It all depends on the current that you haven't revealed to us.
You can use rectifiers, smoothing capacitors plus linear regulators (ICs). But to regulate 15V from 30C means that the voltage regulator will have to dissipate the heat from 15V (difference between 30V and 15V) times the output current.If for example your output current is 0.5A, the power lost in the regulator is 15V*0,5A=7.5W. That is inefficient and calls for a large heat sink.
A much better option would be to use a switch mode regulator. At 80% efficiency this regulator will dissipate only 7.5W(1/80%-1)=1.9W, a considerable improvement. However, I doubt that building your own switch mode regulator is something you're up to.
I can envision two options:
- get a transformer with ~17 V output and build a simple linear regulator
- get a kit for a switch mode regulator.
