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A device that is mostly used to change the voltage in an alternating current (AC); however, it can also be used to maintain the same voltage but act as an electrical isolator. The most common type is the laminated core transformer found in power supplies. Made of steel laminations wrapped with two coils of wire, the ratio of windings between the "primary" input coil and the "secondary" output coil determines the voltage change. For example, if the primary has 1,000 windings and the secondary 100, an input of 120 volts is changed to 12v.

Via ElectroMagnetic Induction
There are numerous transformer architectures, and they span the size gamut. Small ones are used in the myriad black boxes that plug into the wall and create low DC voltage for every electronic gadget, while transformers weighing tons are used to transmit 50,000 volts of AC over the nation's power grid. However, they all work via electromagnetic induction. The changing current in the primary coil induces a voltage across the secondary coil.

Switching Power Supplies
The greater the current needed to power the device, the thicker the wire in the coils and the larger the transformer. However, if a high frequency is used, the number of windings can be reduced to make the transformer small. To accomplish this, the incoming voltage is converted to DC (rectified), and a high-frequency oscillator pulses a transistor that passes the rectified voltage as square waves into a "pulse transformer." The on/off DC pulses cause the changing current in the primary coil just the same as AC does. This square wave generation turns a power supply into a "switching power supply." See power adapter, power supply and wall wart.

The Switching Power Supply
In order to reduce the number of windings in transformer coils, a high frequency pulse transformer is used. This is a hypothetical example; voltages and frequencies vary. For example, the oscillator can generate frequencies from 1 kHz to 200 kHz. Following is a simplified circuit diagram of this power supply.

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