Hi there,
Assuming you want an AC output...
If you have a transformer that you put in 120vac and you get out 12vac that's
a 10:1 ratio, so if you put in 12vac (secondary) you can get 120vac out of
the primary, at least approximately.
The only problem is that when we put in 120vac that has a 170v peak, and
the output has a 17v peak. Thus, if we put in a waveform with a 12v peak
(secondary) we would get a waveform out (primary) that is only 120v peak,
not 170v peak. The other thing is that it will be a square wave if it is
driven with a square wave multivibrator circuit. This means the rms output
will still be 120v, but it will lack the 170v peaks of a sine wave with the same
rms value.
The idea used in many low cost inverter schemes like this is to cut back the
waveform to a single pulse (rather than a real square wave) whos rms value
is equal to the original rms value, 120v. The only problem is, we cant get
a pulse like this out of the transformer primary without putting in a voltage
pulse that is higher than 12v.
One way around this is to remove turns from the secondary, but then we
run the risk that the transformer core saturates, so another way is to
add turns to the primary, which would boost the output enough so that
we can get the required peak for the modified pulse output we intend to use.
This can be hard to do too since the transformer might not have room for
more turns on it, so another idea is to find another transformer with the
right rating and ratio and connect the second transformer (primary or secondary,
as required for the turns ratio) in parallel to the secondary of the first
transformer, then connect the two outputs in series. This boosts the output
to a level that will indeed produce 170v peak, or perhaps a little lower.
If your app can stand 160v peaks you can try to boost the output by 40v
this way and that will get you there. This means you would require another
transformer that can put out 40v with 12v input, which could be a transformer
with 120vac input and 36vac output, also used in reverse. Since it would
be a good idea to have some regulation room (ie wiggle room) you might
want to think about going with a 120vac to 24vac and use in reverse and
also incorporate some feedback for output regulation.