Transformers, # of secondaries and VA

Hi all, I need to buy some transformers for a PIC project I'm making for a train station model, but there's something that puzzles me.

Here are the data of two of them:

Secondary Current VA

12V 0.167A 2VA 12+12V 0.167+0.167A 8VA

Why in transformers with just one secondary the VA is V*A, while in those with two secondaries it's just the half?

I mean, 12V*0.167A is 2W, same as 2VA.. but if I use the two 12V secondaries in serie, to get a 24V secondary, multiplied for the current of 0.167A I get 4W, i.e. half of the 8VA.

Where is my mistake?

When I don't need two secondaries, should I buy a transformer with just one secondary for maximum power/money ratio?

Moreover, if the secondary gives 9V AC, after the diodes bridge and capacitor, I should expect 11.3V, right? ( i.e. 9*1.414-(0.7*2) ) The capacitor is what accounts for the *1.414, and the diodes for the the 2*0.7 voltage drop. But what should I expect under load, even assuming a low impedance secondary? My fear is that the *1.414 will be significantly lower.

I have The Art Of Electronics by Horowitz and Hill, but I didn't really understand this issue well (and it's a simple, basic one!).

One last question.. is there any way to get, from one single AC source, the effect of two secondaries? I know I could use two resistors, but that would mean a lot of power loss. Any better ways?

Thanks, Mike

You're not comparing apples with apples. Compare a 12V non-center-tapped secondary to a 6V+6V seconary. (Or 24V to 12V+12V).

Transformer design involves a number of constraints, and I think the one you're running across is wire size in the secondary (which is *a* limit on the current, there are potentially many others).

??? What "effect" do you want? Isolation voltage? Resistors won't get you that. The ability to do series and parallel for efficient 2-speed (high speed/low torque + low speed/high torque) DC motor driving? If the second, large DC train traction motors traditionally had two windings in the motor that were series-connected or paralleled at operator command. I don't know of any commonly available small motors with this "feature". More recent traction motors convert the third rail DC to AC and use power semiconductors with AC drive to give superior performance.

Tim.