Power transformer selection/ calculations

[warpie]

Can somebody explain me why (let's say) for a -/+28VDC power supply, a 2x30V power transformer is recommended?

If my maths are correct a 2x30VAC would result in:

30 x 1.414 = 42.4
42.4 - 2V (diodes drop) =40.2V

So, the regulator's input would have to dissipate around 12V in order to output 28Vdc.

Even if we consider the Vo <= Vi +3Volts(min) requirement for a LM317 and also some mains sag, that's still a bit too high, no?

 

[gyraf]

depending on transformer's rated secondary current there may be more or less resistance in secondary - resulting in more loss at a given loading for smaller rating transformers

tl:dr The smaller the transformer, the more overvoltage required

 

[warpie]

tl:dr?

 

[merlin]

2x30Vac does sound way too much. 2x25V would make more sense

 

[warpie]

2x30Vac does sound way too much. 2x25V would make more sense

This is also my understanding but I've seen many people disagree with this.