Why is the center tap of the transformer connected to the positive terminal of the 12V DC battery?
We need an alternating current through the transformer.



simulate this circuit &ndash; Schematic created using CircuitLab
Figure 1. If we redraw the primary circuit like this it may become a little more clear.

When SW1 is closed current flows from right to left although only on half the winding.
When SW2 is closed current flows from left to right in the other half of the winding.

The effect on the secondary is that a voltage with alternating polarity is produced. This arrangement is the simplest method of achieving that and only two &quot;switches&quot; are required.