It's a very good question that What happens when I power to 230v to 12v transformer secondary side? Its answer will clear your a big confusion about Transformer basic working principle.

First of all, let's know what is a transformer?
A transformer is an electrical machine that transfers electrical energy by increasing or decreasing the voltage but keeping constant power and frequency. Transformer works on the principle of electromagnetic induction such as self-induction, mutual induction, etc.

### Primary and Secondary winding of Transformer

The winding of a transformer that is connected with a power supply is called primary winding. The primary winding of a step-down transformer carrying high voltage and low current than the secondary winding and vice versa for a step-up transformer.

On the other hand, the winding of a transformer that is connected with load is called secondary winding. The secondary winding of a step-down transformer carrying low voltage and high current than primary winding and vice versa for a step-up transformer.

Now, your question is what will happen if we put 230V to 12V transformer secondary side that means you telling about a step-down transformer(230V/12V) whose primary winding is designed to carry 230V and the secondary winding is design to carry 12V.

So in this transformer when we apply 230V to its primary winding, we will get 12V in its secondary winding or if we apply 12V in its secondary winding we will get 230V in its primary winding.

So, now you may understand, if we put just 12V in its secondary winding it will create 230V in its primary winding, which means it stepping up the voltage at the ratio 230/12 = 19.17. So if we apply 230V in its secondary winding it will create a very high voltage in its primary winding almost 230*19.17 = 4.4KV

But practically it is not possible because, when we apply 230V to the secondary winding (which is designed to carry 12V only) it will be burned.