First post! :) I am building a Geiger counter with this circuit: http://www.pocketmagic.net/wp-content/uploads/2011/03/geiger-simple-1b.jpg in the blog the author used a regular EI transformer from a power supply, but I tried disassembling them and broke the ferrite core 4 time now, so I am planning on making a toroid transformer from an inductor. I have done a lot of research and lot of people are telling me that a toroidal transformer CAN be used instead of an EI one in an inverter circuit such as the one in the Geiger counter. So my question is : Should I use insulation between layers like in EI transformers or should I just wrap the layers on each other? Also, does it matter if the secondary was wound first rather that the primary/feedback?