Step–up transformers are used at power stations to produce the very high voltages needed to transmit electricity through the National Grid power lines. These high voltages are too dangerous to use in the home, so step–down transformers are used locally to reduce the voltage to safe levels.
Thereof, why can't you use DC in a transformer?
A transformer needs an alternating current that will create a changing magnetic field. A changing magnetic field also induces a changing voltage in a coil. This is the basis of how a transformer works: This induces an alternating current (AC) in the circuit connected to the secondary coil.
What does a turns ratio tell us about a transformer?
The ratio of the transformers primary and secondary windings with respect to each other produces either a step-up voltage transformer or a step-down voltage transformer with the ratio between the number of primary turns to the number of secondary turns being called the “turns ratio” or “transformer ratio”.
Why do we use a step down transformer in a rectifier?
Transformer is a device which reduces or increases the AC voltage. So we use step–down transformer in half wave rectifier. However, in some cases, we use a step-up transformer. In the step–down transformer, the primary winding has more turns than the secondary winding.