Hi there. I want to make a transformer from 230 v AC to 40 v AC .I am doing it around a plastic cylinder and I want to know if it is wrong to put the secondary winding first and then the main one? Or it does not matter?

 

Sounds a bit of a dodgy project!
N0rmally the primary is wound first, what do you intend this for, a mains transformer is not normally wound on a cylinder, what do you intend using as a core?
Max.

 

I want to use it for water electrolysis .at the end to connect the secondary winding to a bridge rectifier .I think 40 v is safer to use than 230 v and i have 1n5400 diodes for this amount of voltage.the core is made of plastic

 

You are not going to get too much transformation using plastic!!??
Max.

 

Why not just use a cheap wall-wart supply?

What is the end goal you are trying to accomplish? Getting a bunch of hydrogen and oxygen, or building the necessary circuitry from scratch to show you can do it?