Serial and parallel combinations of diodes: equivalence formulae and   their do-main of validity
  Serial and parallel combinations of diodes are studied by first establishing approached equivalence formulae. Sorting diodes enlights the true dispersion of their characteristics. This simple exercice helps students to realize that "identical" components may have in fact slightly different behaviour, especially in analogic electronic. Using very common components like diodes and simple electronics assembly, students can then face and test the notion of practical domain of validity for a theoretically established formula. Using diodes is especially interesting to introduce these notions, because the domain of application of the proposed formulae strongly depends on the components combination (parallel or serial), because of their non-linear behaviour. 
Raymond Laagel, Olivier Haeberl\'e