Transformers only work with AC or transient voltages. They can step the input voltage up or down. The results are based on the ratio of its primary and secondary windings. No transformer operates without loss but let's assume it is perfect for the sake of expressing the concept.

If the number of windings in the primary and secondary are equal, its ratio as 1:1. That's to say if you have a 20VAC input, you'll get 20VAC output. Power remains the same.

If the primary has twice as many windings than the secondary, its ratio is 2:1. It's a step down transformer. That's to say if you have a 20VAC input, you'll get a 10VAC output. Power remains the same.

If the secondary has twice as many windings than the primary, its ratio is 1:2. It's a step up transformer. That's to say if you have a 20VAC input, you'll get a 40VAC output. Power remains the same.

The Power (Watts = Volts x Current) the primary is drawing is roughly equivalent to the power being drawn from the secondary. It can transform the Voltage:Current ratio but it can't create more power. In fact, there is always some loss.

Power doesn't magically appear so if the output voltage increases, then the output current must decrease in order for the power to remain the same. And vica versa.

There are several other quantities and considerations but that's a basic answer to your question. I hope this helped.

Regards

Arun (askIITians forum expert)