Alternating current adapters convert the 120-volt AC power from your wall outlet to a smaller voltage direct current power for your electronic device. They accomplish this in two steps, first scaling the voltage and then converting the AC to DC.
Voltage
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The AC adapter scales the 120 volts from your wall down to a voltage in the area of 6 to 18 volts using a small transformer. A transformer consists of two coils of wires interspersed around an iron core. By electromagnetic law, the ratio of the number of coil turns in one wire to the other will equal the ratio of the voltages in the two wires.
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Rectifying
After converting the power to the desired voltage level, the adapter converts the AC to DC using a setup called a rectifier. The rectifier uses a combination of diodes, one-way electronic components, to make constant DC power from alternating AC power.
Example
A 12-volt adapter performs the following steps. First, the 120 volts of AC power from the wall goes into one end of the transformer, which has 10 coil windings on the input for every 1 on the output, and induces 12-volt AC power. Next, the AC power goes through a series of diodes and comes out as 12 volts DC, ready to power your laptop, your home telephone, or to charge your cellphone.