented Jan 10, 2014 at 21:10

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What is going to be the voltage on the secondary terminals of the transformer?

If the transformer has a 1:1 winding ratio, then the primary voltage equals the secondary voltage at all times (neglecting leakage and other non-ideal stuff). In the more general case, the voltage ratio between the primary and secondary sides is equal to the the winding ratio.

Will we be also transmitting a ground reference to the receiving end? Or, it will just be a pair of differnetial signals?

If the receiving end is connected to the secondary, and you don't otherwise connect the secondary to ground, then there's no ground reference. That's often the point of using transformer-coupled signaling.

do we always need a reference voltage when we send a differential signal? If not then how does the receive end knows the reference voltage?

The "reference" is the other half of the differential pair.

schematic

simulate this circuit – Schematic created using CircuitLab

Here, VM1 will mirror V1, even if the two sides are connected to different grounds. The ground noise, which might represent natural variations in Earth potential between two distant points, has no effect on the signal. For signal transmission across appreciable distances, this is a really big win.

I've drawn it here with two transformers to make it symmetrical, and so you can clearly see how there's a loop of wire in the middle with no connection to anything else, but which caries the signal anyway. However, even with one transformer, the ground noise is still isolated, although each side is referenced to its own, different idea of "ground".

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  • \$\begingroup\$ +1, but it might be good to point out that two transformers are not necessary to achieve isolation between V1 and VM1 in your diagram. \$\endgroup\$
    – Olin Lathrop
    Commented Jan 10, 2014 at 20:25
  • \$\begingroup\$ Thanks. This makes sense. But then if coming out the ethenet chip is already differntial, why can't we just connect it directly to the receive end. Becasue regardless of how the groudning is, since we have a differential signal, we can always get the data, correct? Then what is the main purpose of transformas in this case? \$\endgroup\$
    – Rudy01
    Commented Jan 10, 2014 at 20:57
  • \$\begingroup\$ @Rudy01 I don't know ethernet specifically well enough to answer that. Also check out this recent question on balance for audio. Ethernet has the same problem, and I bet it uses transformers to make the balanced pair, because transformers do it better than most other solutions. \$\endgroup\$
    – Phil Frost
    Commented Jan 10, 2014 at 21:15
  • \$\begingroup\$ @PhilFrost: You pointed out to a nice article, which sorts of explains. But now I got even confused on the other article, because I still don't think that "hot" and "cold" need to be flipped of each other. As long as we can assure they are treated equally, and ending up in having the same impedence, I think it is totally fine :) \$\endgroup\$
    – Rudy01
    Commented Jan 10, 2014 at 22:13
  • \$\begingroup\$ @Rudy01 that's exactly right. Of course, if you have two transformers like this, then it doesn't matter if you think of one as flipped, or one as zero, because there's no ground. There's only the other half of the pair. \$\endgroup\$
    – Phil Frost
    Commented Jan 10, 2014 at 22:19

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