# What is an Autotransformer

WHAT IS AN AUTOTRANSFORMER? HOW IS IT IMPLEMENTED?
An autotransformer is a type of transformer in which a single inductor coil is used. It has a single continuous winding of coil, with a connection point between the primary and secondary sides. This connection point is called tap. The tap has its own characteristics: an adjustable joint that is used vary the number of turns and  influence the turn ratio, to either step up and step down the voltage across the device. This feature can be used to provide a variable voltage to a load connected to the transformer.

Construction:
The main difference in the construction of autotransformers for a step down or step up voltages lies in the connection between the voltage source and the connected load with the single inductor coil. Variable voltage is produced in this way.

Some key mathematical expressions for step down and step up transformers are given below:
•    For a step down transformer circuit, the voltage ratio is given by:
V1/V2 = (N1+N2) / N2
•    And the current ratio is given as:
I1/I2 = N2 / (N1+N2)
•    For a step up transformer circuit, the voltage ratio is given by:
V1/V2 = N1 / (N1+N2)
•    And the current ratio is given as:
I1/I2 = (N1+N2) / N1

Where    V1 = Voltage across the inductor coil of an autotransformer
V2 = voltage across the load connected to the autotransformer
I1 = Electric current passing through the inductor coil
I2 = Electric current passing through the load
N1 = Number of turns of coil on the primary side
N2 = Number of turns of coil on the secondary side

Comparing an autotransformer with a conventional transformer:
An autotransformer is a type of power transformer. It can transfer a larger apparent power than a two-inductor coil-wound transformer. When compared to standard transformers, autotransformers tend to be smaller in size and lighter in weight. However, a major disadvantage of an autotransformer is its lack of electrical isolation between primary and secondary windings due to the presence of a single winding inductor coil. This property of lacking electrical isolation does not make an autotransformer a useless device - application where electrical isolation is not required do exist.
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