How to hold a specific voltage

[bountyhunter]

Yes, if I need a current of 3 amps I would not begin my design with a transformer only rated for 2 amps, I would likely look for a transformer rated at 4 or 5 amps to allow headroom and so it goes.

Ron

The good rule of thumb is this:

If you use a full wave bridge on the output of the transformer, assume the transformer secondary (AC RMS) current will be about 1.8X times the DC load current the regulator is supplying. So a 3A supply needs a transformer rated for at least 5.4A. Higher is always better.

If you use a full wave center tap configuration, I believe the AC secondary current of the transformer is around 1.4X times the DC load current.