i use to read (from english cientific articles) participle adjectives at the end of the sentence like "...the scheme used." but i had learn that the adjectives should be priori the nouns (...used scheme.).

which is the correct form? is there a rule?

If you are using 'used' as an adjective it would generally come before a noun, e.g. 'used car salesman'. I have never heard of a 'used scheme' before which makes me think that you are using 'used' as a verb, in which case it is all getting a little confused...!