Dear Frances Wo,

Are you trying to build some super stabilizer ?! 12V to 9V to 5V - one normally does this if he has a use for the 12V and 9V also on the board. And even then seperate transformer windings for each voltage level is required - otherwise the 7812 can get hot enough to shutdown.

Normally to get 5VDC / 1A you use a transformer that can give about 7 to 9VAC / 1 x 1.414 Amps when using a full bridge rectifier.

As to the filter capacitor, generally about 1000 MFD / Amp is the guideline - and in any case dont go below 470MFD. More than this the capacitor should have a working voltage that is atleast twice the value of unregulated DC value if you want to avoid a blow up ( I dont know if you have seen electrolytics blowing. I have.)

To avoid the 7805 breaking into oscillations a 0.1Mfd across input / ground is normally provided and so also a 10MFD tantalum across output/ground.

Bye

Raghu