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J

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Why do you say that?But it would not finish up being exactly 1 : 1. It would probably be something like 1: 0.9

If the output is unloaded, then with identical transformers, it should be very close to 1:1.

The voltage will decrease with load, of course, and the load should probably be no more than about 50% of the transformer VA rating for a tolerable voltage drop.

J

But to allow for say a 1 volt drop due to losses at full load the ratio would have to be adjusted to give 13 volts off load which would be a ratio of 240 : 13. So when the two transformers were used back to back the first transformer would be outputting 12 volts at full load.

The second transformer has a ratio of 13 : 240 (As it is used in reverse.) so (12 x 240)/13 = 221.5 volts. You would than also have resistive losses in the second transformer which would cause a further drop in the output voltage when it's 240 volt winding is loaded.

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If the transformers are identical, the unloaded output voltage will essentially equal the input voltage regardless of their turns ratio.

So the main voltage drop of concern would be due to the load current through the winding resistances.

This means the percentage voltage drop for a given load would be twice that of one transformer.

That's why I recommended loading the transformers no more than 50% of their VA rating.