Step-up Step-down Transformers:

        Not only is a transformer employed to transfer electric energy
from one circuit to another, it can also be used to raise or lower
the voltage to meet certain operating requirements. For example, in
practically all alternating current power lines, the voltage is
"stepped up" at the power station, carried long distances, and then
"stepped down" at the place where it is used. Also in practically
all electronic apparatus designed to operate from 120 volt ac circuits
transformers are used to lower the voltage for some circuits and raise
it for others.

        The induced secondary voltage is affected by four factors:
The speed of cutting the lines of force.
The angle of cutting
The strength of the flux. 
The length of the conductor being cut
The speed of cutting cannot be changed in a transformer because it is
controlled by the rate at which the magnetic field of the primary
builds up and collapses. When the primary circuit is closed and
opened, the change in magnetic flux takes place as rapidly as possible.
        The angle of cutting cannot be changed, since the coils are
arranged permanently on the core. Also, the strength of the field can
be ruled out because it is determined by the core material and the
ampere-turns of the primary coil, which is made up of a specific
number of turns and with a certain resistance, so that it carries a
definite current.
        Only the length factor is left. Suppose that the secondary is
wound with twice as many turns as the primary. Then, the same number
of magnetic lines, moving at the speed and cutting at the angle, but
cutting twice the length of wire, produces twice the voltage in the
secondary. The reverse is also true. If the secondary is wound with 
half as many turns as the primary, then the same number of magnetic
lines, moving at the same speed and cutting at the same angle, but
cutting half the length of wire, produces half the voltage in the
secondary.

        As a general rule, the relationship between the number of 
        turns and the voltage of each winding can be stated as:

        In a transformer, the secondary voltage divided by that
        of the primary is the same as the number of turns in the
        secondary divided by those in the primary.

        The ratio between the number of turns in the primary and
        the secondary  is called the Turns Ratio.

Remember that transformers may be used to raise or lower voltage.
A transformer with more turns of wire in the secondary than in the
primary, is known as a Step-up transformer, since it increases
the voltage.

One with fewer turns in the secondary  than in the primary, causes
a decrease in the voltage, and therefore, it is known as a Step-down
transformer.

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