Maximum Power Transfer Formula

chech here http://en.wikibooks.org/wiki/Circuit_Theory/Maximum_Power_Transfer

Rulrs of thumb for one off second hand transformer winding.

1) Milk the previous design, if still working the primary can be salvaged, low voltage secondries turns noted and turns per volt calculated. If the ratings are known the total VA can be noted.

2) If burnt out and specification known primary turns can be counted; & VA from 3) if uncertain.

3) For a bare core the rules are:- Is it scrapless laminations? Take two E shaped laminations, put them leg to leg, the I lams should have come from the winding spaces. They are commonest so here is your answer:-leg area =stack height X tongue. At 50Hz 50 X A squared = watts; 8 sq ins per volt(divide 8 sq" by leg area).

4)Make primary build 35% of window height and secondary loss (in wire heating) similar to primary.