Ok, now i understand perfectly the situation.
I want to thank you for your precious informations (and patience 
) for explaining this to me !
The problem i'm now facing is that i need a constant current source of 700mA (max) but that can handle a lot of watts ! (more than the ones we can find on eBay from shenzen ...)
By the way, i think i will not be able to find what i want because more than 50v DC is dangerous for human body !
Now ... i'm lost