Differential use of transformer secondary ?

[Sorento]

I thought I knew most things about transformers, but here is something I cannot figure out with certainty:

Let's assume I had a transformer with 2 secondays, 30v and 12v.
30v is way too high, 12v too low for my application.
But the 18v between the top of the 2 windings would be spot on when I connected them at the bottom.
Questions:
1) are there any adverse effects to be expected when I loaded the 18v ?
some sort of "magnetic short" causing excessive heat in the core maybe ?
2) can I load the 18v to the full transformer rating ?
provided both windings are spec'd for the same current ?
3) could I still use the 30v and 12v as usual ?
provided the total power drawn does not exceed the ratings ?

Thanks ...

 

[Elvee]

1) are there any adverse effects to be expected when I loaded the 18v ?

Poor regulation, severely reduced power rating

some sort of "magnetic short" causing excessive heat in the core maybe ?

No

2) can I load the 18v to the full transformer rating ?
provided both windings are spec'd for the same current ?

The rating would be reduced to 18/42 of the original

3) could I still use the 30v and 12v as usual ?
provided the total power drawn does not exceed the ratings ?

Depends what you mean by "as usual", but it would severely restrict the rectifier topologies, grounds and regulators arrangements (in practice, it would be almost unusable).

 

[DF96]

Yes, you can do this but you can't draw more current than would be possible from a more conventional use of the windings. That means that your VA rating goes down, as Elvee says. The issue with transformer secondaries is current, creating heat in the winding resistance (and in the primary too).

If you use the two secondaries separately too (I can't imagine how that would work for a PSU, but OK for lighting lamps etc.) then you may get some current cancellation in one of them which will work to your advantage. However, you would need to get your sums right if you decide to exploit this.

 

[Sorento]

Thanks, Elvee and DF96