This is the block diagram of the most of regulated power supply.
Notes:
- Transformer – steps down 230V AC mains to low voltage AC.
- Rectifier – converts AC to DC, but the DC output is varying.
- Smoothing – smooth the DC from varying greatly to a small ripple.
- Regulator – eliminates ripple by setting DC output to a fixed voltage.
Some power supply circuits can be adjusted by add some additional components. The most of them had been placed inside the regulator block.
source: http://www.kpsec.freeuk.com/study.htm
