Special Kinds of Transformers

Tapped Transformer

Tapped transformers have an additional connection, or tap, on their primary and/or secondary windings. An additional tap lead on the secondary gives a transformer three possible output voltages.

Multiple-Winding Transformer

It is often useful to have a number of different secondary windings, each of which is electrically isolated from others (unlike a taped transformer).
Each of the secondary coils will have a voltage that is proportional to its number of turns. The secondary windings can be connected in series-aiding (voltage is summed) or series-opposing (the voltage is the difference) configurations. Dots are often used to indicate the terminals that have the same phase.

 

Autotransformer

This device only uses a single coil and a tap to
make a primary and secondary. Autotransformers
can be used to step up or step down voltages; however, they are not used for isolation application because the primary and secondary are on the same coil (there is no
electrical isolation between the two). These devices are frequently used in impedancematching applications.

 

 

 

Continuous Tapped Transformer (Variac)

This device contains a variable tap that slides up and down the secondary coil to control the secondary coil length and hence the secondary voltage.