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A transformer is a device that changes the voltage, also called a power transformer. Performs the step-down and step-up of AC voltage, and makes adjustments necessary for use in electronic devices. The transformer has a structure in which a primary coil and a secondary coil are wound around an iron core. Depending on the position of the iron core and the coil, it is divided into an inner iron type where the coil covers the iron core and an outer iron type where the iron core is outside the coil. Wiring methods are classified into two types: single-phase, which is a 100 V power supply used in offices, and three-phase, which is a 200 V power supply that sends a large amount of power. The winding of the coil is usually compound wound in which the primary and secondary sides are insulated, but there are also smaller auto transformers. There are also types that can be connected with the cover attached, types that can be used overseas and in Japan, portable types that are suitable for on-site construction with different power and voltage, etc.