Originally Posted by cavorite
Nice project, but a reminder to be very careful with microwave oven transformers (as the author notes on his hot-wire cutter page). Unmodified, they are seriously lethal devices and should be handled with due care and caution.
Those large, red font, printed stickers inside the microwave that say: "Danger High Voltage" should be a big hint about being careful.
Despite that, posts on the 'net can be found about people being electrocuted by those high voltage transformers.
Evidently they either couldn't read, or thought that warning label did not apply to them.
The key thing to remember, is that high voltage secondary is going to be cut away and replaced by the DIY person.
WITHOUT ANY "lets just plug it in to see if it still works"
One of the things I have noticed in the on-net posts about re-winding the secondary of the transformer for low voltage, high current output: Nobody is paying any attention to "packing density" of the hand wound coil.
Using common wire with its large area (relative to the copper area) taken up by the heavy (and low temperature) insulation.
A better way - make a "tape wound" secondary.
Bare metal copper strip, high temperature insulating tape.
With tape wider than the copper strip.
"Wire" does not have to be round, it can be rectangular in cross-section.
Its current handling (or rating) is determined by its cross-section area.
For a "good" packing density, you want a much thicker copper "tape" than insulating "tape".
But for a high current winding, that is what you need to use anyway (a thick copper tape, so the cross-section area is large enough for your purpose).
An example of insulating tape to use:
The bare copper "tape" can be found at any metal supplier that sells slit sheets, such as:
And, if you only need fewer turns than will fit in the core opening when "tape wound" - keep in mind you can wind multiple secondaries in that space that will be used in parallel.
Lots of ways to get a re-wound transformer, specifically suited to your needs, when doing a "tape wound" coil.