power supply design (5v and 12v)

[eng_boody]

hi i want to design a 12 volt and 5 volt power supply from 16 volts..i'll use two regulators LM7805,LM7812 my question here should i put the regulators after a single bridge or use double bridges , one for each regulator , how to calculate power dissipation and sink used

note that am using single ac source

thanks

 

[crutschow]

There's no reason to use two bridges.

Is this a 16Vac output transformer? If so the filtered DC output of the bridge will be about 20V with a good size filter capacitor under load. The dissipation in each regulator will be the bridge output voltage minus the regulator output voltage times the load current.

From that you want a heat sink with a low enough thermal resistance to keep the regulator case temperature to probably no more than about 75C at a sink ambient of 25C (heat sink rise of 50C).