mm wrote:

On Sun, 08 Feb 2009 01:10:22 -0600, Matt
wrote:

mm wrote:
On Sat, 07 Feb 2009 18:34:52 -0600, 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.

Why do you want to do this, btw. Is there no 110 outlet at that
location? Maybe you can run one.


Trying to switch a relay that has a 120VAC coil using a nominal 24VAC
signal from a digital HVAC thermostat. I don't have a relay with a
24VAC coil handy.

Well, you can measure the resistance of the 120volt relay, and from
that do a pretty good calculation of the current it will draw, and
compare that with what I said would come out of the reversed
transformer.

I think I said 300 ma and 8 watts, but you can check. But that was
when I guessed the first transformer was 1500 ma, and someone said it
was half that.

So that should give you a good idea of whether it would work, and then
you can try it and see. It shouldn't take long to try it.

And then if you have to you can get a 24vac relay. I think they are
easily found.

Coil current with AC applied will be much lower than the value
calculated from DC resistance ... by perhaps a factor of ten or more.
Inductive reactance is the dominant contributor to impedance here
and it is proportional to frequency and isn't there at DC.

I agree with mm's comment about 24vac relays. Your HVAC thermostat
may have a solid-state relay driver that won't be happy with the
unexpected load.