Radio and Electronics (DED Philippinen, 66 p.) 
10. BLOCKS OF RADIOS / 1 / POWER SUPPLIES 

A REMARK FOR YOUR SECURITY:
As long as a transformer is used to step down from a voltage exceeding 50 Volts to an extra low voltage it is necessary to make sure that it is consisting of two separate coils which are sufficiently insulated from each other (testing potential 1500 V!!)
IT IS NOT AT ALL ALLOWED TO USE SO CALLED AUTOTRANSFORMER IN SUCH A CASE.
PRINCIPLE OF TRANSFORMERS
was taught in Electrical Science Form 2. The rough formula for the ratio of the input and output voltage is:
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But this formula takes not in account the considerable voltage drop which takes place in case current is drawn from the transformer.
TRANSFORMER PART 2
If it is necessary to built a transformer yourself, the following formulas are helpful: At first it is necessary to find the total output power of the transformer. Therefore sum up all the powers drawn from all the secondary coils (sometimes transformers have more than only one secondary coil).
P_{sec} = P_{1} + P_{2} + P_{3} .......... + P_{n}
Now it is possible to determine the inputpower for the transformer by a rough estimation which tells us:
 for a transformer below 20 W we must expect losses of .... 20 %.
 for a transformer up to 100 W we have to expect losses of 10 %
 for a transformer bigger than 100 W we have to expect losses of 5%.
P_{prim} = P_{sec} + losses
The crossection of the iron core is mainly depending on the total power. The rough formula for the AFe shown, related to transformers which reach a magnetic flux density of 1,2 Vs/m which will not be possible to reach with selfmade sheets.
A_{Fe} (cm^{2}) = P (W)
Therefore it will be better in case of selfmade sheets to enlarge the crossectional area for some per cents. Now it is possible to determine the number of turns with the formula for N.
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But if normal iron core is used and the frequency of the mains is 50 Hz it is possible to use a simplified formula N...
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For taking in account the losses you can either use the secondary windings formula or decrease the voltage of the primary coil for the percentage of losses.
N = N_{nom} + (1 + percentage of losses)
Now you are still lagging the crossection of the wires, which can be obtained by:
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If you plan to produce the iron core yourself now you have to calculate the crossection of the whole coil additional the space necessary for the coilformers, the insulation and the space lost by using round wires.
figure
This is giving you the area necessary between the iron sheets.