"Matt" wrote in message
...
Suppose I have a plug-in transformer that reduces 120VAC power to some
lower AC voltage (say 12VAC or 24VAC), and suppose I want to do the
reverse---increase a low-voltage source back up to 120VAC.

Would there be a problem in connecting the low-voltage power to the
plug-in prongs of the wall wart (instead of the normal use: plugging the
prongs into the wall outlet) and expecting to get high-voltage power back
out the other end?

I guess my question is answered if I know that there are no components
other than the transformer inside the wall wart.

My immediate need is to use a 24VCT (center-tapped, 3-wire, 12VAC or
24VAC) to convert 24VAC back up to 120VAC.

If you attached 12 volts to the line side of the transformer, you'd get 1.2
volts out. You'd want to go the other way to step up , but there are other
considerations, such as the wattage you need the transformer to produce and
what the wall wart is capable of producing