It means that the ends of the transformers' secondary winding are equal in amplitude, but 180° out of phase with each other.So does that mean each end would be 4.5V and the center 0v?
If you grounded one end of the secondary winding, the other end would show a voltage alternating above and below ground.
If you grounded the center tap, the ends would show a voltage alternating above and below ground, 1/2 the voltage measured before, and they would be 180° out of phase with each other.
Roughly, divide the VA output by the voltage output. That will give you the current output. For resistive loads like lightbulbs and resistors, it's the same as Watts. For non-power-factor corrected loads (like some computer power supplies) VA can vary significantly from wattage rating. Google is your friend here.Also, how do current rating work on a transformer. I see alot of them(on jameco anyways) listed with a VA Cap. How does that work into Max Current Output?
Well, transformers exchange voltage for current, and vice versa. However, with standard transformers, how much current they can output is related to their physical size and weight. If you want a transformer that can output 3A @ 20V, you're probably looking at a transformer that weighs in the neighborhood of 6 to 8 lbs.From what I have read the lower the voltage out the higher current output you should have, since transformers don't waste any power.