This device works by providing two energized grids. When an insect flies between the grids it allows the potential energy to overcome the air resistance by travelling through the insect’s body. The zapper uses a transformer from a microwave oven to source that potential. The transformer produces 2.4 kV and the current is limited by a floodlight fitted inside the microwave. The side effect of using the lamp as a limiter is that it lights up with each bug zapped, providing a bit of a light show.
source : http://hackaday.com/2012/08/17/a-huge-microwave-powered-bug-zapper/#more-82990
