Calculation of the primitive of the W-shaped transformer is best illustrated by example. Suppose you need to calculate the transformer parameters: mains voltage U1=220; output voltage (the voltage at the secondary winding) U2=12V; load current i2=0,5 A. First, determine the power output: P2=U2*i2=12*0,5=6W. For such power you can take a magnetic core section of about four square centimeter (S=4)
Next, calculate the number of turns for one volt. For the W-shaped transformer is the formula:=50/S=50/4=12.5 turns per volt.
Then, calculate the number of turns of the primary winding: W1=U1*K=220*12,5=2750 turns. And the number of turns of the secondary winding is: W2=U2*K=12*12,5=150 turns.
After that, determine the current in the primary winding: i1=(1,1*P2)/U1=(1,1*6)/220=30mA. And then will be able to calculate the diameter of the wire of the primary winding without insulation. The fact that the maximum current for copper wire is 5 ampere per square millimeter, therefore: d1=5A/(1/i1)=5A/(1/0,03 A)=0.15 mm.
And last, calculate the diameter of the wire of the secondary winding according to the formula d2=0,025*the square root of i2, the i2 value in this formula, substitute in milliamperes: d2=0,025*22,4=0,56 mm.