I'm having some problems designing a power supply for two voltage regulators. They are needed to power a lamp at 12V and a HT12D chip at 5V.

I am told that the peak voltage of the transformer is 22v @50hz , with an output resistance 0.9ohm. So I then full wave rectify this and am left with 20.6V. I then tried to calculate my value for the smoothing capactior using the formula below to get a 10% ripple:

C= 5xIo

Vsxf

Using this calculation I got a value of 77.27mF ( this seems like a big capacitor but I've never done anything like this before so it could be correct )

I calculated the Io by using ohm's law ( Transformer RMS / 0.9 ohm ) giving 17A

This is where my problems start to begin, I think

I'm using the 7805 and the 7815. Each are rated P<15W. Am I right in assuming that the voltage after the diode rectifier is 22-1.4= 20.6V.

I know that I'll need a resistor to limit the current inputted to the chips to ensure P<15W. I assumed the current inputted to the regulator was 0.6A with a voltage drop of 3 across the resistor ( 17.6 x .6 = 10.56W )

Now I'm pretty sure that this is wrong but as I said I haven't done anything like this before really.

Any help would be much appreciated, I think I've layed this out as clearly as I can, and thanks to anyone who replies!

Damien