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Do you mean voltage which is effected directly, or Direct Current as distinct from Alternating Current? Transformers "transform" voltage from one level to another by means of electromagnetic inductance, but to change from alternating current to direct one uses a device termed a rectifier.
Transformers can "step up" voltage from lower to higher power, or the other way around. Electric circuits generate magnetic fields, and when such a field changes voltage is produced. If voltage runs through one wire and another wire is close enough, current flows into the second wire as the field changes. Voltage and current are not the same thing, of course. Voltage is the pressure of electrons moving from one atom to another, current is the rate of flow of those electrons. It's like water in a hose, the voltage would be like the pressure pushing the water, current as the rate of flow of the water. Direct current is the flow running in one direction always. In alternating current the flow changes back and forth, which involves a process called inductance, and introduces time concepts such as frequency and angle changes, which are not relevant to this explanation.
A transformer is a passive device consisting of a ferromagnetic core and two (or more) coils of wire called "windings." Changing the current in the primary winding creates an alternating magnetic field in that core, which multiplies the field and couples the flux (or most of it) through the secondary winding(s). By means of this device voltage is stepped down (or up) from the input of the transformer to the output. Outside your house on power poles transformers step down the (in the US usually) 7,200 volts in the residential lines to 220-240 volts to your house. This is broken down into 110 to 120 in your house, all AC.
Inside most electrical devices, such as a stereo or whatever, the 110 volts (or so) is stepped down with a transformer and then rectified into direct current at 12 volts. Transformers are used to transform the level of alternating current voltage, not current. Modern rectifiers tend to consist of diodes and resistors, although tubes used to be used and still are in some applications. A rectifier shuttles half of the AC power signal to ground and passes the other half on as DC, which is then conditioned by other components called capacitors.
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