Back to back transformers.

[leadbelly]

I know this has been discussed many times but I can't find the threads I remember in a search.

On the topic of using back to back transformers for HV supplies (say 120VAC:12VAC wired to a 12VAC:120VAC), I am trying to remember all the aspects of getting it right.

The one I remember is that the transformer hooked up to the mains should have a larger VA rating to accomodate it's own mag current plus the draw from the other transformer.

The one I can't remember is the caveat about dissimilar voltage ratings or ratios (i.e. a 120VAC:12VAC transformer feeding a 6VAC or 9VAC:120VAC transformer). What's the "gotcha" for that situation?

[ErikdeBest]

The problem of feeding 12V to a 6 or 9V winding is that you will be overloading the second transformer... too high a magnetization flux (?).

Conversely, if you feed the 12V to a 15V winding you will be 'undermagnetizing' the second core...you can do it, but you should derate the VA rating of the second transformer.

And now some EE can come and explain it over again

[aardvarkash10]

watchouts for me would be: in either case you are going ot have losses - so two identical transformers back to back will not produce the same output as input. THe loss factor depends on the transformer design and quality, but I would imagine up to 30% end to end.

Your second situation implies that your output winding is going to be at a higher voltage than it was designed for. Question here is will the insulation hold out? Otherwise, as above applies - and look to stay withing the original the VA rating, which means your current output will be lower than original spec (cos the voltage is higher...) MTCW.