Q.: Resonant Transformers

[james]

> What, exactly, is a resonant transformer?

Would you mean a ferroresonant transformer?

Ferros are a unique design that will regulate voltage output regardless
(within reason) of input voltage. They are available as stand-alone
devices and as part of high-end UPS's. They really do work very well,
but are hot, inefficient and smell (I suspect the smell is hot
windings). They are also somewhat noisy (hums).

I could waste bandwidth explaining their design, but these guys do it
better.

http://www.allaboutcircuits.com/vol_2/chpt_9/6.html