In this article I am going to present my struggling about rewinding a microwave oven transformer (MOT) in order to make a low voltage (20-30V) but high current (15-30A) transformer. I have done these experiments in order to make a cheap but a very handy transformer for a bench supply. These types of transformers 500-100 VA transformers cost hundreds of USD where I live, but these microwave oven transformers can be purchased for a few dollars used, because they can be only used for special applications (like where it comes from, microwaves). They usually get shorted or interrupted on the secondary winding due to the high voltage, poor insulation, thin aluminium cross section wire. Many times the primary winding remains intact.
Before starting with the practical work, we have to settle a few theoretical consideration, and proving them in small scale.