Hi Tony,

On most of my projects where I've used a 6v ac transformer, my psu consisted of four 1N4001 to rectify the ac, then a 7805 to regulate the dc to 5v, and place a 100uf across the dc in to the regulator, and a 10uf across the dc out. This works fine for me.

I don't know if there are any formula regarding the "correct" values and any further requirements to use Zener diodes to further regulate the supply. Basically the higher value of uf the capacitor are the smoother the dc will be, but there can be a trade off as really large values tend to be physically larger.