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How to build a Spot Welder with a microwave oven transformer. Never connect the transformer to the grid before removing the secondary coil, because it produces 2000 Volts at 500 mA, this is LETHAL. A secondary with more turns than primary rises the voltage. On the other hand, a secondary with less turns lowers the voltage and put more amperes to draw. With a thick cable we make a 2 turns coil that delivers 2 or 3 volts and many many amperes. This is why the cable has to be very thick.