What is transformer and what is step up and step down transformer?

There is lots to learn about transformers and they are very useful

A transformer is a device for changing AC voltage to a higher or lower value
It has at least two windings surrounded by iron. Extra windings can give different voltages out in one device.

A step up transformer increases the voltage This transformer has more windings on the "secondary" side of the transformer, with fewer windings of thicker wire on the primary or input coil. If the secondary winding has 3000 turns and the primary has 1000 turns, the output voltage will be 3 times the input voltage

A step down transformer decreases the voltage. This transformer has fewer windings, using thicker wire on the secondary or output coil, with more windings of thinner wire on the primary or input coil If the secondary winding has 1,200 turns and the primary has 11,000 turns, the output voltage will be 1/10 of the input voltage.

This transformer could be used with a rectifier and a control circuit to make a battery charger for the USA, reducing the 110 volt ac supply to 12 volts ac.

This 12 volts swings between 16.97 positive and 16.97 negative with a RMS value of 12 volts. The 110 volts RMS ac input is a sine wave which swings between 155.6 volts positive and 155.6 volts negative, 60 times per second.

Any transformer can be used in reverse, so if a 12 volt battery was connected to the secondary winding of the step down transformer, using a circuit to rapidly interrupt the current, an output voltage of 38.9 volts ac would be produced on the "primary" coil. The voltage produced is 38.9 volts RMS because the supply swings between 12 volts and zero on the input, with a RMS value of 4.24 volts
RMS means Root Mean Square

Hope I didn't confuse you too much

A transformer is a device used to change alternating voltage.
A step up transformer increases alt. voltage while a step down transformer decreases alt. voltage

An electrical transformer is a static device which is used for the transformation of AC electrical signal in one circuit to the electrical signal of the same frequency in another circuit with a little loss of power. The voltage in a circuit can be increased or decreased, but with a proportional increase or decrease in the current ratings.

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