Matt wrote:

Ralph Mowery wrote:
"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 it is an AC output and not a DC output wall wart then you can connect
12 volts to it and get out about 120 volts if it is normally a 12 volt
output. Just put the 12 volts to the normal 12 volt output and the prongs
will have
120 volts on them . You have to check to see how much current or how
many
watts the transformer is rated for as not to overload it. If your wall
wart is good for 12 volts at one amp, then about all you can draw at 120
volts is 1/10 of an amp.

Usuallly transformers do not care if they are connected up either way.
Sometimes the output voltage will exceed the insulation of the wiring.


Thanks.

I am measuring about 25.7 VAC at the output when the input is 117.5 VAC.

The thing is labeled "Plug-in class 2 transformer, Input: AC 120V 60Hz
35W, Output: AC 24VCT 700 mA".

So it is producing well over its nominal voltage. Can I conclude that
it is unregulated---that it is just a plain old transformer?

Can I expect the same _power_ capabilities in normal and reverse
direction? Your remark about current seems to indicate that.

It has a 3-prong (grounded) plug. What could that tell somebody about
its innards?

It is very likely just a transformer; i.e., does not provide any
sort of voltage regulation.

A transformer may be operated in either direction - either winding
can be used as the input. Don't exceed the voltage or current ratings
of either winding. It is not necessarily true that the center tap
on the 24V winding is rated for 700ma, but it probably is.

The ground is probably connected to the transformer core. It might
also ground the center tap of the 24V winding. Why not use your
ohmmeter or a battery and bulb (sigh, LED) tester to find out?