transformer caLCULATIONS

think i have done it in ma olden days.i don't know whether its correct.input is 240v thats is a ac supply and output that you need is 16 to 20 volt so the transformer that you need is a step down transformer. the equations for transformer is input voltage divide by output voltage is equal to the number of turns in primary to the number of turn in secondary.that is e1/e2=n1/n2. which gives you the unknown quantity.power can be found out from voltage into current into power factor. Hope this solves your question.