Transformers are classified according to the size, the power, handled in the circuit and the number of turns in the primary and secondary coils. As far as the size is concerned transformers range from small-sized, found in a microphone to gigawatt, used in connection of national power gids. By the power handled in the circuit transformers are classified into signal transformers, small power transformers, power transformers and large power transformers. The types of transformers according to the number of turns in the coils include step up transformers, step down transformers, isolating and variable auto transformers.
Auto transformers have a single winding, in which one part serves as both primary and secondary coils. They can act as step up or step down transformers. Autotransformers are small, light, less costly and more efficient than two-winding transformers. For this reason they are widely used. Auto transformers are usually preferred to isolation transformers, which have two separate windings. One of the reasons for such preference is better voltage stability and overload tolerance of auto transformers. Functions performed by auto transformers are much the same as those of the transformers used by electric company to supply buildings with electricity. However variable auto transformers don't provide isolation from line current. In this case they cannot be substituted for isolated transformers.
Auto transformers are able to solve many problems connected with size, cost and the type of transformer required by a definite appliance. It is possible to use any conventional transformer as an auto transformer. The ratio of secondary voltage to primary determines the amount of the voltage, which can be changed on the transformer.
Auto transformers are used to convert the local voltage to the voltage, which is needed for a particular appliance. Frequently this conversion is from 250 volts to 125 volts and vice versa.
In case you want to reduce the line voltage, one of the secondary leads should be connected to primary and the voltage should be measured between the other two leads. If you observe the reduction of voltage it means you got it right. In case of the increased voltage the secondary leads should be reversed. A secondary current rating of the auto transformer must be equal to the primary current rating of the subject transformer. It means that if a subject transformer has the primary current of 5 amps, it should be used with an autotransformer, which has the secondary current of at least 5 amps.
Sometimes auto transformers are used to boost voltage over subject transformers. In this case it is necessary to limit the VA of the latter to the special rating. It is not advisable to boost voltage of the transformer by more than 20%. When the primary voltage of the subject transformer is reduced, the current rating of its secondary shouldn't be increased, since it may cause the reduction of total VA by the percentage of the voltage reduction.