What do you want to power? There are lightweight "hi power" travel
adapters - which just have a rectifier and a fuse inside. They are
suitable for a lot of 110v kit, eg electric frypans, waffle makers, etc.
Unfortunately, that doesn't work. Usually it will result
in some combination of smoke, sparks, and a bang;-)
If you take 240V RMS and half-wave rectify it, you will
end up with an RMS voltage of 240/sqrt(2) = 170V. This
will run a 120V appliance at exactly twice it's power
rating, and whilst overloading it in this way, also stress
its insulation to twice its design voltage.
To get the power rating right, you would need to half-wave
rectify and then let only alternate pulses through by some
other means. You still have the peak voltage problem, and
a peak current which is 4 times the design rating.
I built a switcher to solve this problem some years back.
I use a pair of MOSFETs to switch the mains. What I do is
switch off during the middle portion of each positive and
negative going cycle. (IIRC, the switching angle is 43/137
degrees to produce 120V RMS from a 240V RMS sine wave,
but that's actually quite a bastard to work out with
integral calculas, although going the other way is easy.)
This has a number of benefits. The higher voltage part
of the sine wave is removed, reducing the excess stress
on the insulation of the 120V product (although it's still
higher than when running on 120V RMS sine wave). It also
keeps the instantaneous peak current as low as possible,
although the power factor will only be 0.5. It also draws
power only from the portion of the sine wave which is under-
utilised by the very many power supplies which only draw
current near the peaks, so although it generates a nasty
harmonic current, it's cancelling out the nasty harmonic
currents most other loads generate!
In my case, this runs a 1050W 120V frying pan on 240V
mains, and is a big improvement over the transformer which
we used to lug around.
