Re: REQ:24 VDC to 12 VDC
Depending on the current requirements using a 7812 will end up with a lot of wasted energy due to the large input overhead and inefficiency of the linear regulator
For 24 -> 12V DC-DC conversion up to say 4 amps, I would suggest either the NS simple switcher devices as they provide on-line simulation and design software or if low current (<300mA) some better cost/PCB real estate solutions are available from Linear tech.
The 7812 at high current will need some serious heatsinking and thermal management, with the correct parts selection from NS or LT and PCB layout (thermals) you might get away without the heatsink, NS and LT have PCB Heatsink layout recomendations on their sites for free.